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6-Bit, Programmable 2-, 3-, 4-Phase Synchronous Buck Controller ADP3168
FEATURES
Selectable 2-, 3-, or 4-phase operation at up to 1 MHz per phase 10 mV worst-case differential sensing error over temperature Logic-level PWM outputs for interface to external high power drivers Active current balancing between all output phases Built-in Power Good/crowbar blanking supports On-the-fly VID code changes 6-bit digitally programmable 0.8375 V to 1.6 V output Programmable short-circuit protection with programmable latch-off delay
FUNCTIONAL BLOCK DIAGRAM
VCC
28
RAMPADJ
14
RT
13
ADP3168
UVLO SHUTDOWN AND BIAS
11
OSCILLATOR SET EN
27
EN
19
CMP
RESET
GND
DAC +150mV CMP RESET CSREF CURRENTBALANCING CIRCUIT CMP DAC -250mV CMP RESET 2-, 3-, 4-PHASE DRIVER LOGIC RESET
PWM1
26
PWM2
25
PWM3
24
PWM4
APPLICATIONS
Desktop PC power supplies for: Next generation Intel(R) processors VRM modules
10
DELAY
CROWBAR CURRENT LIMIT
23
PWRGD
SW1
22
SW2
GENERAL DESCRIPTION
The ADP3168 is a highly efficient, multiphase, synchronous buck switching regulator controller optimized for converting a 12 V main supply into the core supply voltage required by high performance Intel processors. It uses an internal 6-bit DAC to read a voltage identification (VID) code directly from the processor, which is used to set the output voltage between 0.8375 V and 1.6 V, and uses a multimode PWM architecture to drive the logic-level outputs at a programmable switching frequency that can be optimized for VR size and efficiency. The phase relationship of the output signals can be programmed to provide 2-, 3-, or 4-phase operation, allowing for the construction of up to four complementary buck switching stages. The ADP3168 also includes programmable no-load offset and slope functions to adjust the output voltage as a function of the load current so that it is always optimally positioned for a system transient. The ADP3168 also provides accurate and reliable short-circuit protection, adjustable current limiting, and a delayed Power Good output that accommodates on-the-fly output voltage changes requested by the CPU. The device is specified over the commercial temperature range of 0C to 85C and is available in a 28-lead TSSOP package.
Rev. B
Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. Trademarks and registered trademarks are the property of their respective owners.
15
21
ILIMIT EN CURRENTLIMIT CIRCUIT
12
SW3
20
SW4
17
CSSUM
16
CSREF
18
DELAY SOFTSTART
CSCOMP
8 9
FB
COMP
PRECISION REFERENCE
VID DAC
03258-001
7
1
2
3
4
5
6
FBRTN
VID4
VID3
VID2
VID1
VID0
VID5
Figure 1.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781.329.4700 www.analog.com Fax: 781.326.8703 (c) 2004 Analog Devices, Inc. All rights reserved.
ADP3168
TABLE OF CONTENTS
Specifications..................................................................................... 3 Absolute Maximum Ratings............................................................ 5 ESD Caution.................................................................................. 5 Pin Configuration and Function Descriptions............................. 6 Typical Performance Characteristics and Test Circuits............... 7 Theory of Operation ........................................................................ 9 Number of Phases......................................................................... 9 Master Clock Frequency.............................................................. 9 Output Voltage Differential Sensing .......................................... 9 Output Current Sensing .............................................................. 9 Active Impedance Control Mode............................................. 10 Current-Control Mode and Thermal Balance........................ 10 Voltage Control Mode................................................................ 10 Soft Start ...................................................................................... 10 Current-Limit, Short-Circuit, and Latch-Off Protection...... 11 Dynamic VID.............................................................................. 12 Power-Good Monitoring........................................................... 12 Output Crowbar ......................................................................... 12 Output Enable and UVLO ........................................................ 12 Application Information................................................................ 13 Setting the Clock Frequency ..................................................... 13 Soft Start and Current Limit Latch-Off Delay Times............ 13 Inductor Selection ...................................................................... 13 Designing an Inductor............................................................... 15 Selecting a Standard Inductor................................................... 15 Output Droop Resistance.......................................................... 15 Inductor DCR Temperature Correction ................................. 16 Output Offset .............................................................................. 16 COUT Selection ............................................................................. 17 Power MOSFETs......................................................................... 18 Ramp Resistor Selection............................................................ 19 COMP Pin Ramp ....................................................................... 19 Current-Limit Set Point............................................................. 19 Feedback Loop Compensation Design.................................... 19 CIN Selection and Input Current di/dt Reduction.................. 21 Tuning Procedure for the ADP3168 ........................................ 21 Layout and Component Placement.............................................. 23 General Recommendations....................................................... 23 Power Circuitry .......................................................................... 23 Signal Circuitry........................................................................... 23 Outline Dimensions ....................................................................... 24 Ordering Guide .......................................................................... 24
REVISION HISTORY
11/04--Rev. A to Rev. B Changes to Specifications ................................................................ 3 Updated Outline Dimensions ....................................................... 24 Changes to Ordering Guide .......................................................... 24 4/03--Data Sheet Changed from Rev. 0 to Rev. A. Changes to Specifications ................................................................ 2
Rev. B | Page 2 of 24
ADP3168 SPECIFICATIONS
All limits at temperature extremes are guaranteed via correlation using standard statistical quality control (SQC). VCC = 12 V, FBRTN = GND, TA = 0C to 85C, unless otherwise noted. Table 1.
Parameter ERROR AMPLIFIER Output Voltage Range Accuracy Symbol VCOMP VFB Conditions Min 0.5 -10 Typ Max 3.5 +10 Unit V mV
Line Regulation Input Bias Current FBRTN Current Output Current Gain Bandwidth Product Slew Rate VID INPUTS Input Low Voltage Input High Voltage Input Current, Input Voltage Low Input Current, Input Voltage High Pull-Up Resistance Internal Pull-Up Voltage VID Transition Delay Time1 No CPU Detection Turn-Off Delay Time OSCILLATOR Frequency Range1 Frequency Variation
VFB IFB IFBRTN IO(ERR) GBW(ERR)
Relative to nominal DAC output, Referenced to FBRTN, CSSUM = CSCOMP; see Figure 10 VCC = 10 V to 14 V
14 FB forced to VOUT - 3% COMP = FB CCOMP = 10 pF
0.05 15.5 90 500 20 25
17 120
% A A A MHz V/s V V A A k V ns ns
VIL(VID) VIH(VID) IIL(VID) IIH(VID) RVID
0.4 0.8 VID(X) = 0 V VID(X) = 1.25 V 35 0.825 400 400 -20 15 60 1.00 -30 25 115
VID code change to FB change VID code change to 11111 to PWM going low fOSC fPHASE
Output Voltage RAMPADJ Output Voltage RAMPADJ Input Current Range CURRENT SENSE AMPLIFIER Offset Voltage Input Bias Current Gain Bandwidth Product Slew Rate Input Common-Mode Range Positioning Accuracy Output Voltage Range Output Current CURRENT-BALANCE CIRCUIT Common-Mode Range Input Resistance Input Current Input Current Matching
VRT VRAMPADJ IRAMPADJ VOS(CSA) IBIAS(CSA) GBW(CSA)
TA = 25C, RT = 250 k, 4-phase TA = 25C, RT = 115 k, 4-phase TA = 25C, RT = 75 k, 4-phase RT = 100 k to GND RAMPADJ - FB
0.25 155
1.9 -50 0 -1.5 -50
200 400 600 2.0
4 245
2.1 +50 100 +1.5 +50
MHz kHz kHz kHz V mV A mV nA MHz V/s V mV V A mV k A %
CSSUM - CSREF; see Figure 5
VFB ICSCOMP VSW(X)CM RSW(X) ISW(X) ISW(X)
CCSCOMP = 10 pF CSSUM and CSREF See Figure 6 ICSCOMP = 100 A
10 10 0 -77 0.05 -80 500 -600 20 4 -5 +200 40 10 +5 3 -83 3.3
SW(X) = 0 V SW(X) = 0 V SW(X) = 0 V
30 7
1
Guaranteed by design, not tested in production.
Rev. B | Page 3 of 24
ADP3168
Parameter CURRENT-LIMIT COMPARATOR ILIMIT Output Voltage Normal Mode Shutdown Mode Output Current, Normal Mode Current-Limit Threshold Voltage Current-Limit Setting Ratio DELAY Normal Mode Voltage DELAY Overcurrent Threshold Latch-Off Delay Time SOFT START Output Current, Soft-Start Mode Soft-Start Delay Time ENABLE INPUT Input Low Voltage Input High Voltage Input Current, Input Voltage Low Input Current, Input Voltage High POWER-GOOD COMPARATOR Undervoltage Threshold Overvoltage Threshold Output Low Voltage Power-Good Delay Time VID Code Changing VID Code Static Crowbar Trip Point Crowbar Reset Point Crowbar Delay Time VID Code Changing VID Code Static PWM OUTPUTS Output Voltage Low Output Voltage High SUPPLY DC Supply Current UVLO Threshold Voltage UVLO Hysteresis Symbol Conditions Min Typ Max Unit
VILIMIT(NM) VILIMIT(SD) IILIMIT(NM) VCL VDELAY(NM) VDELAY(OC) tDELAY IDELAY(SS) tDELAY(SS)
EN > 1.7 V, RILIMIT = 250 k EN > 0.8 V, IILIMIT = -100 A EN > 1.7 V, RILIMIT = 250 k VCSREF - VCSCOMP, RILIMIT = 250 k VCL/IILIMIT
2.9
3 12 125 10.4 3 1.8 600 20 350
3.1 400 145 3.1 1.9
105 2.9 1.7
RDELAY = 250 k, CDELAY = 4.7 nF During startup, DELAY < 2.8 V RDELAY = 250 k, CDELAY= 4.7 nF VID code = 011111 15
V mV A mV mV/A V V s A s
25
VIL(EN) VIH(EN) IIL(EN) IIH(EN) VPWRGD(UV) VPWRGD(OV) VOL(PWRGD)
0.4 EN = 0 V EN = 1.25 V Relative to nominal DAC output Relative to nominal DAC output IPWRGD(SINK) = 4 mA 0.8 -1 10 -200 90 -250 150 225 250 200 150 550 250 400 160 5.0 5 6.9 0.9 500 +1 25 -325 200 400
V V A A mV mV mV s ns mV mV s ns mV V mA V V
100 VCROWBAR tCROWBAR Relative to nominal DAC output Relative to FBRTN Overvoltage to PWM going low 90 450 100
200 650
VOL(PWM) VOH(PWM)
IPWM(SINK) = 400 A IPWM(SOURCE) = 400 A
4.0
VUVLO
VCC rising
6.5 0.7
8 7.3 1.1
Rev. B | Page 4 of 24
ADP3168 ABSOLUTE MAXIMUM RATINGS
Table 2.
Parameter VCC FBRTN VID0 to VID5, EN, DELAY, ILIMIT, CSCOMP, RT, PWM1 to PWM4, COMP SW1-SW4 All Other Inputs and Outputs Operating Ambient Temperature Range Operating Junction Temperature Storage Temperature Range Junction to Air Thermal Resistance (JA) Lead Temperature (Soldering, 10 sec) Vapor Phase (60 sec) Infrared (15 sec) Rating -0.3 V to +15 V -0.3 V to +0.3 V -0.3 V to +5.5 V -5 V to +25 V -0.3 V to VCC + 0.3 V 0C to 85C 125C -65C to +150C 100C/W 300C 215C 220C
Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; functional operation of the device at these or any other conditions above those listed in the operational sections of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. Absolute maximum ratings apply individually only, not in combination. Unless otherwise specified, all other voltages are referenced to GND.
ESD CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on the human body and test equipment and can discharge without detection. Although this product features proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance degradation or loss of functionality.
Rev. B | Page 5 of 24
ADP3168 PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
VID4 1 VID3 2 VID2 3 VID1 4 VID0 5 VID5 6 FBRTN 7 FB 8 COMP 9 PWRGD 10 EN 11 DELAY 12 RT 13 RAMPADJ 14 TOP VIEW (Not to Scale)
28 VCC 27 PWM1 26 PWM2
ADP3168
25 PWM3 24 PWM4 23 SW1 22 SW2 21 SW3 20 SW4 19 GND 18 CSCOMP 17 CSSUM 16 CSREF 15 ILIMIT
03258-B-002
Figure 2. Pin Configuration
Table 3. Pin Function Descriptions
Pin No. 1 to 6 Mnemonic VID4 to VID0, VID5 Function Voltage Identification DAC Inputs. These six pins are pulled up to an internal reference, providing a Logic 1 if left open. When in normal operation mode, the DAC output programs the FB regulation voltage from 0.8375 V to 1.6 V. Leaving VID4 through VID0 open results in the ADP3168 going into a no CPU mode, shutting off its PWM outputs. Feedback Return. VID DAC and error amplifier reference for remote sensing of the output voltage. Feedback Input. Error amplifier input for remote sensing of the output voltage. An external resistor between this pin and the output voltage sets the no -load offset point. Error Amplifier Output and Compensation Point. Power Good Output. Open-drain output that pulls to GND when the output voltage is outside the proper operating range. Power Supply Enable Input. Pulling this pin to GND disables the PWM outputs. Soft-Start Delay and Current Limit Latch-Off Delay Setting Input. An external resistor and capacitor connected between this pin and GND set the soft-start ramp-up time and the overcurrent latch-off delay time. Frequency Setting Resistor Input. An external resistor connected between this pin and GND sets the oscillator frequency of the device. PWM Ramp Current Input. An external resistor from the converter input voltage to this pin sets the internal PWM ramp. Current Limit Set Point/Enable Output. An external resistor from this pin to GND sets the current limit threshold of the converter. This pin is actively pulled low when the ADP3168 EN input is low or when VCC is below its UVLO threshold to signal to the driver IC that the driver high-side and low-side outputs should go low. Current Sense Reference Voltage Input. The voltage on this pin is used as the reference for the currentsense amplifier and the Power Good and crowbar functions. This pin should be connected to the common point of the output inductors. Current-Sense Summing Node. External resistors from each switch node to this pin sum the average inductor currents to measure the total output current. Current Sense Compensation Point. A resistor and a capacitor from this pin to CSSUM determine the slope of the load line and the positioning loop response time. Ground. All internal biasing and the logic output signals of the device are referenced to this ground. Current Balance Inputs. Inputs for measuring the current level in each phase. The SW pins of unused phases should be left open. Logic-Level PWM Outputs. Each output is connected to the input of an external MOSFET driver, such as the ADP3413 or ADP3418. Connecting the PWM3 and/or PWM4 outputs to GND causes that phase to turn off, allowing the ADP3168 to operate as a 2-, 3 -, or 4 -phase controller. Supply Voltage for the Device.
7 8 9 10 11 12
FBRTN FB COMP PWRGD EN DELAY
13 14 15
RT RAMPADJ ILIMIT
16
CSREF
17 18 19 20 to 23 24 to 27
CSSUM CSCOMP GND SW4 to SW1 PWM4 to PWM1
28
VCC
Rev. B | Page 6 of 24
ADP3168 TYPICAL PERFORMANCE CHARACTERISTICS AND TEST CIRCUITS
4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 SEE EQUATION 1 FOR FREQUENCIES NOT ON THIS GRAPH
03258-B-003
5.3 TA = 25C 4-PHASE OPERATION 5.2
MASTER CLOCK FREQUENCY (MHz)
SUPPLY CURRENT (mA)
5.1
5.0
4.9
4.8
4.7
0
50
100
150 RT VALUE (k)
200
250
300
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
MASTER CLOCK FREQUENCY (MHz)
Figure 3. Master Clock Frequency vs. RT
Figure 4. Supply Current vs. Master Clock Frequency
ADP3168
1
ADP3168
VID4 VID3 VID2 VID1 VID0 VID5 FBRTN FB COMP PWRGD EN DELAY VCC 28 PWM1 27 PWM2 26 PWM3 25 PWM4 24 SW1 23 SW2 22 SW3 21 SW4 20 GND 19 CSCOMP 18 20k
12
12V
28
VCC
2
+ 1F
12V 100nF
18
CSCOMP
3
6-BIT CODE
4
39k
100nF
17
CSSUM
5 6
1k
16
CSREF VOS = CSCOMP - 1V 40
7 8 9
19
GND
03258-B-005
1.0V
1k
10
Figure 5. Test Circuit 1, Current Sense Amplifier VOS
ADP3168
12V
28 VCC
1.25V
11
100nF
CSSUM 17 CSREF 16 ILIMIT 15 250k
03258-B-007
4.7nF
250k 13 RT
14
RAMPADJ
8 FB
10k
9 COMP
Figure 7. Test Circuit 3, Closed-Loop Output Voltage Accuracy
200k
18
CSCOMP
200k
100nF
17
CSSUM
V
16
CSREF
1.0V
03258-B-006
19 GND
VFB = FBV = 80mV - FBV = 0mV
Figure 6. Test Circuit, Positioning Voltage
Rev. B | Page 7 of 24
03258-B-004
0
4.6
ADP3168
Table 4. Output Voltage vs. VID Code (X = Don't Care)
VID4 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 VID3 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 VID2 1 0 0 0 0 0 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 1 1 1 1 1 1 0 0 0 0 VID1 1 1 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 0 0 0 0 1 1 0 0 0 0 1 1 1 1 VID0 1 0 0 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 0 0 1 1 0 0 1 1 0 0 VID5 X 0 0 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 VOUT(NOM) No CPU 0.8375 V 0.850 V 0.8625 V 0.875 V 0.8875 V 0.900 V 0.9125 V 0.925 V 0.9375 V 0.950 V 0.9625 V 0.975 V 0.9875 V 1.000 V 1.0125 V 1.025 V 1.0375 V 1.050 V 1.0625 V 1.075 V 1.0875 V 1.100 V 1.1125 V 1.125 V 1.1375 V 1.150 V 1.1625 V 1.175 V 1.1875 V 1.200 V 1.2125 V VID4 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 VID3 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 VID2 0 0 0 0 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 0 0 0 VID1 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 VID0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 VID5 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 VOUT(NOM) 1.225 V 1.2375 V 1.250 V 1.2625 V 1.275 V 1.2875 V 1.300 V 1.3125 V 1.325 V 1.3375 V 1.350 V 1.3625 V 1.375 V 1.3875 V 1.400 V 1.4125 V 1.425 V 1.4375 V 1.450 V 1.4625 V 1.475 V 1.4875 V 1.500 V 1.5125 V 1.525 V 1.5375 V 1.550 V 1.5625 V 1.575 V 1.5875 V 1.600 V
Rev. B | Page 8 of 24
ADP3168 THEORY OF OPERATION
The ADP3168 combines a multimode, fixed frequency PWM control with multiphase logic outputs for use in 2-, 3-, and 4-phase synchronous buck CPU core supply power converters. The internal 6-bit VID DAC conforms to Intel's VRD/VRM 10 specifications. Multiphase operation is important for producing the high currents and low voltages demanded by today's microprocessors. Handling the high currents in a single-phase converter would place high thermal demands on system components such as inductors and MOSFETs. The multimode control of the ADP3168 ensures a stable, high performance topology for * Balancing currents and thermals between phases * High speed response at the lowest possible switching frequency and output decoupling * Minimizing thermal switching losses due to lower frequency operation * Tight load-line regulation and accuracy * High current output resulting from having up to a 4-phase operation * Reduced output ripple due to multiphase cancellation * PC board layout noise immunity * Ease of use and design due to independent component selection * Flexibility in operation for tailoring design to low cost or high performance The PWM outputs become logic-level devices once normal operation starts. The detection is normal and is intended for driving external gate drivers such as the ADP3418. Because each phase is monitored independently, operation approaching 100% duty cycle is possible. Also, more than one output can be on at any given time for overlapping phases.
MASTER CLOCK FREQUENCY
The clock frequency of the ADP3168 is set with an external resistor connected from the RT pin to ground. The frequency follows the graph in Figure 3. To determine the frequency per phase, the clock is divided by the number of phases in use. If PWM4 is grounded, divide the master clock by 3 for the frequency of the remaining phases. If PWM3 and PWM4 are grounded, divide by 2. If all phases are in use, divide by 4.
OUTPUT VOLTAGE DIFFERENTIAL SENSING
The ADP3168 combines differential sensing with a high accuracy VID DAC and reference and a low offset error amplifier to maintain a worst-case specification of 10 mV differential sensing error with a VID input of 1.6000 V over its full operating output voltage and temperature range. The output voltage is sensed between the FB and FBRTN pins. FB should be connected through a resistor to the regulation point, usually the remote sense pin of the microprocessor. FBRTN should be connected directly to the remote sense ground point. The internal VID DAC and precision reference are referenced to FBRTN, which has a minimal current of 90 A to allow accurate remote sensing. The internal error amplifier compares the output of the DAC to the FB pin to regulate the output voltage.
NUMBER OF PHASES
The number of operational phases and their phase relationship is determined by the internal circuitry that monitors the PWM outputs. Normally, the ADP3168 operates as a 4-phase PWM controller. Grounding the PWM4 pin programs 3-phase operation; grounding the PWM3 and PWM4 pins programs 2-phase operation. When the ADP3168 is enabled, the controller outputs a voltage on PWM3 and PWM4 of approximately 550 mV. An internal comparator checks each pin's voltage vs. a threshold of 400 mV. If the pin is grounded, the voltage is below the threshold and the phase is disabled. The output resistance of the PWM pin is approximately 5 k during this detection time. Any external pull-down resistance connected to the PWM pin should be at least 25 k to ensure proper operation. The phase detection is made during the first two clock cycles of the internal oscillator. After this time, if the PWM output is not grounded, the 5 k resistance is removed and switches between 0 V and 5 V. If the PWM output was grounded, it remains off.
OUTPUT CURRENT SENSING
The ADP3168 provides a dedicated current sense amplifier (CSA) to monitor the total output current for proper voltage positioning vs. load current and for current limit detection. Sensing the load current at the output gives the total average current being delivered to the load, which is an inherently more accurate method than peak current detection or sampling the current across a sense element such as the low-side MOSFET. This amplifier can be configured several ways, depending on the objectives of the system: * * * Output inductor ESR sensing without thermistor for lowest cost Output inductor ESR sensing with thermistor for improved accuracy with tracking of inductor temperature Sense resistors for most accurate measurements
Rev. B | Page 9 of 24
ADP3168
The positive input of the CSA is connected to the CSREF pin, which is connected to the output voltage. The inputs to the amplifier are summed together through resistors from the sensing element (such as the switch node side of the output inductors) to the inverting input, CSSUM. The feedback resistor between CSCOMP and CSSUM sets the gain of the amplifier, and a filter capacitor is placed in parallel with this resistor. The gain of the amplifier is programmable by adjusting the feedback resistor to set the load line required by the microprocessor. The current information is then given as the difference of CSREF - CSCOMP. This difference signal is used internally to offset the VID DAC for voltage positioning and as a differential input for the current-limit comparator. To provide the best accuracy for the current sensing, the CSA was designed to have a low offset input voltage. Also, the sensing gain is determined by external resistors so that it can be made extremely accurate. Resistors RSW1 through RSW4 (see the typical applica-tion circuit in Figure 11) can be used for adjusting thermal balance. It is best to add these resistors during the initial design, so make sure placeholders are provided in the layout. To increase the current in any given phase, make RSW for that phase larger. (Make RSW = 0 for the hottest phase and do not change during balancing.) Increasing RSW to only 500 makes a substantial increase in phase current. Increase each RSW value by small amounts to achieve balance, starting with the coolest phase first.
VOLTAGE CONTROL MODE
A high gain bandwidth voltage mode error amplifier is used for the voltage-mode control loop. The control input voltage to the positive input is set via the VID 6-bit logic code, according to the voltages listed in Table 4. This voltage is also offset by the droop voltage for active positioning of the output voltage as a function of current, commonly known as active voltage positioning. The output of the amplifier is the COMP pin, which sets the termination voltage for the internal PWM ramps. The negative input (FB) is tied to the output sense location with a resistor, RB, and is used for sensing and controlling the output voltage at this point. A current source from the FB pin flowing through RB is used for setting the no-load offset voltage from the VID voltage. The no-load voltage is negative with respect to the VID DAC. The main loop compensation is incorporated into the feedback network between FB and COMP.
ACTIVE IMPEDANCE CONTROL MODE
For controlling the dynamic output voltage droop as a function of output current, a signal proportional to the total output current at the CSCOMP pin can be scaled to be equal to the droop impedance of the regulator times the output current. This droop voltage is then used to set the input control voltage to the system. The droop voltage is subtracted from the DAC reference input voltage directly to tell the error amplifier where the output voltage should be. This differs from previous implementations and allows enhanced feed-forward response.
CURRENT-CONTROL MODE AND THERMAL BALANCE
The ADP3168 has individual inputs that are used for monitoring the current in each phase. This information is combined with an internal ramp to create a current-balancing feedback system that has been optimized for initial current balance accuracy and dynamic thermal balancing during operation. This current-balance information is independent of the average output current information used for positioning described previously. The magnitude of the internal ramp can be set to optimize the transient response of the system. It also monitors the supply voltage for feed-forward control for changes in the supply. A resistor connected from the power input voltage to the RAMPADJ pin determines the slope of the internal PWM ramp. Detailed information about programming the ramp is given in the Application Information section. External resistors can be placed in series with individual phases, for example, to create an intentional current imbalance so one phase may have better cooling and can support higher currents.
SOFT START
The power-on ramp-up time of the output voltage is set with a capacitor and a resistor in parallel from the DELAY pin to ground. The RC time constant also determines the current-limit latch-off time, as explained in the following section. In UVLO or when EN is a logic low, the DELAY pin is held at ground. After the UVLO threshold is reached and EN is a logic high, the DELAY capacitor is charged up with an internal 20 A current source. The output voltage follows the ramping voltage on the DELAY pin, limiting the inrush current. The soft-start time depends on the values of VID DAC and CDLY, with a secondary effect from RDLY. Refer to the Application Information section for detailed information on setting CDLY. When the PWRGD threshold is reached, the soft-start cycle is stopped and the DELAY pin is pulled up to 3 V. This ensures that the output voltage is at the VID voltage when the PWRGD signals to the system that the output voltage is good. If EN is taken low or VCC drops below UVLO, the DELAY capacitor is reset to ground to be ready for another soft-start cycle. Figure 8 shows a typical start-up sequence for the ADP3168.
Rev. B | Page 10 of 24
ADP3168
The latch-off function can be reset either by removing and reapplying VCC to the ADP3168, or by pulling the EN pin low for a short time. To disable the short-circuit latch-off function, the external resistor to ground should be left open, and a high value (>1 M) resistor should be connected from DELAY to VCC. This prevents the DELAY capacitor from discharging, so the 1.8 V threshold is never reached. The resistor has an impact on the soft-start time because the current through it adds to the internal 20 A current source.
Figure 8. Start-Up Waveforms, Circuit of Figure 12. Channel 1--PWRGD, Channel 2--VOUT, Channel 3--High-Side MOSFET VGS, Channel 4--Low-Side MOSFET VGS
CURRENT-LIMIT, SHORT-CIRCUIT, AND LATCH-OFF PROTECTION
The ADP3168 compares a programmable current-limit set point to the voltage from the output of the current-sense amplifier. The level of current limit is set with the resistor from the ILIMIT pin to ground. During normal operation, the voltage on ILIMIT is 3 V. The current through the external resistor is internally scaled to give a current-limit threshold of 10.4 mV/A. If the difference in voltage between CSREF and CSCOMP rises above the current-limit threshold, the internal current-limit amplifier controls the internal COMP voltage to maintain the average output current at the limit. After the limit is reached, the 3 V pull-up on the DELAY pin is disconnected, and the external delay capacitor is discharged through the external resistor. A comparator monitors the DELAY voltage and shuts off the controller when the voltage drops below 1.8 V. The current-limit latch-off delay time is therefore set by the RC time constant discharging from 3 V to 1.8 V. The Application Information section discusses the selection of CDLY and RDLY. Because the controller continues to cycle the phases during the latch-off delay time, if the short is removed before the 1.8 V threshold is reached, the controller returns to normal operation. The recovery characteristic depends on the state of PWRGD. If the output voltage is within the PWRGD window, the controller resumes normal operation. However, if a short circuit has caused the output voltage to drop below the PWRGD threshold, a soft-start cycle is initiated.
03258-B-008
Figure 9. Overcurrent Latch-Off Waveforms, Circuit of Figure 11. Channel 1-- PWRGD, Channel 2--VOUT, Channel 3--CSCOMP Pin of ADP3168, Channel 4--High-Side MOSFET VGS
During startup, when the output voltage is below 200 mV, a secondary current limit is active. This is necessary because the voltage swing of CSCOMP cannot go below ground. This secondary current limit controls the internal COMP voltage to the PWM comparators to 2 V. This limits the voltage drop across the low-side MOSFETs through the current-balance circuitry. There is also an inherent per-phase current limit that protects individual phases in the case where one or more phases stop functioning because of a faulty component. This limit is based on the maximum normal mode COMP voltage.
Rev. B | Page 11 of 24
03258-B-009
ADP3168
DYNAMIC VID
The ADP3168 incorporates the ability to dynamically change the VID input while the controller is running. This allows the output voltage to change while the supply is running and supplying current to the load. This is commonly referred to as VID on-the-fly (OTF). A VID OTF can occur under either light load or heavy load conditions. The processor signals the controller by changing the VID inputs in multiple steps from the start code to the finish code. This change can be either positive or negative. When a VID input changes state, the ADP3168 detects the change and ignores the DAC inputs for a minimum of 400 ns. This prevents a false code due to logic skew while the six VID inputs are changing. Additionally, the first VID change initiates the PWRGD and CROWBAR blanking functions for a minimum of 250 s to prevent a false PWRGD or CROWBAR event. Each VID change resets the internal timer. Figure 10 shows VID on-the-fly performance when the output voltage is stepping up and the output current is switching between minimum and maximum values, which is the worst-case situation.
OUTPUT CROWBAR
As part of the protection for the load and output components of the supply, the PWM outputs are driven low (turning on the low-side MOSFETs) when the output voltage exceeds the upper Power-Good threshold. This crowbar action stops once the output voltage has fallen below the release threshold of approximately 450 mV. Turning on the low-side MOSFETs pulls down the output as the reverse current builds up in the inductors. If the output overvoltage is due to a short of the high-side MOSFET, this action current limits the input supply or blows its fuse, protecting the microprocessor from destruction.
OUTPUT ENABLE AND UVLO
The input supply (VCC) to the controller must be higher than the UVLO threshold, and the EN pin must be higher than its logic threshold for the ADP3168 to begin switching. If UVLO is less than the threshold or the EN pin is a logic low, the ADP3168 is disabled. This holds the PWM outputs at ground, shorts the DELAY capacitor to ground, and holds the ILIMIT pin at ground. In the application circuit, the ILIMIT pin should be connected to the OD pins of the ADP3418 drivers. Because ILIMIT is grounded, this disables the drivers so that both DRVH and DRVL are grounded. This feature is important to prevent discharging of the output capacitors when the controller is shut off. If the driver outputs were not disabled, a negative voltage could be generated on the output due to the high current discharge of the output capacitors through the inductors.
Figure 10. VID On-the-Fly Waveforms, Circuit of Figure 12. VID Change = 5 mV, 5 s per Step, 50 Steps, IOUT Change =5 A to 65 A
POWER-GOOD MONITORING
The Power-Good comparator monitors the output voltage via the CSREF pin. The PWRGD pin is an open-drain output whose high level (when connected to a pull-up resistor) indicates that the output voltage is within the nominal limits specified in Table 1 based on the VID voltage setting. PWRGD goes low if the output voltage is outside of this specified range. PWRGD is blanked during a VID OTF event for a period of 250 s to prevent false signals during the time the output is changing.
Rev. B | Page 12 of 24
03258-B-010
ADP3168 APPLICATION INFORMATION
The design parameters for a typical Intel VRD 10 compliant CPU application are as follows: * Input voltage (VIN) = 12 V * VID setting voltage (VVID) = 1.500 V * Duty cycle (D) = 0.125 * Nominal output voltage at no load (VONL) = 1.480 V * Nominal output voltage at 65 A load (VOFL) = 1.3955 V * Static output voltage drop based on a 1.3 m load line (RO) from no load to full load * (VD) = VONL - VOFL = 1.480 V - 1.3955 V = 84.5 mV * Maximum output current (IO) = 65 A * Maximum output current step (IO) = 60 A * Number of phases (n) = 3 * Switching frequency per phase (fSW) = 267 kHz The closest standard value for CDLY is 39 nF. Once CDLY has been chosen, RDLY can be calculated for the current-limit latch-off time using
RDLY = 1.96 x t DELAY C DLY
(3)
If the result for RDLY is less than 200 k, a smaller soft-start time should be considered by recalculating the equation for CDLY, or a longer latch-off time should be used. In no case should RDLY be less than 200 k . In this example, a delay time of 8 ms gives RDLY = 402 k. The closest standard 5% value is 390 k.
INDUCTOR SELECTION
The choice of inductance for the inductor determines the ripple current in the inductor. Less inductance leads to more ripple current, which increases the output ripple voltage and conduction losses in the MOSFETs but allows using smaller inductors and, for a specified peak-to-peak transient deviation, less total output capacitance. Conversely, a higher inductance means lower ripple current and reduced conduction losses but requires larger inductors and more output capacitance for the same peak-to-peak transient deviation. In any multiphase converter, a practical value for the peak-to-peak inductor ripple current is less than 50% of the maximum dc current in the same inductor. Equation 4 shows the relationship between the inductance, oscillator frequency, and peak-to-peak ripple current in the inductor. Equation 5 can be used to determine the minimum inductance based on a given output ripple voltage.
IR = L VVID x (1 - D ) f SW x L
SETTING THE CLOCK FREQUENCY
The ADP3168 uses a fixed-frequency control architecture. The frequency is set by an external timing resistor (RT). The clock frequency and the number of phases determine the switching frequency per phase, which relates directly to switching losses and the sizes of the inductors and input and output capacitors. With n = 3 for three phases, a clock frequency of 800 kHz sets the switching frequency, fSW, of each phase to 267 kHz, which represents a practical trade-off between the switching losses and the sizes of the output filter components. Figure 3 shows that to achieve an 800 kHz oscillator frequency, the correct value for RT is 249 k. Alternatively, the value for RT can be calculated using
RT =
1
(n x
f SW x 5.83 pF) -
1 1.5M
(1)
(4)
where 5.83 pF and 1.5 M are internal IC component values. For good initial accuracy and frequency stability, a 1% resistor is recommended.
VVID x RO x (1 - (n x D )) f SW x V RIPPLE
(5)
SOFT START AND CURRENT LIMIT LATCH-OFF DELAY TIMES
Because the soft-start and current limit latch-off delay functions share the DELAY pin, these two parameters must be considered together. The first step is to set CDLY for the soft-start ramp. This ramp is generated with a 20 A internal current source. The value of RDLY has a second-order impact on the soft-start time because it sinks part of the current source to ground. However, as long as RDLY is kept greater than 200 k, this effect is minor. The value for CDLY can be approximated using
VVID C DLY = 20 A - 2 x RDLY t ss x V VID
Solving Equation 5 for a 10 mV p-p output ripple voltage yields:
L 1.5 V x 1.3 m x (1 - 0.375 ) = 456 nH 267 kHz x 10 mV
If the resulting ripple voltage is less than that designed for, the inductor can be made smaller until the ripple value is met. This allows optimal transient response and minimum output decoupling. The smallest possible inductor should be used to minimize the number of output capacitors. Choosing a 600 nH inductor is a good starting point and gives a calculated ripple current of 8.2 A. The inductor should not saturate at the peak current of 25.8 A and should be able to handle the sum of the power dissipation caused by the average current of 22.7 A in the winding and core loss.
(2)
where tSS is the desired soft-start time. Assuming an RDLY of 390 k and a desired a soft-start time of 3 ms, CDLY is 36 nF.
Rev. B | Page 13 of 24
ADP3168
Another important factor in the inductor design is the DCR, which is used for measuring the phase currents. A large DCR causes excessive power losses, while too small a value leads to
L1 1.6H VIN 12V VIN RTN D1 1N4148WS 470F/16V x 6 NICHICON PW SERIES + C1 + C6 C9 4.7F
increased measurement error. A good rule is to have the DCR be about 1 to 11/2 times the droop resistance (RO). Our example uses an inductor with a DCR of 1.6 m.
U2 C8 ADP3418 100nF
1 2 3 4
D2 1N4148WS
BST IN OD VCC
DRVH 8 SW 7 PGND 6 DRVL 5
Q1 IPD12N03L
L2 600nH/1.6m C10 4.7nF R1 2.2
820F/2.5V x 8 FUJITSU RE SERIES 8m ESR (EACH) + C21 + C28
VCC(CORE) 0.8375V-1.6V 65A AVG, 74A PK VCC(CORE) RTN
C7 4.7F
Q3 IPD06N03L Q2 IPD06N03L 10F x 23MLCC AROUND SOCKET
D3 1N4148WS
1 2 3 4
C12 U3 ADP3418 100nF BST IN OD VCC DRVH 8 SW 7 PGND 6 DRVL 5
C13 4.7F Q4 IPD12N03L
L3 600nH/1.6m C14 4.7nF R2 2.2
C11 4.7F Q6 IPD06N03L Q5 IPD06N03L D4 1N4148WS
1 2 3 4
U4 ADP3418
BST IN OD VCC DRVH 8 SW 7 PGND 6 DRVL 5
C16 100nF
C17 4.7F Q7 IPD12N03L
L4 600nH/1.6m C18 4.7nF R3 2.2
C15 4.7F Q9 Q8 IPD06N03L IPD06N03L
RTH 100k, 5%
R4 10
C19 1F
+ C20 33F RR 383k
1 2 3
U1 ADP3168
VCC 28 PWM1 27 PWM2 26 PWM3 25 PWM4 24 SW1 23 RSW21 SW2 22 RSW31 SW3 21 SW4 20 GND 19 CSCOMP 18 CSSUM 17 CSREF 16 ILIMIT 15 RLIM 200k CCS2 RCS1 1.5nF 35.7k RPH3 124k RCS2 73.2k RPH2 124k RPH1 124k RSW11
VID4 VID3 VID2 VID1 VID0 VID5 FBRTN FB COMP PWRGD EN DELAY RT RAMPADJ
FROM CPU
4 5 6 7 8 9 10 11 12
CB 1.5nF CFB 33pF POWER GOOD ENABLE RB 1.33k CA RA 390pF 16.9k
CDLY 39nF
RDLY 390k
13
CCS1 2.2nF
RT 249k
14
NOTE: 1 FOR A DESCRIPTION OF OPTIONAL R SW RESISTORS, SEE THE THEORY OF OPERATION SECTION.
Figure 11. 65 A Intel Pentium 4-CPU Supply Circuit, VRD 10 Design
Rev. B | Page 14 of 24
03258-011
ADP3168
DESIGNING AN INDUCTOR
Once the inductance and DCR are known, the next step is to either design an inductor or find a standard inductor that comes as close as possible to meeting the overall design goals. It is also important to have the inductance and DCR tolerance specified to control the accuracy of the system. 15% inductance and 8% DCR (at room temperature) are reasonable tolerances that most manufacturers can meet. The first decision in designing the inductor is to choose the core material. There are several possibilities for providing low core loss at high frequencies. Two examples are the powder cores (e.g., Kool-M(R) from Magnetics, Inc. or Micrometals) and the gapped soft ferrite cores (e.g., 3F3 or 3F4 from Philips). Low frequency powdered iron cores should be avoided due to their high core loss, especially when the inductor value is relatively low and the ripple current is high. The best choice for a core geometry is a closed-loop type such as a pot core, PQ, U, or E core or toroid. A good compromise between price and performance is a core with a toroidal shape. There are many useful references for quickly designing a power inductor, such as the following: * * Magnetic Designer Software Intusoft (www.intusoft.com) Designing Magnetic Components for High-Frequency DC-DC Converters, by William T. McLyman, Kg Magnetics, Inc., ISBN 1883107008
OUTPUT DROOP RESISTANCE
The design requires that the regulator output voltage measured at the CPU pins drops when the output current increases. The specified voltage drop corresponds to a dc output resistance (RO). The output current is measured by summing together the voltage across each inductor and passing the signal through a low-pass filter. This summer filter is the CS amplifier configured with resistors RPH(X) (summers), and RCS and CCS (filter). The output resistance of the regulator is set by the following equations, where RL is the DCR of the output inductors:
RO = RCS x RL RPH ( x )
(6)
CCS =
L RL x RCS
(7)
One has the flexibility of choosing either RCS or RPH(X). It is best to select RCS equal to 100 k, and then solve for RPH(X) by rearranging Equation 6.
RPH (x ) = RPH ( x ) = RL x RCS RO
1.6 m x 100 k = 123 k 1.3 m
Next, use Equation 6 to solve for CCS.
CCS = 600 nH = 3.75 nF 1.6 m x 100 k
SELECTING A STANDARD INDUCTOR
The companies listed below can provide design consultation and deliver power inductors optimized for high power applications upon request.
Power Inductor Manufacturers
* Coilcraft (847)639-6400 www.coilcraft.com Coiltronics (561)752-5000 www.coiltronics.com Sumida Electric Company (510) 668-0660 www.sumida.com Vishay Intertechnology (402) 563-6866 www.vishay.com
It is best to have a dual location for CCS in the layout so standard values can be used in parallel to get as close to the value desired. For this example, choosing CCS to be 1.5 nF and 2.2 nF in parallel is a good choice. For best accuracy, CCS should be a 5% or 10% NPO capacitor. The closest standard 1% value for RPH(X) is 124 k.
*
*
*
Rev. B | Page 15 of 24
ADP3168
INDUCTOR DCR TEMPERATURE CORRECTION
With the inductor's DCR being used as the sense element and copper wire being the source of the DCR, one needs to compensate for temperature changes of the inductor's winding. Fortunately, copper has a well-known temperature coefficient (TC) of 0.39%/C. If RCS is designed to have an opposite and equal percentage change in resistance to that of the wire, it cancels the temperature variation of the inductor's DCR. Due to the nonlinear nature of NTC thermistors, resistors RCS1 and RCS2 are needed (see Figure 12) to linearize the NTC and produce the desired temperature tracking.
PLACE AS CLOSE AS POSSIBLE TO NEAREST INDUCTOR OR LOW-SIDE MOSFET RTH TO SWITCH NODES TO VOUT SENSE
4.
Compute the relative values for RCS1, RCS2, and RTH using: ( A - B ) x 1 x 2 x - A x (1 - B ) x 2 + B x (1 - A ) x 1 RCS 2 = A x (1 - B ) x 1 - B x (1 - A ) x 2 - ( A - B )
RCS1 =
(1 - A) 1 A - 1 - RCS2 1 - RCS 2
1 1 1 - 1 - RCS2 RCS1
(8)
RTH =
5.
Calculate RTH = RTH x RCS, then select the closest value of thermistor available. Also compute a scaling factor k based on the ratio of the actual thermistor value used relative to the computed one:
k= RTH ( ACTUAL ) RTH (CALCULATED )
(9)
RPH1
RPH2
RPH3
6.
ADP3168
CSCOMP RCS1
18
RCS2 KEEP THIS PATH AS SHORT AS POSSIBLE AND WELL AWAY FROM SWITCH NODE LINES
03258-B-012
Finally, calculate values for RCS1 and RCS2 using Equation 10: RCS1 = RCS x k x CS1 (10) RCS2 = RCS x ((1 - k ) + (k x CS 2 ))
CCS 1.8nF CSSUM
17
CSREF
16
Figure 12. Temperature Compensation Circuit Values
The following procedure and expressions yield values to use for RCS1, RCS2, and RTH (the thermistor value at 25C) for a given RCS value. 1. Select an NTC based on type and value. Because there is no value yet, start with a thermistor with a value close to RCS. The NTC should also have an initial tolerance of better than 5%. Based on the type of NTC, find its relative resistance value at two temperatures. The temperatures that work well are 50C and 90C. We will call these resistance values A (RTH(50C)/RTH(25C)) and B (RTH(90C)/RTH(25C)). Note that the NTC's relative value is always 1 at 25C. Find the relative value of RCS required for each of these temperatures. This is based on the percentage change needed, which in this example is initially 0.39%/C. These are called r1 (1/(1 + TC x (T1 - 25))) and r2 (1/(1 + TC x (T2 - 25))), where TC = 0.0039, T1 = 50C, and T2 = 90C.
For this example, RCS has been chosen to be 100 k , so we start with a thermistor value of 100 k. Looking through available 0603 size thermistors, we find a Vishay NTHS0603N01N1003JR NTC thermistor with A = 0.3602 and B = 0.09174. From these we compute RCS1 = 0.3796, RCS2 = 0.7195, and RTH = 1.0751. Solving for RTH yields 107.51 k, so we choose 100 k, making k = 0.9302. Finally, we find RCS1 and RCS2 to be 35.3 k and 73.9 k. Choosing the closest 1% resistor values yields a choice of 35.7 k and 73.2 k.
OUTPUT OFFSET
Intel's specification requires that at no load the nominal output voltage of the regulator be offset to a lower value than the nominal voltage corresponding to the VID code. The offset is set by a constant current source flowing out of the FB pin (IFB) and flowing through RB. The value of RB can be found using Equation 11:
RB = VVID - VONL I FB
2.
3.
(11)
RB = 1.5 V - 1.480 V = 1.33 k 15 A
The closest standard 1% resistor value is 1.33 k.
Rev. B | Page 16 of 24
ADP3168
COUT SELECTION
The required output decoupling for the regulator is typically recommended by Intel for various processors and platforms. One can also use some simple design guidelines to determine what is required. These guidelines are based on having both bulk and ceramic capacitors in the system. The first thing is to select the total amount of ceramic capacitance. This is based on the number and type of capacitor to be used. The best location for ceramics is inside the socket, with 12 to 18 of size 1206 being the physical limit. Others can be placed along the outer edge of the socket as well. Combined ceramic values of 200 F to 300 F are recommended, usually made up of multiple 10 F or 22 F capacitors. Select the number of ceramics and then find the total ceramic capacitance (CZ). Next, there is an upper limit imposed on the total amount of bulk capacitance (CX) when one considers the VID on-the-fly voltage stepping of the output (voltage step VV in time tV with error of VERR) and a lower limit based on meeting the critical capacitance for load release for a given maximum load step IO:
L x IO C x ( MIN ) - Cz nx R xV VID O
For our example, 23 10 F 1206 MLC capacitors (CZ = 230 F) were used. The VID on-the-fly step change is 250 mV in 150 s with a setting error of 2.5 mV. Solving for the bulk capacitance yields
600 nH x 60 A C x (MIN ) 3 x 1.3 m x 1.5 V - 230 F = 5.92 mF 600 nH x 250 mV C x (MAX ) x 2 3 x 4.6 2 x (1.3 m ) x 1.5 V
2 150 s x 1.5 V x 3 x 4.6 x 1.3 m - 1 - 230 F 1+ 250 mV x 600 nH = 23.9 mF where k = 4.6
Using eight 820 F A1-Polys with a typical ESR of 8 m each yields CX = 6.56 mF with an RX = 1.0 m. One last check should be made to ensure that the ESL of the bulk capacitors (LX) is low enough to limit the initial high frequency transient spike. This is tested using
Lx C z x R Lx 230 F x (1.3 m ) = 389 pH
2
(12)
(14)
Cx ( MAX )
2 V nKRO - 1 - C z (13) x 1 + tv VID x V L V
L V xV 22 nK RO VVID
In this example, LX is 375 pH for the eight A1-Polys capacitors, which satisfies this limitation. If the LX of the chosen bulk capacitor bank is too large, the number of capacitors must be increased. One should note that for this multimode control technique, all ceramic designs can be used as long as the conditions of Equations 11, 12, and 13 are satisfied.
V where K = 1n ERR V V

To meet the conditions of these expressions and transient response, the ESR of the bulk capacitor bank (RX) should be less than two times the droop resistance, RO. If the CX(MIN) is larger than CX(MAX), the system does not meet the VID on-the-fly specification and may require the use of a smaller inductor or more phases (and may have to increase the switching frequency to keep the output ripple the same).
Rev. B | Page 17 of 24
ADP3168
POWER MOSFETS
For this example, the N-channel power MOSFETs have been selected for one high-side switch and two low-side switches per phase. The main selection parameters for the power MOSFETs are VGS(TH), QG, CISS, CRSS, and RDS(ON). The minimum gate drive voltage (the supply voltage to the ADP3418) dictates whether standard threshold or logic-level threshold MOSFETs must be used. With VGATE ~10 V, logic-level threshold MOSFETs (VGS(TH) <2.5 V) are recommended. The maximum output current IO determines the RDS(ON) requirement for the low-side (synchronous) MOSFETs. The ADP3168, balances currents between phases, thus the current in each lowside MOSFET is the output current divided by the total number of MOSFETs (nSF). With conduction losses being dominant, the following expression shows the total power being dissipated in each synchronous MOSFET in terms of the ripple current per phase (IR) and average total output current (IO):
I PSF = (1 - D ) x O n SF 1 n IR + x 12 n SF
2
MOSFET input capacitance, the following expression provides an approximate value for the switching loss per main MOSFET, where nMF is the total number of main MOSFETs:
PS (MF ) = 2 x f SW x VCC x I O n x RG x MF x C ISS n n MF
(16)
Here, RG is the total gate resistance (2 for the ADP3418 and about 1 for typical high speed switching MOSFETs, making RG = 3 ) and CISS is the input capacitance of the main MOSFET. Note that adding more main MOSFETs (nMF) does not really help the switching loss per MOSFET because the additional gate capacitance slows switching. The best thing to reduce switching loss is to use lower gate capacitance devices. The conduction loss of the main MOSFET is given by the following, where RDS(MF) is the ON resistance of the MOSFET:
I PC (MF ) = D x O nMF 1 n x IR + x 12 nMF
2

2
x RDS (SF ) (15)

2
x RDS (MF ) (17)
Knowing the maximum output current being designed for and the maximum allowed power dissipation, one can find the required RDS(ON) for the MOSFET. For D-PAK MOSFETs up to an ambient temperature of 50C, a safe limit for PSF is 1 W to 1.5 W at 120C junction temperature. Thus, for this example (65 A maximum), we find RDS(SF) (per MOSFET) < 8.7 m. This RDS(SF) is also at a junction temperature of about 120C, so we need to make sure we account for this when making this selection. For this example, we selected two lower-side MOSFETs at 7 m each at room temperature, which gives 8.4 m at high temperature. Another important factor for the synchronous MOSFET is the input capacitance and feedback capacitance. The ratio of the feedback to input needs to be small (less than 10% is recommended) to prevent accidental turn-on of the synchronous MOSFETs when the switch node goes high. Also, the time to switch the synchronous MOSFETs off should not exceed the nonoverlap dead time of the MOSFET driver (40 ns typical for the ADP3418). The output impedance of the driver is about 2 and the typical MOSFET input gate resistances are about 1 to 2 , so a total gate capacitance of less than 6000 pF should be adhered to. Because there are two MOSFETs in parallel, the input capacitance for each synchronous MOSFET should be limited to 3000 pF. The high-side (main) MOSFET must be able to handle two main power dissipation components: conduction and switching losses. The switching loss relates to the amount of time it takes for the main MOSFET to turn on and off, and to the current and voltage that are being switched. Basing the switching speed on the rise and fall time of the gate driver impedance and
Typically, for main MOSFETs, the highest speed (low CISS) device is preferred, but these usually have higher ON resistance. Select a device that meets the total power dissipation (about 1.5 W for a single D-PAK) when combining the switching and conduction losses. For this example, an Infineon IPD12N03L was selected as the main MOSFET (three total; nMF = 3), with a CISS = 1460 pF (max) and RDS(MF) = 14 m (max at TJ = 120C), and an Infineon IPD06N03L was selected as the synchronous MOSFET (six total; nSF = 6), with CISS = 2370 pF (max) and RDS(SF) = 8.4 m (max at TJ = 120C). The synchronous MOSFET CISS is less than 3000 pF, satisfying that requirement. Solving for the power dissipation per MOSFET at IO = 65 A and IR = 8.2 A yields 863 mW for each synchronous MOSFET and 1.44 W for each main MOSFET. These numbers work well considering there is usually more PCB area available for each main MOSFET vs. each synchronous MOSFET. One last thing to consider is the power dissipation in the driver for each phase. This is best described in terms of the QG for the MOSFETs and is given by the following, where QGMF is the total gate charge for each main MOSFET and QGSF is the total gate charge for each synchronous MOSFET:
f PDRV = SW x (n MF x QGMF + nSF x QGSF ) + I CC x VCC (18) 2x n
Also shown is the standby dissipation factor (ICC x VCC) for the driver. For the ADP3418, the maximum dissipation should be less than 400 mW. For our example, with ICC = 7 mA, QGMF = 22.8 nC, and QGSF = 34.3 nC, we find 260 mW in each driver, which is below the 400 mW dissipation limit. See the ADP3418 data sheet for more details.
Rev. B | Page 18 of 24
ADP3168
RAMP RESISTOR SELECTION
The ramp resistor (RR) is used for setting the size of the internal PWM ramp. The value of this resistor is chosen to provide the best combination of thermal balance, stability, and transient response. This expression determines the optimum value: For values of RLIM greater than 500 k, the current limit may be lower than expected, so some adjustment of RLIM may be needed. Here, ILIM is the average current limit for the output of the supply. For our example, choosing 120 A for ILIM, we find RLIM to be 200 k, for which we chose 200 k as the nearest 1% value. The per-phase current limit described earlier has its limit determined by the following:
RR = RR =
AR x L 3 x AD x RDS x C R 0.2 x 600 nH = 381 k 3 x 5 x 4.2 m x 5 pF
(19)
where AR is the internal ramp amplifier gain, AD is the current balancing amplifier gain, RDS is the total low-side MOSFET ON resistance, and CR is the internal ramp capacitor value. The closest standard 1% resistor value is 383 k. The internal ramp voltage magnitude can be calculated using
VR = AR x (1 - D ) x VVID RR x C R x f SW
I PHLIM
VCOMP (MAX ) - VR - VBIAS AD x RDS (MAX )
+
IR 2
(23)
For the ADP3168, the maximum COMP voltage (VCOMP(MAX)) is 3.3 V, the COMP pin bias voltage (VBIAS) is 1.2 V, and the current balancing amplifier gain (AD) is 5. Using VR of 0.63 V and RDS(MAX) of 4.2 m (low-side ON resistance at 150C), we find a per-phase limit of 66 A. This limit can be adjusted by changing the ramp voltage VR, but make sure not to set the per-phase limit lower than the average per-phase current (ILIM/n). There is also a per-phase initial duty cycle limit determined by
0.2 x (1 - 0.125 ) x 1.5 V VR = 383 k x 5 pF x 267 kHz
(20)
The size of the internal ramp can be made larger or smaller. If it is made larger, stability and transient response improve, but thermal balance degrades. Likewise, if the ramp is made smaller, thermal balance improves at the sacrifice of transient response and stability. The factor of three in the denominator of Equation 19 sets a ramp size that gives an optimal balance for good stability, transient response, and thermal balance.
DMAX = D x
VCOMP (MAX ) - VBIAS VRT
(24)
For this example, the maximum duty cycle is found to be 0.42.
COMP PIN RAMP
There is a ramp signal on the COMP pin due to the droop voltage and output voltage ramps. This ramp amplitude adds to the internal ramp to produce the following overall ramp signal at the PWM input.
FEEDBACK LOOP COMPENSATION DESIGN
Optimized compensation of the ADP3168 allows the best possible response of the regulator's output to a load change. The basis for determining the optimum compensation is to make the regulator and output decoupling appear as an output impedance that is entirely resistive over the widest possible frequency range, including dc, and equal to the droop resistance (RO). With the resistive output impedance, the output voltage droops in proportion with the load current at any load current slew rate; this ensures the optimal positioning and allows the minimization of the output decoupling. With the multimode feedback structure of the ADP3168, the feedback compensation must be set to make the converter's output impedance, working in parallel with the output decoupling, meet this goal. There are several poles and zeros created by the output inductor and decoupling capacitors (output filter) that need to be compensated for. A type-three compensator on the voltage feedback is adequate for proper compensation of the output filter. The expressions given in Equations 25 to 29 are intended to yield an optimal starting point for the design; some adjustments may be necessary to account for PCB and component parasitic effects (see the Tuning Procedure for the ADP3168).
VRT =
2 x (1 - n x D ) 1 - nx f xC x R SW X O
VR

(21)
For this example, the overall ramp signal is found to be 0.63 V.
CURRENT-LIMIT SET POINT
To select the current-limit set point, first find the resistor value for RLIM. The current limit threshold for the ADP3168 is set with a 3 V source (VLIM) across RLIM with a gain of 10.4 mV/A (ALIM). RLIM can be found using the following:
RLIM = ALIM x VLIM I LIM x RO
(22)
Rev. B | Page 19 of 24
ADP3168
The first step is to compute the time constants for all of the poles and zeros in the system:
RE = n x RO x AD x RDS +
RL x VRT 2 x L x (1 - n x D ) x VRT + VDID n x C X x RO x VVID
RE = 3 x 1.3 m + 5 x 4.2 m + TA = C X x (RO - R ') +
1.6 m x 0.63 V 2 x 600 nH x (1 - 0.375) x 0.63 V + 37.9 m 1. 5 V 3 x 6.56 mF x 1.3 m x 1.5 V
(25)
L X RO - R ' 375 pH 1.3 m - 0.6 m x = 6.56 mF x (1.3 m - 0.6 m ) + x = 4.79 s RO RX 1.3 m 1.0 m
(26)
TB = (R X + R '- RO ) x C X = (1.0 m + 0.6 m - 1.3 m ) x 6.56 mF = 1.97 s A x RDS VRT x L - D 2 x f SW TC = VVID x RE TD = 5 x 4.2 m 0.63 V x 600 nH = 2 x 267 kHz = 6.2 s = 1.5 V x 37.9 m
(27)
(28)
2 2 C X x C Z x RO 6.56 mF x 230 F x (1.3 m ) = = 521 ns C X x (RO - R') + C Z x RO 6.56 mF x (1.3 m - 0.6 m ) + 230 F x 1.3 m
(29)
where, for the ADP3168, R' is the PCB resistance from the bulk capacitors to the ceramics and where RDS is the total low side MOSFET ON resistance per phase. For this example, AD is 5, VRT equals 0.63 V, R' is approximately 0.6 m (assuming a 4-layer motherboard), and LX is 375 pH for the eight Al-Poly capacitors.
The compensation values can be solved using the following:
CA = n x RO x TA RE x RB
Figure 13 shows the typical transient response using the compensation values.
(30)
3 x 1.3 m x 4.79 s = 371 pF CA = 37.9 m x 1.33 k
RA = CB = C FB = TC 6.2 s = = 16.7 k C A 371 pF TB 1.97 s = = 1.48 nF RB 1.33 k TD 521 ns = = 31.2 pF R A 16.7 k
(31)
(32)
03258-B-013
(33)
Figure 13. Typical Transient Response for Design Example
Choosing the closest standard values for the components yields
C A = 390 pF, R A = 16.9 k, C B = 1.5 nF, C FB = 33 pF
Rev. B | Page 20 of 24
ADP3168
CIN SELECTION AND INPUT CURRENT DI/DT REDUCTION
In continuous inductor current mode, the source current of the high-side MOSFET is approximately a square wave with a duty ratio equal to n x VOUT/VIN and an amplitude of one-nth of the maximum output current. To prevent large voltage transients, a low ESR input capacitor sized for the maximum rms current must be used. The maximum rms capacitor current is given by
I CRMS = D x I O x 1 -1 N xD
TUNING PROCEDURE FOR THE ADP3168
1. 2. Build circuit based on compensation values computed from design spreadsheet. Hook up dc load to circuit, turn on, and verify operation. Also check for jitter at no-load and full-load. Measure output voltage at no-load (VNL). Verify that it is within tolerance. Measure output voltage at full-load cold (VFLCOLD). Let board set for ~10 minutes at full-load and measure output (VFLHOT). If there is a change of more than a couple of millivolts, adjust RCS1 and RCS2 using Equations 35 and 37. Repeat Step 4 until cold and hot voltage measurements remain the same. Measure output voltage from no-load to full-load using 5 A steps. Compute the loadline slope for each change and then average to get overall loadline slope (ROMEAS). If ROMEAS is off from RO by more than 0.05 m, use the following to adjust the RPH values:
RPH ( NEW ) = RPH (OLD ) x ROMEAS RO
DC Loadline Setting
3. 4.
(34)
I CRMS 1 = 0.125 x 65 A x - 1 = 10.5 A 3 x 0.125
5. 6.
Note that the capacitor manufacturer's ripple current ratings are often based on only 2,000 hours of life. This makes it advisable to further derate the capacitor or choose a capacitor rated at a higher temperature than required. Several capacitors may be placed in parallel to meet size or height requirements in the design. In this example, the input capacitor bank is formed by three 2,200 F, 16 V Nichicon capacitors with a ripple current rating of 3.5 A each. To reduce the input current di/dt to a level below the recommended maximum of 0.1 A/s, an additional small inductor (L > 1 H @ 15 A) should be inserted between the converter and the supply bus. That inductor also acts as a filter between the converter and the primary power source.
RCS2( NEW ) = RSC 2(OLD ) x VNL - VFLCOLD VNL - VFLHOT
7.
(36)
(35)
Repeat Steps 6 and 7 to check loadline and repeat adjustments if necessary. 9. Once complete with dc loadline adjustment, do not change RPH, RCS1, RCS2, or RTH for rest of procedure. 10. Measure output ripple at no-load and full-load with scope and make sure it is within specifications.
8.
RCS 2( NEW ) =
RCS1(OLD ) x RTH ( 25C ) + (RCS 1(OLD ) - RCS 2( NEW ) ) x (RCS 1(OLD ) - RTH (25C ) )
100 90 80 70
1 RCS1(OLD ) + RTH (25C )
(37)
- 1 RTH (25C )
EFFICIENCY (%)
60 50 40 30 20 10
03258-B-014
0 0 10 20 30 40 50 60 OUTPUT CURRENT (A)
Figure 14. Efficiency of the Circuit of Figure 11 vs. Output Current
Rev. B | Page 21 of 24
ADP3168
AC Loadline Setting
11. Remove dc load from circuit and hook up dynamic load. 12. Hook up scope to output voltage and set to dc coupling with time scale at 100 s/div. 13. Set dynamic load for a transient step of about 40 A at 1 kHz with 50% duty cycle. 14. Measure output waveform (may have to use dc offset on scope to see waveform). Try to use vertical scale of 100 mV/div or finer. 15. This waveform should look something like Figure 15. Use the horizontal cursors to measure VACDRP and VDCDRP as shown. Do not measure the undershoot or overshoot that happens immediately after the step.
VDROOP
VTRAN1
VTRAN2
Figure 16. Transient Setting Waveform
VACDRP VDCDRP
20. If both overshoots are larger than desired, try making the adjustments described below. (Note: If these adjustments do not change the response, you are limited by the output decoupling.) Check the output response each time you make a change as well as the switching nodes (to make sure the response is still stable). a. b. c. Make ramp resistor larger by 25% (RRAMP). For VTRAN1, increase CB or increase switching frequency. For VTRAN2, increase RA and decrease CA by 25%.
Figure 15. AC Loadline Waveform
16. If the VACDRP and VDCDRP are different by more than a few millivolts, use Equation 38 to adjust CCS. Parallel different values to get the right one because there are limited standard capacitor values available. (Make sure that there are locations for two capacitors in the layout for this.)
C CS ( NEW ) = C CS(OLD ) x V ACDRP V DCDRP
21. For load release (see Figure 17), if VTRANREL is larger than VTRAN1 (see Figure 16), there is not enough output capacitance. You will either need more capacitance or have to make the inductor values smaller. (If you change inductors, you will need to start the design over using the spreadsheet and this tuning procedure.)
(38)
VTRANREL VDROOP
17. Repeat Steps 11 to 13, making adjustments if necessary. Once complete, do not change CCS again in the procedure. 18. Set dynamic load step to maximum step size (do not use a step size larger than needed) and verify that the output waveform is square (which means VACDRP and VDCDRP are equal).Make sure load step slew rate and turn-on are set for a slew rate of ~150 A/s to 250 A/s (for example, a load step of 50 A should take 200 ns to 300 ns) with no overshoot. Some dynamic loads have an excessive turn-on overshoot if a minimum current is not set properly. (This is an issue if using a VTT tool.)
Figure 17. Transient Setting Waveform
Initial Transient Setting
19. With dynamic load still set at maximum step size, expand scope time scale to see 2 s/div to 5 s/div. The waveform may have two overshoots and one minor undershoot (see Figure 16). Here, VDROOP is the final desired value.
Because the ADP3168 turns off all of the phases (switches inductors to ground), there is no ripple voltage present during load release. Thus, you do not have to add headroom for ripple, allowing your load release VTRANREL to be larger than VTRAN1 by the amount of ripple and still meet specifications. If VTRAN1 and VTRANREL are less than the desired final droop, this implies that capacitors can be removed. When removing capacitors, check the output ripple voltage as well to make sure it is still within specifications.
Rev. B | Page 22 of 24
B-017
B-016
ADP3168 LAYOUT AND COMPONENT PLACEMENT
The following guidelines are recommended for optimal performance of a switching regulator in a PC system. Key layout issues are illustrated in Figure 18.
12V CONNECTOR SWITCH NODE PLANES INPUT POWER PLANE
The output capacitors should be connected as close as possible to the load (or connector) that receives the power (e.g., a microprocessor core). If the load is distributed, the capacitors should also be distributed and generally in proportion to where the load tends to be more dynamic. Avoid crossing signal lines over the switching power path loop, as described next.
POWER CIRCUITRY
The switching power path should be routed on the PCB to encompass the shortest possible length in order to minimize radiated switching noise energy (i.e., EMI) and conduction losses in the board. Failure to take proper precautions often results in EMI problems for the entire PC system as well as noise-related operational problems in the power converter control circuitry. The switching power path is the loop formed by the current path through the input capacitors and the power MOSFETs including all interconnecting PCB traces and planes. Using short and wide interconnection traces is critical in this path because it minimizes the inductance in the switching loop, which can cause high energy ringing, and it accommodates the high current demand with minimal voltage loss. Whenever a power dissipating component (e.g., a power MOSFET) is soldered to a PCB, the liberal use of vias, both directly on the mounting pad and immediately surrounding it, is recommended. Two important reasons for this are improved current rating through the vias and improved thermal performance from vias extended to the opposite side of the PCB, where a plane can more readily transfer the heat to the air. Make a mirror image of any pad being used to heat sink the MOSFETs on the opposite side of the PCB to achieve the best thermal dissipation to the air around the board. To further improve thermal performance, use the largest possible pad area. The output power path should also be routed to encompass a short distance. The output power path is formed by the current path through the inductor, the output capacitors, and the load. For best EMI containment, a solid power ground plane should be used as one of the inner layers extending fully under all the power components.
THERMISTOR KEEP-OUT AREA OUTPUT POWER PLANE KEEP-OUT AREA CPU SOCKET KEEP-OUT AREA
KEEP-OUT AREA
Figure 18. Layout Recommendations
GENERAL RECOMMENDATIONS
For good results, a PCB with at least four layers is recommended. This should allow the needed versatility for control circuitry interconnections with optimal placement, power planes for ground, input, and output power, and wide interconnection traces in the rest of the power delivery current paths. Keep in mind that each square unit of 1 ounce copper trace has a resistance of ~0.53 m at room temperature. Whenever high currents are routed between PCB layers, vias should be used liberally to create several parallel current paths so that the resistance and inductance introduced by the current paths is minimized and the via current rating is not exceeded. If critical signal lines (including the output voltage sense lines of the ADP3168) must cross through power circuitry, it is best if a signal ground plane can be interposed between those signal lines and the traces of the power circuitry. This creates a shield to minimize noise injection into the signals at the expense of making signal ground a bit noisier. An analog ground plane should be used around and under the ADP3168 as a reference for the components associated with the controller. This plane should be tied to the nearest output decoupling capacitor ground and should not be tied to any other power circuitry to prevent power currents from flowing in it. The components around the ADP3168 should be located close to the controller with short traces. The most important traces to keep short and away from other traces are the FB and CSSUM pins. See Figure 18 for details on layout for the CSSUM node.
03258-B-018
SIGNAL CIRCUITRY
The output voltage is sensed and regulated between the FB pin and the FBRTN pin, which connects to the signal ground at the load. To avoid differential mode noise pickup in the sensed signal, the loop area should be small. Thus the FB and FBRTN traces should be routed adjacent to each other on top of the power ground plane back to the controller. Connect the feedback traces from the switch nodes as close as possible to the inductor. The CSREF signal should be connected to the output voltage at the nearest inductor to the controller.
Rev. B | Page 23 of 24
ADP3168 OUTLINE DIMENSIONS
9.80 9.70 9.60
28
15
4.50 4.40 4.30 6.40 BSC
1 14
PIN 1 0.65 BSC 0.15 0.05 COPLANARITY 0.10 0.30 0.19 1.20 MAX 8 0 0.75 0.60 0.45
SEATING PLANE
0.20 0.09
COMPLIANT TO JEDEC STANDARDS MO-153AE
Figure 19. 28-Lead Thin Shrink Small Outline Package [TSSOP] (RU-28) Dimensions shown in millimeters
ORDERING GUIDE
Model ADP3168JRU-REEL7 ADP3168JRU-REEL ADP3168JRUZ-REEL1 Temperature Range 0C to 85C 0C to 85C 0C to 85C Package Options RU-28 (TSSOP-28) RU-28 (TSSOP-28) RU-28 (TSSOP-28) Quantity per Reel 1000 2500 2500
1
Z = Pb-free part.
(c) 2004 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. C03258-0-12/04(B)
Rev. B | Page 24 of 24

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