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ISL6539
Data Sheet June 6, 2005 FN9144.4
Wide Input Range Dual PWM Controller with DDR Option
The ISL6539 dual PWM controller delivers high efficiency and tight regulation from two voltage regulating synchronous buck DC/DC converters. It was designed especially for DDR DRAM, SDRAM, graphic chipset applications, and system regulators in high performance applications. Voltage-feed-forward ramp modulation, current mode control, and internal feedback compensation provide fast response to input voltage and output load transients. Input current ripple is minimized by channel-to-channel PWM phase shift of 0, 90, or 180 (determined by input voltage and status of the DDR pin). The ISL6539 can control two independent output voltages adjustable from 0.9V to 5.5V or, by activating the DDR pin, transform into a complete DDR memory power supply solution. In DDR mode, CH2 output voltage VTT tracks CH1 output voltage VDDQ. CH2 output can both source and sink current, an essential power supply feature for DDR memory. The reference voltage VREF required by DDR memory is generated as well. In dual power supply applications the ISL6539 monitors the output voltage of both CH1 and CH2. An independent PGOOD (power good) signal is asserted for each channel after the soft-start sequence has completed, and the output voltage is within PGOOD window. In DDR mode CH1 generates the only PGOOD signal. Built-in overvoltage protection prevents the output from going above 115% of the set point by holding the lower MOSFET on and the upper MOSFET off. When the output voltage decays below the overvoltage threshold, normal operation automatically resumes. Once the soft-start sequence has completed, undervoltage protection latches the offending channel off if the output drops below 75% of its set point value for the dual switcher. Adjustable overcurrent protection (OCP) monitors the voltage drop across the rDS(ON) of the lower MOSFET. If more precise current-sensing is required, an external current sense resistor may be used.
Features
* Provides regulated output voltage in the range 0.9V-5.5V * Complete DDR memory power solution with VTT tracks VDDQ/2 and VDDQ/2 buffered reference output * Supports both DDR-I and DDR2 memory * Lossless rDS(ON) current-sense sensing * Excellent dynamic response with voltage feed-forward and current mode control accommodating wide range LC filter selections * Dual mode operation-operates directly from a 5.0-15V input or 3.3V/5V system rail * Undervoltage lock-out on VCC pin * Power-good, overcurrent, overvoltage, undervoltage protection for both channels * Synchronized 300kHz PWM operation in PWM mode * Pb-Free Plus Anneal Available (RoHS Compliant)
Applications
* Single and Dual Channel DDR Memory Power Systems * Graphics cards - GPU and memory supplies * Supplies for Servers, Motherboards, FPGAs * ASIC power supplies * Embedded processor and I/O supplies * DSP supplies
1
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures. 1-888-INTERSIL or 1-888-468-3774 | Intersil (and design) is a registered trademark of Intersil Americas Inc. Copyright Intersil Americas Inc. 2004, 2005. All Rights Reserved All other trademarks mentioned are the property of their respective owners.
ISL6539 Ordering Information
PART NUMBER ISL6539CA ISL6539CA-T ISL6539CAZ (Note) ISL6539CAZ-T (Note) ISL6539IA ISL6539IA-T ISL6539IAZ (Note) ISL6539IAZ-T (Note) TEMP. RANGE (C) 0 to 70 PACKAGE 28 Ld SSOP PKG. DWG. # M28.15
GND LGATE1 PGND1 PHASE1 UGATE1 BOOT1 1 2 3 4 5 6 7 8 9
Pinout
ISL6539 (SSOP) TOP VIEW
28 VCC 27 LGATE2 26 PGND2 25 PHASE2 24 UGATE2 23 BOOT2 22 ISEN2 21 EN2 20 GND 19 VSEN2 18 OCSET2 17 SOFT2 16 PG2/REF 15 PG1
28 Ld SSOP Tape and Reel 0 to 70 28 Ld SSOP (Pb-Free) M28.15
28 Ld SSOP Tape and Reel (Pb-Free) -40 to 85 28 Ld SSOP M28.15
ISEN1 EN1 GND
28 Ld SSOP Tape and Reel -40 to 85 28 Ld SSOP (Pb-Free) M28.15
VSEN1 10 OCSET1 11 SOFT1 12 DDR 13 VIN 14
28 Ld SSOP Tape and Reel (Pb-Free)
NOTE: Intersil Pb-free plus anneal products employ special Pb-free material sets; molding compounds/die attach materials and 100% matte tin plate termination finish, which are RoHS compliant and compatible with both SnPb and Pb-free soldering operations. Intersil Pb-free products are MSL classified at Pb-free peak reflow temperatures that meet or exceed the Pb-free requirements of IPC/JEDEC J STD-020.
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FN9144.4 June 6, 2005
ISL6539 Generic Application Circuits
VIN 3.3V OR 5.0V TO 15V OCSET1 Q1 L1 VOUT1 PWM1 Q2 EN1 EN2 5V VCC DDR OCSET2 Q4 C2 PWM2 + Q3 L2 C1 +
VOUT2
ISL6539 APPLICATION CIRCUIT FOR TWO CHANNEL POWER SUPPLY
VIN 3.3V OR 5.0V TO 15V OCSET1 Q1 L1 VDDQ +
PWM1 Q2 EN1 EN2 5V VREF VCC DDR PG2/VREF PWM2 C1
Q3
L2 VTT
OCSET2 Q4 C2
+
ISL6539 APPLICATION CIRCUIT FOR COMPLETE DDR MEMORY POWER SUPPLY
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FN9144.4 June 6, 2005
ISL6539
Absolute Maximum Ratings
Bias Voltage, VCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +6.5V Input Voltage, VIN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +18.0V PHASE, UGATE . . . . . . . . . . . . . . . . . . .GND-5V (Note 1) to +24.0V BOOT, ISEN . . . . . . . . . . . . . . . . . . . . . . . . . . . GND-0.3V to +24.0V BOOT with respect to PHASE . . . . . . . . . . . . . . . . . . . . . . . . . + 6.5V All Other Pins . . . . . . . . . . . . . . . . . . . . . . GND -0.3V to VCC + 0.3V ESD Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Class 2
BOOT
Thermal Information
Thermal Resistance (Typical, Note 2)
JA (C/W)
SSOP Package . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 Maximum Junction Temperature (Plastic Package). . . . . . . . . 150C Maximum Storage Temperature Range . . . . . . . . . . . -65C to 150C Maximum Lead Temperature (Soldering 10s) . . . . . . . . . . . . . 300C (SSOP - Lead Tips Only)
Recommended Operating Conditions
Bias Voltage, VCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +5.0V 5% Input Voltage, VIN . . . . . . . . . . . . . . . . . . . . +3.3V or 5.0V to +18.0V Ambient Temperature Range, Commercial . . . . . . . . . . 0C to 70C Junction Temperature Range, Commercial . . . . . . . . . 0C to 125C Ambient Temperature Range, Industrial. . . . . . . . . . . .-40C to 85C Junction Temperature Range, Industrial . . . . . . . . . .-40C to 125C
CAUTION: Stresses above those listed in "Absolute Maximum Ratings" may cause permanent damage to the device. This is a stress only rating and operation of the device at these or any other conditions above those indicated in the operational sections of this specification is not implied.
NOTES: 1. 250ns transient. See Confining The Negative Phase Node Voltage Swing in Application Information Section. 2. JA is measured with the component mounted on a high effective thermal conductivity test board in free air. See Tech Brief TB379 for details.
Electrical Specifications
PARAMETER VCC SUPPLY Bias Current Shut-down Current VCC UVLO Rising VCC Threshold Falling VCC Threshold VIN Input Voltage Pin Current (Sink) Shut-down Current OSCILLATOR Oscillator Frequency Ramp Amplitude, pk-pk Ramp Amplitude, pk-pk Ramp Offset Ramp/VIN Gain Ramp/VIN Gain REFERENCE AND SOFT-START Internal Reference Voltage Reference Voltage Accuracy Soft-Start Current During Start-up Soft-Start Complete Threshold
Recommended Operating Conditions, Unless Otherwise Noted. SYMBOL TEST CONDITIONS MIN TYP MAX UNITS
ICC ICCSN
LGATEx, UGATEx Open, VSENx forced above regulation point, DDR = 0, VIN > 5V
-
1.8 -
3.0 1
mA A
VCCU VCCD
4.30 4.00
4.45 4.14
4.50 4.34
V V
IVIN IVINS
-
-
35 1
A A
fOSC VR1 VR2 VROFF GRB1 GRB2 Vin pin voltage = 16V, by design Vin pin voltage = 5V, by design By design Vin pin voltage > 4.2V, by design Vin pin voltage 4.1V, by design
255 -
300 2 0.625 1 125 250
345 -
kHz V V V mV/V mV/V
VREF
-1.0
0.9 4.5 1.5
+1.0 -
V % A V
ISOFT VST By design
-
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ISL6539
Electrical Specifications
PARAMETER PWM CONVERTERS Load Regulation VSEN pin bias current Minimum Duty Cycle Maximum Duty Cycle Undervoltage Shut-Down Level Overvoltage Protection GATE DRIVERS Upper Drive Pull-Up Resistance Upper Drive Pull-Down Resistance Lower Drive Pull-Up Resistance Lower Drive Pull-Down Resistance R2UGPUP R2UGPDN R2LGPUP R2LGPDN VCC = 5V VCC = 5V VCC = 5V VCC = 5V 4 2.3 4 1.1 8 4 8 3 IVSEN DMIN DMAX VUVL VOVP1 Fraction of the set point; ~2ms noise filter Fraction of the set point; ~2ms noise filter 0.0mA < IVOUT1 < 5.0A; 5.0V < VIN < 15.0V By design -2.0 70 110 80 4 87 75 115 +2.0 80 % nA % % % % Recommended Operating Conditions, Unless Otherwise Noted. (Continued) SYMBOL TEST CONDITIONS MIN TYP MAX UNITS
POWER GOOD AND CONTROL FUNCTIONS Power Good Lower Threshold Power Good Higher Threshold PGOODx Leakage Current PGOODx Voltage Low ISEN sourcing current OCSET sourcing current range EN - Low (Off) EN - High (On) DDR - Low (Off) DDR - High (On) DDR REF Output Voltage DDR REF Output Current VDDREF IDDREF DDR = 1, IREF = 0...10mA DDR = 1. Guaranteed by design. VPGVPG+ IPGLKG VPGOOD Fraction of the set point; ~3ms noise filter Fraction of the set point; ~3ms noise filter. VPULLUP = 5.5V IPGOOD = -4mA By design 84 110 2 2.0 3 0.99* VOC2 89 115 0.5 VOC2 10 92 120 1 1 260 20 0.8 0.8 1.01* VOC2 12 % % A V A A V V V V V mA
Functional Pin Description
GND (Pin 1, 9, 20)
Signal ground for the IC. All three ground pins must be connected to ground for proper IC operation. Connect to the ground plane through a path as low in inductance as possible.
PHASE1, PHASE2 (Pin 4, 25)
The PHASE1 and PHASE2 points are the junction points of the upper MOSFET sources, output filter inductors, and lower MOSFET drains. Connect these pins to the respective converter's upper MOSFET source.
LGATE1, LGATE2 (Pin 2, 27)
Connect these pins to the gates of the corresponding lower MOSFETs. These pins provide the PWM-controlled gate drive for the lower MOSFETs.
UGATE1, UGATE2 (Pin 5, 24)
Connect these pins to the gates of the corresponding upper MOSFETs. These pins provide the PWM-controlled gate drive for the upper MOSFETs.
PGND1, PGND2 (Pin 3, 26)
These pins provide the return connection for lower gate drivers, and are connected to sources of the lower MOSFETs of their respective converters. These pins must be connected to the ground plane through a path as low in inductance as possible. 5
BOOT1, BOOT2 (Pin 6, 23)
These pins power the upper MOSFET drivers of the PWM converter. Connect these pins to the junction of the bootstrap capacitor with the cathode of the bootstrap diode. The anode of the bootstrap diode is connected to the VCC voltage.
FN9144.4 June 6, 2005
ISL6539
ISEN1, ISEN2 (Pin 7, 22)
These pins are used to monitor the voltage drop across the lower MOSFET for current feedback and overcurrent protection. For precise current detection these inputs can be connected to the optional current sense resistors placed in series with the source of the lower MOSFETs. comparator is proportional to the voltage on this pin (see Table 1 and Table 2 for details). While the DDR pin is high (in the DDR application) and when the VIN pin voltage is tied to 5V, it commands 90 out-of-phase channel synchronization, with the second channel lagging the first channel, to reduce inter-channel interference. While the DDR pin is high (in the DDR application) and when the VIN pin voltage is tied to ground, it commands in-phase channel synchronization.
EN1, EN2 (Pin 8, 21)
These pins enable operation of the respective converter when high. When both pins are low, the chip is disabled and only low leakage current is taken from Vcc and Vin. EN1 and EN2 can be used independently to enable either Channel 1 or Channel 2, respectively.
PG1 (Pin 15)
PGOOD1 is an open drain output used to indicate the status of the output voltage. This pin is pulled low when the first channel output is out of 11% of the set value.
VSEN1, VSEN2 (Pin 10, 19)
These pins are connected to the resistive dividers that set the desired output voltage. The PGOOD, UVP, and OVP circuits use this signal to report output voltage status.
PG2/REF (Pin 16)
This pin has a double function, depending on the mode of operation. When the chip is used as a dual channel PWM controller (DDR= 0), the pin provides an open drain PGOOD2 function for the second channel the same way as PG1. The pin is pulled low when the second channel output is out of 11% of the set value. In DDR mode (DDR = 1), this pin is the output of the buffer amplifier that takes VDDQ/2 voltage applied to OCSET2 pin from the resistor divider. It can source a typical 10mA current.
OCSET1 (Pin 11)
This pin is a buffered 0.9V internal reference voltage. A resistor from this pin to ground sets the overcurrent threshold for the first controller.
SOFT1, SOFT2 (Pin 12, 17)
These pins provide soft-start function for their respective controllers. When the chip is enabled, the regulated 5A pull-up current source charges the capacitor connected from the pin to ground. The output voltage of the converter follows the ramping voltage on the SOFT pin in the soft-start process with the SOFT pin voltage as reference. When the SOFT pin voltage is higher than 0.9V, the error amplifier will use the internal 0.9V reference to regulate output voltage. In the event of undervoltage and overcurrent shutdown, the soft-start pin is pulled down through a 2k resistor to ground to discharge the soft-start capacitor.
OCSET2 (Pin 18)
In a dual channel application with DDR = 0, a resistor from this pin to ground sets the overcurrent threshold for the second channel controller. Its voltage is the buffered internal 0.9V reference. In the DDR application with DDR = 1, this pin connects to the center point of a resistor divider tracking the VDDQ/2. This voltage is then buffered by an amplifier voltage follower and sent to the PG2/REF pin. It sets the reference voltage of Channel 2 for its regulation.
DDR (Pin 13)
When the DDR pin is low, the chip can be used as a dual switcher controller. The output voltage of the two channels can be programmed independently by VSENx pin resistor dividers. The PWM signals of Channel 1 and Channel 2 will be synchronized 180 degrees out-of-phase. When the DDR pin is high, the chip transforms into a complete DDR memory solution. The OCSET2 pin becomes an input through a resistor divider tracking to VDDQ/2. The PG2/REF pin becomes the output of the VDDQ/2 buffered voltage. The VDDQ/2 voltage is also used as the reference to the error amplifier by the second channel. The channel phase-shift synchronization is determined by the VIN pin when DDR = 1 as described in VIN (Pin 14) below.
VCC (Pin 28)
VCC provides the bias supply for the ISL6539. The supply to VCC should be locally bypassed using a ceramic capacitor.
Typical Application
Figures 1 and 2 show the application circuits of a dual channel DC/DC converter. The power supply in Figure 1 provides +V2.5 and +V1.8 voltages for memory and the graphics interface chipset from a 5.0-15VDC input rail. Figure 2 illustrates the application circuit for a DDR memory power solution. The power supply shown in Figure 2 generates +2.5V VDDQ voltage. The +1.25V VTT termination voltage tracks VDDQ/2 and is derived from +2.5V VDDQ. To complete the DDR memory power requirements, the +1.25V reference voltage is provided through the PG2 pin. In this application circuit shown, two output 220F capacitors are used at the outputs.
VIN (Pin 14)
This pin has multiple functions. When connected to the input voltage, it provides a feed-forward input to the oscillator for the rejection of input voltage variation. The ramp of the PWM
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ISL6539
VIN Cdc D1 4.7F BAT54W Cin1 10F Lo1 4.7H Co13 220F Co11 4.7F FDS6912A Q1 Rs1 2.0K Cb11 0.15F GND Rbt1 0 BOOT1 UGATE1 V1 (2.5V) Rfb11 17.8K Cfb1 0.01 F PHASE1 ISEN1 LGATE1 PGND1 GND VSEN1 Rfb12 10K PG1 EN1 SOFT1 Csoft1 0.01F OCSET1 Rset1 100K U1 BOOT2 UGATE2 PHASE2 ISEN2 LGATE2 PGND2 GND VSEN2 PG2 EN2 SOFT2 OCSET2 Rset2 100K Csoft2 0.01F Rfb22 10K FDS6912A Rs2 2.0K Q2 4.7H Co21 Co22 220F 4.7F Rfb21 10K VIN VCC DDR Rbt2 0 Lo2 VCC (5V)
D2 BAT54W Cbt2 0.15F Cin2 10F
V2 (1.8V)
Cfb2 0.01F
ISL6539
FIGURE 1. TYPICAL APPLICATION CIRCUIT AS DUAL SWITCHER, VOUT1 = 2.5V, VOUT2 = 1.8V
Vin Cdc 4.7F VIN VCC DDR Rbt2 BOOT1 UGATE1 Q1 Co13 220F Rfb1 17.8K Co11 4.7F FDS6912A PHASE1 Rs1 2.0K ISEN1 LGATE1 PGND1 GND VSEN1 Rfb12 10K PG1 EN1 SOFT1 Csoft1 0.01F OCSET1 Rset1 100K SOFT2 Csoft2 (N/U) ISL6539 0.01F Cf 0.1F U1 BOOT2 UGATE2 PHASE2 ISEN2 LGATE2 PGND2 VSEN2 PG2_REF GND EN2 OCSET2_VDDQ/2 FDS6912A Rs2 1.0K Q2 1.5H 0 Cbt2 0.15F Lo2 D2 BAT54W VDDQ GND Cin1 10F Lo1 4.6F Cbt1 0.15F Rbt1 0
VCC (5V)
D1 BAT54W
Cin2 4.7F VTT (1.25V)
VDDQ (2.5V)
Cfb1 0.01F
Co21 Co22 220F 4.7F
Vref (VDDQ/2) Cref 4.7F VDDQ Rd1 10K VDDQ/2 Rd2 10K
FIGURE 2. TYPICAL APPLICATION AS DDR MEMORY POWER SUPPLY, VOUT1 = 2.5V, VOUT2 = 1.25V
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FN9144.4 June 6, 2005
Block Diagram
BOOT1 UGATE1 PHASE1 PGND1 LGATE1 VCC ADAPTIVE DEAD-TIME DIODE EMULATION V/I SAMPLE TIMING DDR = 0 DDR = 1 ADAPTIVE DEAD-TIME DIODE EMULATION V/I SAMPLE TIMING PG1 EN1 VCC GND EN2 REF/PG2 BOOT2 UGATE2 PHASE2 PGND2 LGATE2 VCC
POR ENABLE BIAS SUPPLIES REFERENCE FAULT LATCH SOFT-START 16.7pF OV UV PGOOD DDR MODE CONTROL OV UV PGOOD 16.7pF 1M
8
1M 500k VSEN1 300k
ISL6539
500k 1.25pF 4.4k OC1 DDR OC2 PWM2
+
1.25pF
+
300k (200k, DDR = 1)
VSEN2
+
PWM1
+
4.4k ERROR AMP 2
SOFT2
+ 0.9V REF SOFT1
ERROR AMP 1
DUTY CYCLE RAMP GENERATOR PWM CHANNEL PHASE CONTROL DDR EN1 1 1 EN2 1 1 5V VIN 15.0V VIN = 5V VIN = GND CH1 CH2 180 90 0 CURRENT SAMPLE
DDR = 0 + DDR = 1 0.9V REF 140 CURRENT SAMPLE ISEN2
ISEN1
140 CURRENT SAMPLE
0
+
CURRENT SAMPLE
1
+
OCSET1
DDR = 0
OCSET2
+
0.9V REFERENCE
0.9V REFERENCE
+
DDR = 1
+
1/2.9 OCSET1 1/33.1 ISEN1
OC1
OC2
+ + DDR VREF BUFFER AMP
VIN SAME STATE FOR 8 CLOCK CYCLES REQUIRED TO LATCH OVERCURRENT FAULT
DDR
VCC SAME STATE FOR 8 CLOCK CYCLES REQUIRED TO LATCH OVERCURRENT FAULT
1/33.1 ISEN2
1/2.9 OCSET2
+ DDR VTT REFERENCE
FN9144.4 June 6, 2005
ISL6539 Theory of Operation
Operation
The ISL6539 is a dual channel PWM controller intended for use in power supplies for graphic chipsets, SDRAM, DDR DRAM, or other power applications. The IC integrates two control circuits for two synchronous buck converters. The output voltage of each controller can be set in the range of 0.9V to 5.5V by an external resistive divider. The synchronous buck converters can operate from either an unregulated DC source with a voltage ranging from 5.0V to 15V, or from a regulated system rail of 3.3V or 5V. In either operational mode the controller is biased from the +5V source. The controllers operate in the current mode with input voltage feed-forward which simplifies feedback loop compensation and rejects input voltage variation. An integrated feedback loop compensation dramatically reduces the number of external components. The ISL6539 has a special means to rearrange its internal architecture into a complete DDR solution. When the DDR pin is set high, the second channel can provide the capability to track the output voltage of the first channel. The buffered reference voltage required by DDR memory chips is also provided.
EN 1
0.9V 2
1.5V SOFT
VOUT 3 PGOOD 4
Ch1 5.0V Ch3 1.0V
Ch2 2.0V Ch4 5.0V
M1.00ms
FIGURE 3. START UP
Even though the soft-start pin voltage continues to rise after reaching 1.5V, this voltage does not affect the output voltage. The soft-start time (the time from the moment when EN becomes high to the moment when PGOOD is reported) is determined by the following equation:
1.5 V x C sof t TS OFT = --- --- -- --- --- ----- --- --- --- --- -5 A
Initialization
The ISL6539 initializes if at least one of the enable pins is set high. The Power-On Reset (POR) function continually monitors the bias supply voltage on the VCC pin, and initiates soft-start operation when EN1 or EN2 is high after the input supply voltage exceeds 4.45V. Should this voltage drop lower than 4.14V, the POR disables the chip.
The time it takes the output voltage to come into regulation can be obtained from the following equation.
TR ISE = 0.6 x T SOFT
During soft-start, before the PGOOD pin is enabled, the undervoltage protection is prohibited. The overvoltage and overcurrent protection functions are enabled. If the output capacitor has residue voltage before start-up, both lower and upper MOSFETs are in off-state until the softstart capacitor charges equal the VSEN pin voltage. This will ensure the output voltage starts from its existing voltage level.
Soft-Start
When soft-start is initiated, the voltage on the SOFT pin of the enabled channel starts to ramp up gradually with the internal 5A current charging the soft-start capacitor. The output voltage follows the soft-start voltage. When the SOFT pin voltage reaches 0.9V, the output voltage comes into regulation. When the SOFT voltage reaches 1.5V, the power good (PGOOD) is enabled. The soft-start process is depicted in Figure 3.
Output Voltage Program
The output voltage of either channel is set by a resistive divider from the output to ground. The center point of the divider is connected to the VSEN pin as shown in Figure 4. The output voltage value is determined by the following equation.
0.9 V * ( R1 + R 2 ) --- --- --- --- --- --- --- VO = --- --- --- --- --- --- --- --R2
where 0.9V is the value of the internal reference. The VSEN pin voltage is also used by the controller for the power good function and to detect undervoltage and overvoltage conditions.
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ISL6539
Vin Q1 UGAT E ISEN Q2 LGAT E VOUT VSEN OCSET C1 Cz R1 L1
Feedback Loop Compensation
Both channel PWM controllers have internally compensated error amplifiers. To make internal compensation possible several design measures were taken. * The ramp signal applied to the PWM comparator has been made proportional to the input voltage by the VIN pin. This keeps the product of the modulator gain and the input voltage constant even when the input voltage varies. * The load current proportional signal is derived from the voltage drop across the lower MOSFET during the PWM off time interval, and is subtracted from the error amplifier output signal before the PWM comparator input. This effectively creates an internal current control loop. The resistor connected to the ISEN pin sets the gain in the current sensing. The following expression estimates the required value of the current sense resistor, depending on the maximum continuous load current, and the value of the MOSFETs rDSON, assuming the ISEN pin sources 75A current.
I MAX * R DS ( ON ) R CS = -------------------------------------------- - 140 75A
RCS
ISL6539
ROC
R2
FIGURE 4. THE INTERNAL COMPENSATOR
Current Sensing
The current on the lower MOSFET is sensed by measuring its voltage drop within its on-time. In order to activate the current sampling circuitry, two conditions need to be met. (1) the LGATE is high and (2) the phase pin sees a negative voltage for regular buck operation, which means the current is freewheeling through lower MOSFET. For the second channel of the DDR application, the phase pin voltage needs to be higher than 0.1V to activate the current sensing circuit for bidirectional current sensing. The current sampling finishes at about 400ns after the lower MOSFET has turned on. This current information is held for current mode control and overcurrent protection. The current sensing pin can source up to 260A. The current sense resistor and OCSET resistor can be adjusted simultaneously for the same overcurrent protection level; however, the current sensing gain will be changed only according to the current sense resistor value, which will affect the current feedback loop gain. The middle point of the ISEN current can be at 75A, but it can be tuned up and down to fit application needs. If another channel is switching at the moment the current sample is finishing, it could cause current sensing error and phase voltage jitter. In the design stage, the duty cycles and synchronization have to be analyzed for all the input voltage and load conditions to reduce the chance of current sensing error. The relationship between the sampled current and MOSFET current is given by:
I SEN ( R CS + 140 ) = r DS ( ON ) I D
Because the current sensing circuit is a sample-and-hold type, the information obtained at the last moment of the sampling is being used. This current sensing circuit samples the inductor current very close to its peak value. The current feedback essentially injects a resistor Ri in series with the original LC filter as shown in Figure 5, where the sampleand-hold effect of the current loop has been ignored. Vc and Vo are small signal components extracted from its DC operation points.
Ri Lo DCR Co Gm*Vc
+
Ro Vo
+ -
ESR
FIGURE 5. THE EQUIVALENT CIRCUIT OF THE POWER STAGE WITH CURRENT LOOP INCLUDED
The value of the injected resistor can be estimated by:
V IN r DS ( ON ) R i = ---------------- --------------------------- * 4.4k V ramp R CS + 140
Which means the current sensing pin will source current to make the voltage drop on the MOSFET equal to the voltage generated on the sensing resistor, plus the internal resistor, along the ISEN pin current flowing path.
Ri is in k, and RDS and RCS are in . Vin divided by Vramp, is defined as Gm, which is a constant 8 or 18dB for both channels in dual switcher applications, when Vin is above 3V. Refer to Tables 1 and 2 for the ramp amplitude in different Vin pin connections. The feed-forward effect of the Vin is reflected in Gm. Vc is defined as the error amplifier output voltage.
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ISL6539
TABLE 1. PWM COMPARATOR RAMP AMPLITUDE FOR DUAL SWITCHER APPLICATION VIN PIN CONNECTIONS Ch1 and Ch2 Input Voltage Input voltage >4.2V Input voltage <4.2V GND VRAMP AMPLITUDE Vin/8 1.25V 1.25V
TO PWM COMPARATOR 1M 16.7pF 300K Vc
the internal compensator and makes it possible to accommodate many applications having a wide range of parameters. The schematics for the internal compensator is shown in Figure 6.
1.25pF 500K
TABLE 2. PWM COMPARATOR RAMP VOLTAGE AMPLITUDE FOR DDR APPLICATION VIN PIN CONNECTION Ch1 Input Voltage Input voltage >4.2V Input voltage <4.2V GND Ch2 Input voltage >4.2V GND VRAMP AMPLITUDE Vin/8 1.25V 1.25V 0.625V 1.25V
4.4K ISEN
+
VSEN 0.9V
FIGURE 6. THE INTERNAL COMPENSATOR
Its transfer function can be written as the following:
5 ss1.857 * 10 -------------- + 1 -------------- + 1 2f 2f z1 z2 Gcomp ( s ) = -------------------------------------------------------------------------------------------s s -------------- + 1 2f p1
The small signal transfer function from the error amplifier output voltage Vc to the output voltage Vo can be written in the following expression:
s -------- + 1 Wz Ro G ( s ) = G m --------------------------------------- --------------------------------------------------------R i + DCR + R o s s ------------ + 1 ------------ + 1 Wp1 Wp2
where fz1 = 6.98kHz, fz2 = 380kHz, and fp1 = 137kHz Outside the ISL6539 chip, a capacitor Cz can be placed in parallel with the top resistor in the feedback resistor divider, as shown in Figure 4. In this case the transfer function from the output voltage to the middle point of the divider can be written as:
sR 1 C z + 1 R2 Gfd ( s ) = -------------------- --------------------------------------------R 1 + R 2 s ( R 1 || R 2 )C z + 1
The dc gain is derived by shorting the inductor and opening the capacitor. There is one zero and two poles in this transfer function. The zero is related to ESR and the output capacitor. The first pole is a low frequency pole associated with the output capacitor and its charging resistors. The inductor can be regarded as short. The second pole is the high frequency pole related to the inductor. At high frequency the output cap can be regarded as a short circuit. By approximation, the poles and zero are inversely proportional to the time constants, associated with inductor and capacitor, by the following expressions:
1 Wz = ----------------------ESR*C o 1 Wp1 = -----------------------------------------------------------------------------( ESR + ( R i + DCR ) || R o )*C o R i + DCR + ESR || R o Wp2 = --------------------------------------------------------Lo
The ratio of R1 and R2 is determined by the output voltage set point; therefore, the position of the pole and zero frequency in the above equation may not be far apart; however, they can improve the loop gain and phase margin with the proper design. The Cz can bring the high frequency transient output voltage variation directly to the VSEN pin to cause the PGOOD drop. Such an effect should be considered in the selection of Cz. From the analysis above, the system loop gain can be written as:
Gloop ( s ) = G ( s ) * Gcomp ( s ) * Gfd ( s )
Since the current loop separates the LC resonant poles into two distant poles, and ESR zero tends to cancel the high frequency pole, the second order system behaves like a first order system. This control method simplifies the design of
Figure 7 shows the composition of the system loop gain. As shown in the graph, the power stage became a well damped second order system compared to the LC filter characteristics. The ESR zero is so close to the high frequency pole that they cancel each other out. The power stage behaves like a first order system. With an internal compensator, the loop gain transfer function has a cross
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over frequency at about 30kHz. With a given set of parameters, including the MOSFET rDSON, current sense resistor RCS, output LC filter, and feedback network, the system loop gain can be accurately analyzed and modified by the system designers based on the applications requirements.
60 50 40 30 20
GAIN (dB)
Voltage Monitor and Protections
The converter output is monitored and protected against extreme overload, short circuit, overvoltage, and undervoltage conditions. A sustained overload on the output sets the PGOOD low and latches off the offending channel of the chip. The controller operation can be restored by cycling the VCC voltage or toggling both enable (EN) pins to low to clear the latch.
LC FILTER COMPENSATOR VO/VC LOOP GAIN
Power Good
In the soft-start process, the PGOOD is established after the soft pin voltage is at 1.5V. In normal operation, the PGOOD window is 100mV below the 0.9V and 135mV higher than 0.9V. The VSEN pin has to stay within this window for PGOOD to be high. Since the VSEN pin is used for both feedback and monitoring purposes, the output voltage deviation can be coupled directly to the VSEN pin by the capacitor in parallel with the voltage divider as shown in Figure 4. In order to prevent false PGOOD drop, capacitors need to parallel at the output to confine the voltage deviation with severe load step transient. The PGOOD comparator has a built-in 3s filter. PGOOD is an open drain output.
10 0 -10 -20 -30 -40 -50 -60 100
1*103
1*104 FREQUENCY (Hz)
1*105
1*106
FIGURE 7. THE BODE PLOT OF THE LC FILTER, COMPENSATOR, CONTROL TO OUTPUT VOLTAGE TRANSFER FUNCTION, AND SYSTEM LOOP GAIN
Overcurrent Protection
In dual switcher application, both PWM controllers use the lower MOSFETs on-resistance rDSON, to monitor the current for protection against shorted outputs. The sensed current from the ISEN pin is compared with a current set by a resistor connected from the OCSET pin to ground:
10.3V R SET = -------------------------------------------------------I OC * r DS ( ON ) -------------------------------------- + 8A R CS + 140
Gate Control Logic
The gate control logic translates generated PWM signals into gate drive signals providing necessary amplification, level shift, and shoot-through protection. It bears some functions that help to optimize the IC performance over a wide range of the operational conditions. As MOSFET switching time can vary dramatically from type to type, and with the input voltage, the gate control logic provides adaptive dead time by monitoring real gate waveforms of both the upper and the lower MOSFETs.
Dual-Step Conversion
The ISL6539 dual channel controller can be used either in power systems with a single-stage power conversion or in systems where some intermediate voltages are initially established. The choice of the approach may be dictated by the overall system design criteria, or the approach may be a matter of voltages available to the system designer. When the output voltage is regulated from low voltage such as 5V, the feed-forward ramp may become too shallow, creating the possibility of duty-factor jitter; this is particularly relevant in a noisy environment. Noise susceptibility, when operating from low level regulated power sources, can be improved by connecting the VIN pin to ground, by which the feed-forward ramp generator will be internally reconnected from the VIN pin to the VCC pin, and the ramp slew rate will be doubled.
where IOC is a desired overcurrent protection threshold and RCS is the value of the current sense resistor connected to the ISEN pin. The 8A is the offset current added on top of the sensed current from the ISEN pin for internal circuit biasing. If the lower MOSFET current exceeds the overcurrent threshold, a pulse skipping circuit is activated. The upper MOSFET will not be turned on as long as the sensed current is higher than the threshold value, limiting the current supplied by the DC voltage source. The current in the lower MOSFET will be continuously monitored until it is lower than the OC threshold value, then the following UGATE pulse will be released and normal current sample resumes. This kind of operation remains for eight clock cycles after the overcurrent comparator was tripped for the first time. If after the first eight clock cycles the current exceeds the overcurrent threshold again, in a time interval of another eight clock cycles, the overcurrent protection latches and disables the offending channel. If the overcurrent condition goes away during the first eight clock cycles, normal operation is restored and the overcurrent circuit resets itself at the end of sixteen clock cycles (See Figure 8).
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disengaged. The MOSFET driver will restore its normal operation. When the OVP occurs, the PGOOD will drop to low as well.
8 CLK SHUTDOWN 2 VOUT
1 IL
PGOOD
This OVP scheme provides a `soft' crowbar function, which helps clamp the voltage overshoot, and does not invert the output voltage when otherwise activated with a continuously high output from lower MOSFET driver--a common problem for OVP schemes with a latch.
3 Ch1 5.0V Ch3 1.0A Ch2 100mV M 10.0s
DDR Application
High throughput Double Data Rate (DDR) memory chips are expected to take the place of traditional memory chips. A novel feature associated with this type of memory are the referencing and data bus termination techniques. These techniques employ a reference voltage, VREF, that tracks the center point of VDDQ and VSS voltages, and an additional VTT power source where all terminating resistors are connected. Despite the additional power source, the overall memory power consumption is reduced compared to traditional termination. The added power source has a cluster of requirements that should be observed and considered. Due to the reduced differential thresholds of DDR memory, the termination power supply voltage, VTT, closely tracks VDDQ/2 voltage. Another very important feature of the termination power supply is the capability to operate at equal efficiency in sourcing and sinking modes. The VTT supply regulates the output voltage with the same degree of precision when current is flowing from the supply to the load, and when the current is diverted back from the load into the power supply. The ISL6539 dual channel PWM controller possesses several important enhancements that allow re-configuration for DDR memory applications, and provides all three voltages required in a DDR memory compliant computer. To reconfigure the ISL6539 for a complete DDR solution, the DDR pin should be set high permanently to the VCC rail. This activates some functions inside the chip that are specific to DDR memory power needs. In the DDR application presented in Figure 2, the first controller regulates the VDDQ rail to 2.5V. The output voltage is set by external dividers Rfb1 and Rfb12. The second controller regulates the VTT rail to VDDQ/2. The OCSET2 pin function is now different, and serves as an input that brings VDDQ/2 voltage, created by the Rd1 and Rd2 divider, inside the chip, effectively providing a tracking function for the VTT voltage. The PG2 pin function is also different in DDR mode. This pin becomes the output of the buffer, whose input is connected to the center point of the R/R divider from the VDDQ output by the OCSET2 pin. The buffer output voltage serves as a 1.25V reference for the DDR memory chips. Current capability of this pin is 10mA (12mA max).
FIGURE 8. OVERCURRENT PROTECTION
Due to the nature of the current sensing technique, and to accommodate a wide range of the rDSON variation, the value of the overcurrent threshold should set at about 180% of the nominal load value. If more accurate current protection is desired, a current sense resistor placed in series with the lower MOSFET source may be used. The inductor current going through the lower MOSFET is sensed and held at 400ns after the upper MOSFET is turned off; therefore, the sensed current is very close to its peak value. The inductor peak current can be written as:
( V IN - V out ) * V out I peak = -------------------------------------------------- + I load 2L * F SW * V IN
As seen from the equation above, the inductor peak current changes with the input voltage and the inductor value once an output voltage is selected. After overcurrent protection is activated, there are two ways to bring the offending channel back: (1) Both EN1 and EN2 have to be held low to clear the latch, (2) To recycle the Vcc of the chip, the POR will clear the latch.
Undervoltage Protection
In the process of operation, if a short circuit occurs, the output voltage will drop quickly. Before the overcurrent protection circuit responds, the output voltage will fall out of the required regulation range. The chip comes with undervoltage protection. If a load step is strong enough to pull the output voltage lower than the undervoltage threshold, the offending channel latches off immediately. The undervoltage threshold is 75% of the nominal output voltage. Toggling both enables to low, or recycling Vcc, will clear the latch and bring the chip back to operation.
Overvoltage Protection
Should the output voltage increase over 115% of the normal value due to the upper MOSFET failure, or for other reasons, the overvoltage protection comparator will force the synchronous rectifier gate driver high. This action actively pulls down the output voltage. As soon as the output voltage drops below the threshold, the OVP comparator is 13
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For the VTT channel where output is derived from the VDDQ output, some control and protective functions have been significantly simplified. For example, the overcurrent, and overvoltage, and undervoltage protections for the second channel controller are disabled when the DDR pin is set high. As the VTT channel tracks the VDDQ/2 voltage, the soft-start function is not required, and the SOFT2 pin may be left open, in the event both channels are enabled simultaneously. However, if the VTT channel is enabled later than the VDDQ, the SOFT2 pin must have a capacitor in place to ensure soft-start. In case of overcurrent or undervoltage caused by short circuit on VTT, the fault current will propagate to the first channel and shut down the converter. The VREF voltage will be present even if the VTT is disabled.
Application Information
Design Procedures
GENERAL A ceramic decoupling capacitor should be used between the VCC and GND pin of the chip. There are three major currents drawn from the decoupling capacitor: 1. the quiescent current, supporting the internal logic and normal operation of the IC 2. the gate driver current for the lower MOSFETs 3. and the current going through the external diodes to the bootstrap capacitor for upper MOSFET. In order to reduce the noisy effect of the bootstrap capacitor current to the IC, a small resistor, such as 10, can be used with the decoupling cap to construct a low pass filter for the IC, as shown in Figure 9.
TO BOOT 5V 10 VCC
Channel Synchronization in DDR Applications
The presence of two PWM controllers on the same die requires channel synchronization, to reduce inter-channel interference that may cause the duty factor jitter and increased output ripple. The PWM controller is at greatest noise susceptibility when an error signal on the input of the PWM comparator approaches the decision-making point. False triggering may occur, causing jitter and affecting the output regulation. A common approach used to synchronize dual channel converters is out-of-phase operation. Out-of-phase operation reduces input current ripple and provides a minimum interference for channels that control different voltage levels. When used in a DDR application with cascaded converters (VTT generated from VDDQ), several methods of synchronization are implemented in the ISL6539. When the DDR pin is connected to GND for dual switcher applications, the channels operate 180 out-of-phase. In the DDR mode, when the DDR pin is connected to VCC, the channels operate either with 0 phase shift, when the VIN pin is connected to the GND, or with 90 phase shift if the VIN pin is connected to a voltage higher than 4.2V. The following table lists the different synchronization schemes and their usage:
DDR PIN 0 1 1 VIN PIN Vin pin >4.2V Vin pin voltage <4.2V Vin pin voltage >4.1V SYNCHRONIZATION 180 out of phase 0 phase 90 phase shift
FIGURE 9. INPUT FILTERING FOR THE CHIP
The soft-start capacitor and the resistor divider setting the output voltage is easy to select as discussed in the "Block Diagram" on page 8.
Selection of the Current Sense Resistor
The value of the current sense resistor determines the gain of the current sensing circuit. It affects the current loop gain and the overcurrent protection setpoint. The voltage drop on the lower MOSFET is sensed within 400ns after the upper MOSFET is turned off. The current sense pin has a 140 resistor in series with the external current sensing resistor. The current sense pin can source up to a 260A current while sensing current on the lower MOSFET, in such a way that the voltage drop on the current sensing path would equal to the voltage on the MOSFET.
I SOURCING ( 140 + R CS ) = I D r DS ( ON )
ID can be assumed to be the inductor peak current. In a worst case scenario, the high temperature rDSON could increase to 150% of the room temperature level. During overload condition, the MOSFET drain current ID could be 130% higher than the normal inductor peak. If the inductor has 30% peak-to-peak ripple, ID would equal to 115% of the load current. The design should consider the above factors so that the maximum ISOURCING will not saturate to 260A under worst case conditions. To be safe, ISOURCING should be less than 100A in normal operation at room temperature. The formula in the earlier discussion assumes
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a 75A sourcing current. Users can tune the sourcing current of the ISEN pin to meet the overcurrent protection and the change the current loop gain. The lower the current sensing resistor, the higher gain of the current loop, which can damp the output LC filter more. A higher current-sensing resistor will decrease the current sense gain. If the phase node of the converter is very noisy due to poor layout, the sensed current will be contaminated, resulting in duty cycle jittering by the current loop. In such a case, a bigger current sense resistor can be used to reduce both real and noise current levels. This can help damp the phase node waveform jittering. Sometimes, if the phase node is very noisy, a resistor can be put on the ISEN pin to ground. This resistor together with the RCS can divide the phase node voltage down, seen by the internal current sense amplifier, and reduce noise coupling. Based on the previous description and functional block diagram, the OC set resistor can be calculated as:
10.3V R set = --------------------------------------------------I OC r DS ( ON ) -------------------------------- + 8A R CS + 140
IOC is the inductor peak current and not the load current. Since inductor peak current changes with input voltage, it is better to use an oscilloscope when testing the overcurrent setting point to monitor the inductor current, and to determine when the OC occurs. To get consistent test results on different boards, it is best to keep the MOSFET at a fixed temperature. The MOSFET will not heat-up when applying a very low frequency and short load pulses with an electronic load to the output. As an example, assume the following: * the maximum normal operation load current is 1, * the OC set point is 10% higher than the maximum load current, * the inductor peak current is 1.15-1.3 times higher than the load current, depending on the inductor value and the input voltage, * and the rDSON has a 45% increase at higher temperature. IOC should set at least 1.8 to 2 times higher than the maximum load current to avoid nuisance overcurrent trip.
Sizing the Overcurrent Setpoint Resistor
The internal 0.9V reference is buffered to the OCSET pin with a voltage follower (refer to the equivalent circuit in Figure 10). The current going through the external overcurrent set resistor is sensed from the OCSET pin. This current, divided by 2.9, sets up the overcurrent threshold and compares with the scaled ISEN pin current going through RCS with an 8A offset. Once the sensed current is higher than the threshold value, an OC signal is generated. The first OC signal starts a counter and activates a pulse skipping function. The inductor current will be continuously monitored through the phase node voltage after the first OC trip. As long as the sensed current exceeds the OC threshold value, the following PWM pulse will be skipped. This operation will be the same for 8 switching cycles. Another OC occurring between 8 to 16 switching cycles would result in a latch off with both upper and lower drives low. If there is no OC within 8 to 16 switching cycles, normal operation resumes.
Selection of the LC Filter
The duty cycle of a buck converter is a function of the input voltage and output voltage. Once an output voltage is fixed, it can be written as:
V OUT D ( V IN ) = --------------V IN
PHASE RCS
_
ISEN
140 140
The switching frequency, Fsw, of ISL6539 is 300kHz. The peak-to-peak ripple current going through the inductor can be written as:
_ +
8A + + 8uA
rRdson DS(ON)
+
V OUT ( 1 - D ( V IN ) ) I pp = ------------------------------------------------F sw L
IIsense SENSE
/ 33.1 OCSET Amplifier AMPLIFIER _ + Rset 0.9 V REFERENCE Reference + _ OC
COMPARATOR Comparator
/ 2.9
FIGURE 10. EQUIVALENT CIRCUIT FOR OC SIGNAL GENERATOR
As higher ripple current will result in higher switching loss and higher output voltage ripple, the peak-to-peak current of the inductor is generally designed with a 20%-40% peak-topeak ripple of the nominal operation current. Based on this assumption, the inductor value can be selected with the above equation. In addition to the mechanical dimension, a shielded ferrite core inductor with a very low DC resistance, DCR, is preferred for less core loss and copper loss. The DC copper loss of the inductor can be estimated by:
P copper = I load DCR
2
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The inductor copper loss can be significant in the total system power loss. Attention has to be given to the DCR selection. Another factor to consider when choosing the inductor is its saturation characteristics at elevated temperature. Saturated inductors could result in nuisance OC, or OV trip. Output voltage ripple and the transient voltage deviation are factors that have to be taken into consideration when selecting an output capacitor. In addition to high frequency noise related MOSFET turn-on and turn-off, the output voltage ripple includes the capacitance voltage drop and ESR voltage drop caused by the AC peak-to-peak current. These two voltages can be represented by:
I pp V c = -----------------8CF sw V esr = I pp ESR
The maximum RMS current required by the regulator may be closely approximated through the following equation:
2 OUT 2 (D - D ) + I 2D ----12
I
Cin ( RMS )
=
I
Ripple ( p - p )
In addition to the bulk capacitance, some low ESL ceramic decoupling is recommended to be used between the drain terminal of the upper MOSFET and the source terminal of the lower MOSFET, in order to clamp the parasitic voltage ringing at the phase node in switching.
Choosing MOSFETs
For a maximum input voltage of 15V, at least a minimum 30V MOSFETs should be used. The design has to trade off the gate charge with the rDSON of the MOSFET: * For the lower MOSFET, before it is turned on, the body diode has been conducting. The lower MOSFET driver will not charge the miller capacitor of this MOSFET. * In the turning off process of the lower MOSFET, the load current will shift to the body diode first. The high dv/dt of the phase node voltage will charge the miller capacitor through the lower MOSFET driver sinking current path. This results in much less switching loss of the lower MOSFETs. The duty cycle is often very small in high battery voltage applications, and the lower MOSFET will conduct most of the switching cycle; therefore, the lower the rDSON of the lower MOSFET, the less the power loss. The gate charge for this MOSFET is usually of secondary consideration. The upper MOSFET does not have this zero voltage switching condition, and because the upper MOSFET conducts for less time compared to the lower MOSFET, the switching loss tends to be dominant. Priority should be given to the MOSFETs with less gate charge, so that both the gate driver loss, and switching loss, will be minimized. For the lower MOSFET, its power loss can be assumed to be the conduction loss only.
P lower ( V IN ) ( 1 - D ( V IN ) )I load rDS ( ON )Lower
2
These two components constitute a large portion of the total output voltage ripple. Several capacitors have to be paralleled in order to reduce the ESR and the voltage ripple. If the output of the converter has to support another load with high pulsating current, such as the first channel in Figure 2, it feeds into the VTT channel which draws high pulsating current. More capacitors are needed in order to reduce the equivalent ESR and suppress the voltage ripple to a tolerable level. To support a load transient that is faster than the switching frequency, more capacitors have to be used to reduce the voltage excursion during load step change. Another aspect of the capacitor selection is that the total AC current going through the capacitors has to be less than the rated RMS current specified on the capacitors, to prevent the capacitor from overheating. For DDR applications, as the second channel draws pulsate current directly from the first channel, it is recommended to parallel capacitors for output of the first channel to reduce ESR and smooth ripple. Excessive high ripple voltage at the output can propagate into the output of the error amplifier and cause too much phase voltage jitter.
Input Capacitor Selection
The important parameters for the bulk input capacitance are the voltage rating and the RMS current rating. For reliable operation, select bulk capacitors with voltage and current ratings above the maximum input voltage and largest RMS current required by the circuit. Their voltage rating should be at least 1.25 times greater than the maximum input voltage, while a voltage rating of 1.5 times is a conservative guideline. For most cases, the RMS current rating requirement for the input capacitor of a buck regulator is approximately 1/2 the DC load current.
For the upper MOSFET, its conduction loss can be written as:
P uppercond ( V IN ) = D ( V IN )I load rDS ( ON )upper
2
and its switching loss can be written as:
V IN I vally T on F sw V IN I peak T off F sw Puppersw ( V IN ) = --------------------------------------------- + ---------------------------------------------2 2
The peak and valley current of the inductor can be obtained based on the inductor peak-to-peak current and the load current. The turn-on and turn-off time can be estimated with the given gate driver parameters in the Electrical
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Specification Table on page 4. For example, if the gate driver turn-on path MOSFET has a typical on-resistance of 4, its maximum turn-on current is 1.2A with 5V Vcc. This current would decay as the gate voltage increased. With the assumption of linear current decay, the turn-on time of the MOSFETs can be written with:
2Q gd T on = ---------------I driver
between BOOT and PHASE when the phase became negative. A resistor can be placed between the cathode of the boot strap diode and BOOT pin to increase the charging time constant of the boot cap. This resistor will not affect the turn-on and off of the upper MOSFET. A schottky diode can reduce the reverse recovery of the lower MOSFET when transitioning from freewheeling to blocking, therefore, it is generally good practice to have a schottky diode closely parallel with the lower MOSFET. B340LA, from Diodes, Inc.(R), can be used as the external schottky diode.
Qgd is used because when the MOSFET drain-to-source voltage has fallen to zero, it gets charged. Similarly, the turnoff time can be estimated based on the gate charge and the gate drivers sinking current capability. The total power loss of the upper MOSFET is the sum of the switching loss and the conduction loss. The temperature rise on the MOSFET can be calculated based on the thermal impedance given on the datasheet of the MOSFET. If the temperature rise is too much, a different MOSFET package size, layout copper size, and other options have to be considered to keep the MOSFET cool. The temperature rise can be calculated by:
T rise = jaPtotalpower loss
Tuning the Turn-on of Upper MOSFET
The turn-on speed of the upper MOSFET can be adjusted by the resistor connecting the boot cap to the boot pin of the chip. This resistor can confine the voltage ringing on the boot capacitor from coupling to the boot pin. This resistor slows down only the turn-on of the upper MOSFET. If the upper MOSFET is turned on very fast, it could result in a very high dv/dt on the phase node, which could couple into the lower MOSFET gate through the miller capacitor, causing momentous shoot-through. This phenomenon, together with the reverse recovery of the body diode of the lower MOSFET, can overshoot the phase node voltage to beyond the voltage rating of the MOSFET. However, a bigger resistor will slow the turn-on of the MOSFET too much and lower the efficiency. Trade-offs need to be made in choosing such a resistor.
The MOSFET gate driver loss can be calculated with the total gate charge and the driver voltage Vcc. The lower MOSFET only charges the miller capacitor at turn-off.
P driver = V cc Q gs F sw
System Loop Gain and Stability
The system loop gain is a product of three transfer functions: 1. the transfer function from the output voltage to the feedback point, 2. the transfer function of the internal compensation circuit from the feedback point to the error amplifier output voltage, 3. and the transfer function from the error amplifier output to the converter output voltage. These transfer functions are written in a closed form in the Theory of Operation section. The external capacitor, in parallel with the upper resistor of the resistor divider, Cz, can be used to tune the loop gain and phase margin. Other component parameters, such as the inductor value, can be changed for a wider cross-over frequency of the system loop gain. A body plot of the loop gain transfer function with a 45 degree phase margin (a 60 degree phase margin is better) is desirable to cover component parameter variations.
Based on the above calculation, the system efficiency can be estimated by the designer.
Confining the Negative Phase Node Voltage Swing with Schottky Diode
At each switching cycle, the body diode of the lower MOSFET will conduct before the MOSFET is turned-on, as the inductor current is flowing to the output capacitor. This will result in a negative voltage on the phase node. The higher the load current, the lower this negative voltage. This voltage will ring back less negative when the lower MOSFET is turned on. A total 400ns period is given to the current sample-and-hold circuit on the ISEN pin to sense the current going through the lower MOSFET after the upper MOSFET turns off. An excessive negative voltage on the lower MOSFET will be treated as overcurrent. In order to confine this voltage, a schottky diode can be used in parallel with the lower MOSFET for high load current applications. PCB layout parasitics should be reduced in order to reduce the negative ringing of phase voltage. The second concern for the phase node voltage going into negative is that the boot strap capacitor between the BOOT and PHASE pin could get be charged higher than VCC voltage, exceeding the 6.5V absolute maximum voltage
Testing the Overvoltage on Buck Converters
For synchronous buck converters, if an active source is used to raise the output voltage for the overvoltage protection test, the buck converter will behave like a boost converter and dump energy from the external source to the input. The overvoltage test can be done on ISL6539 by connecting the VSEN pin to an external voltage source or signal generator through a diode. When the external voltage, or signal generator voltage, is tuned to a higher level than the overvoltage threshold (the lower MOSFET will be on), it
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indicates the overvoltage protection works. This kind of overvoltage protection does not require an external schottky in parallel with the output capacitor. wide, and away from other traces. There should be no other weak signal traces in parallel with these traces on any layer.
Pin 3 and Pin 26, PGND1 and PGND2
Each pin should be laid out to the negative side of the relevant output cap with separate traces. The negative side of the output capacitor must be close to the source node of the bottom MOSFET. These traces are the return path of LGATE1 and LGATE2.
Layout Considerations
Power and Signal Layer Placement on the PCB
As a general rule, power layers should be close together, either on the top or bottom of the board, with signal layers on the opposite side of the board. For example, prospective layer arrangement on a 4 layer board is shown below: 1. Top Layer: ISL6539 signal lines 2. Signal Ground 3. Power Layers: Power Ground 4. Bottom Layer: Power MOSFET, Inductors and other Power traces It is a good engineering practice to separate the power voltage and current flowing path from the control and logic level signal path. The controller IC will stay on the signal layer, which is isolated by the signal ground to the power signal traces.
Pin 4 and Pin 25, the PHASE Pin
These traces should be short, and positioned away from other weak signal traces. The phase node has a very high dv/dt with a voltage swing from the input voltage to ground. No trace should be in parallel with these traces. These traces are also the return path for UGATE1 and UGATE2. Connect these pins to the respective converters' upper MOSFET source.
Pin 5 and Pin 24, the UGATE1 and UGATE2
These pins have a square shape waveform with high dv/dt. It provides the gate drive current to charge and discharge the top MOSFET with high di/dt. This trace should be wide, short, and away from other traces similar to the LGATEx.
Component Placement
The control pins of the two-channel ISL6539 are located symmetrically on two sides of the IC; it is desirable to arrange the two channels symmetrically around the IC. The power MOSFET should be close to the IC so that the gate drive signal, the LAGTEx, UGATEx, PHASEx, BOOTx, and ISENx traces can be short. Place the components in such a way that the area under the ISL6539 has fewer noise traces with high dv/dt and di/dt, such as gate signals and phase node signals.
Pin 6 and Pin 23, the BOOT1 and BOOT2
These pins di/dt are as high as that of the UGATEx; therefore, the traces should be as short as possible.
Pin 7 and Pin 22, the ISEN1 and ISEN2
The ISEN trace should be a separate trace, and independently go to the drain terminal of the lower MOSFET. The current sense resistor should be close to ISEN pin. The loop formed by the bottom MOSFET, output inductor, and output capacitor, should be very small. The source of the bottom MOSFET should tie to the negative side of the output capacitor in order for the current sense pin to get the voltage drop on the rDSON.
Signal Ground and Power Ground Connection
At minimum, a reasonably large area of copper, which will shield other noise couplings through the IC, could be used as signal ground beneath the ISL6539. The best tie-point between the signal ground and the power ground is at the negative side of the output capacitor on each channel, where there is less noise. Noisy traces beneath the ISL6539 are not recommended.
Pin 8 and Pin 21, the EN1 and EN2
These pins stay high in enable mode and low in idle mode and are relatively robust. Enable signals should refer to the signal ground.
Pin 10 and Pin 19, VSEN1 and VSEN2
There is usually a resistor divider connecting the output voltage to this pin. The input impedance of these two pins is high because they are the input to the amplifiers. The correct layout should bring the output voltage from the regulation point to the SEN pin with kelvin traces. Build the resistor divider close to the pin so that the high impedance trace is shorter.
GND and VCC Pins
At least one high quality ceramic decoupling cap should be used across these two pins. A via can tie GND to signal ground. Since Pin 1 (GND) and Pin 28 (VCC) are close together, the decoupling cap can be put close to the IC.
Pin 2 and Pin 27, the LGATE1 and LGATE2
These are the gate drive signals for the bottom MOSFETs of the buck converter. The signal going through these traces have both high dv/dt and high di/dt, with high peak charging and discharging current. These two traces should be short,
Pin 11 and Pin 18, the OCSET1 and OCSET2
In dual switcher mode operation, the overcurrent set resistor should be put close to this pin. In DDR mode operation, the voltage divider, which divides the VDQQ voltage in half,
18
FN9144.4 June 6, 2005
ISL6539
should be put very close to this pin. The other side of the OC set resistor should connect to signal ground.
Decoupling Capacitor for Switching MOSFET
It is recommended that ceramic caps be used closely connected to the drain side of the upper MOSFET, and the source of the lower MOSFET. This capacitor reduces the noise and the power loss of the MOSFET. Refer to Figure 11 for the power component placement.
Pin 12 and Pin 17, SOFT1 and SOFT2
The soft-start capacitors should be laid out close to this pin. The other side of the soft-start cap should tie to signal ground.
Pin 15 and Pin 16, PG1 and PG2 for Dual Switcher Operation
For dual switcher operations, these two lines are less noise sensitive. For DDR applications, a capacitor should be placed to the PG2/REF pin.
Pin 13, DDR
This pin should connect to VCC in DDR applications, and to signal ground in dual switcher applications.
Pin 14, VIN
This pin connects to battery voltage, and is less noise sensitive.
Copper size for the Phase Node
Big coppers on both sides of the Phase node introduce parasitic capacitance. The capacitance of PHASE should be kept very low to minimize ringing. If ringing is excessive, it could easily affect current sample information. It would be best to limit the size of the PHASE node copper in strict accordance with the current and thermal management of the application.
FIGURE 11. A GOOD EXAMPLE POWER COMPONENT REPLACEMENT. IT SHOWS THE NEGATIVE OF INPUT AND OUTPUT CAP AND THE SOURCE OF THE MOSFET ARE TIED AT ONE POINT.
Identify the Power and Signal Ground
The input and output capacitors of the converters, the source terminals of the bottom switching MOSFET PGND1, and PGND2, should be closely connected to the power ground. The other components should connect to signal ground. Signal and power ground are tied together at the negative terminal of the output capacitors.
19
FN9144.4 June 6, 2005
ISL6539 Shrink Small Outline Plastic Packages (SSOP) Quarter Size Outline Plastic Packages (QSOP)
N INDEX AREA H E -B1 2 3 SEATING PLANE -AD -CA 0.25 0.010 h x 45 L GAUGE PLANE 0.25(0.010) M BM
M28.15
28 LEAD SHRINK SMALL OUTLINE PLASTIC PACKAGE (0.150" WIDE BODY) INCHES SYMBOL A A1 A2 B C D
A2 C 0.10(0.004) C AM BS
MILLIMETERS MIN 1.35 0.10 0.20 0.18 9.81 3.81 5.80 0.26 0.41 28 0 MAX 1.75 0.25 1.54 0.30 0.25 10.00 3.98 6.19 0.49 1.27 8 NOTES 9 3 4 5 6 7 Rev. 1 6/04
MIN 0.053 0.004 0.008 0.007 0.386 0.150 0.228 0.0099 0.016 28 0
MAX 0.069 0.010 0.061 0.012 0.010 0.394 0.157 0.244 0.0196 0.050 8
A1
e
B 0.17(0.007) M
E e H h L N
0.025 BSC
0.635 BSC
NOTES: 1. Symbols are defined in the "MO Series Symbol List" in Section 2.2 of Publication Number 95. 2. Dimensioning and tolerancing per ANSI Y14.5M-1982. 3. Dimension "D" does not include mold flash, protrusions or gate burrs. Mold flash, protrusion and gate burrs shall not exceed 0.15mm (0.006 inch) per side. 4. Dimension "E" does not include interlead flash or protrusions. Interlead flash and protrusions shall not exceed 0.25mm (0.010 inch) per side. 5. The chamfer on the body is optional. If it is not present, a visual index feature must be located within the crosshatched area. 6. "L" is the length of terminal for soldering to a substrate. 7. "N" is the number of terminal positions. 8. Terminal numbers are shown for reference only. 9. Dimension "B" does not include dambar protrusion. Allowable dambar protrusion shall be 0.10mm (0.004 inch) total in excess of "B" dimension at maximum material condition. 10. Controlling dimension: INCHES. Converted millimeter dimensions are not necessarily exact.
All Intersil U.S. products are manufactured, assembled and tested utilizing ISO9000 quality systems. Intersil Corporation's quality certifications can be viewed at www.intersil.com/design/quality
Intersil products are sold by description only. Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without notice. Accordingly, the reader is cautioned to verify that data sheets are current before placing orders. Information furnished by Intersil is believed to be accurate and reliable. However, no responsibility is assumed by Intersil or its subsidiaries for its use; nor for any infringements of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Intersil or its subsidiaries.
For information regarding Intersil Corporation and its products, see www.intersil.com 20
FN9144.4 June 6, 2005

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