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19-1175; Rev 0; 12/96
High-Accuracy Step-Down Controller with Synchronous Rectifier for CPU Power
_______________General Description
The high-performance, step-down DC-DC converter provides main CPU power in battery-powered systems. This buck controller achieves 96% efficiency by using synchronous rectification and Maxim's proprietary Idle ModeTM control scheme to extend battery life at full-load (up to 10A) and no-load outputs. The MAX798's high accuracy meets the demanding requirements of the latest-generation CPUs. Excellent dynamic response corrects output transients caused by the latest dynamic-clock CPUs within five 300kHz clock cycles. Unique bootstrap circuitry drives inexpensive N-channel MOSFETs, reducing system cost and eliminating the crowbar switching currents found in some PMOS/NMOS switch designs. The MAX798 has a logic-controlled and synchronizable fixed-frequency pulse-width-modulating (PWM) operating mode, which reduces noise and RF interference in sensitive mobile-communications and pen-entry applications. The SKIP override input allows automatic switchover to idle-mode operation (for high-efficiency pulse skipping) at light loads, or forces fixed-frequency mode for lowest noise at all loads. MAX798
____________________________Features
o 96% Efficiency o 4.5V to 30V Input Range o 1.6V to 6V Adjustable Precision Output o o o o o 0.4% Max Total Load-Regulation Error 0.06%/V Max Line-Regulation Error 5V Linear-Regulator Output Precision 2.505V Reference Output Automatic Bootstrap Circuit
MAX798
o 150kHz/300kHz Fixed-Frequency PWM Operation o Programmable Soft-Start o 1.2mA Typical Quiescent Current (VIN = 12V, VOUT = 2.5V) o 1A Typical Shutdown Current
______________Ordering Information
PART MAX798ESE TEMP. RANGE -40C to +85C PIN-PACKAGE 16 Narrow SO
________________________Applications
Notebook and Subnotebook Computers PDAs and Mobile Communicators
__________Typical Operating Circuit
INPUT 4.5V TO 30V
__________________Pin Configuration
TOP VIEW
V+ VL BST N
5V, 25mA 4.7F
SS 1 SKIP 2 REF 3 GND 4 SYNC 5 SHDN 6 FB 7 CSH 8
16 DH 15 LX 14 BST
DH 0.1F
RSENSE VOUT
MAX798
LX ON SHDN DL PGND CSH CSL N
MAX798
13 DL 12 PGND 11 VL 10 V+ 9 CSL
OFF
SO
GND
FB
Idle Mode is a trademark of Maxim Integrated Products. U.S. and foreign patents pending.
________________________________________________________________ Maxim Integrated Products 1
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800
High-Accuracy Step-Down Controller with Synchronous Rectifier for CPU Power MAX798
ABSOLUTE MAXIMUM RATINGS
V+ to GND .................................................................-0.3V, +36V GND to PGND........................................................................2V VL to GND ...................................................................-0.3V, +7V BST to GND ...............................................................-0.3V, +36V DH to LX ........................................................-0.3V, (BST + 0.3V) LX to BST.....................................................................-7V, +0.3V SHDN to GND............................................................-0.3V, +36V SYNC, SS, REF, SKIP, DL to GND ...................-0.3V, (VL + 0.3V) CSH, CSL to GND .......................................................-0.3V, +7V VL Short Circuit to GND..............................................Momentary REF Short Circuit to GND ...........................................Continuous VL Output Current ...............................................................50mA Continuous Power Dissipation (TA = +70C) SO (derate 8.70mW/C above +70C) ........................696mW Operating Temperature Range MAX798ESE....................................................-40C to +85C Storage Temperature Range .............................-65C to +160C Lead Temperature (soldering, 10sec) .............................+300C
Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(V+ = +15V, GND = PGND = 0V, IVL = IREF = 0A, TA = 0C to +85C, unless otherwise noted.) PARAMETER STEP-DOWN CONTROLLER Input Supply Range Nominal Adjustable Output Voltage Range Feedback Voltage Load Regulation Line Regulation Current-Limit Voltage SS Source Current SS Fault Sink Current INTERNAL REGULATOR AND REFERENCE VL Output Voltage VL Fault Lockout Voltage VL/CSL Switchover Voltage Reference Output Voltage Reference Fault Lockout Voltage Reference Load Regulation CSL Shutdown Leakage Current V+ Shutdown Current V+ Off-State Leakage Current Dropout Power Consumption Quiescent Power Consumption SHDN = 2V, 0mA 2
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High-Accuracy Step-Down Controller with Synchronous Rectifier for CPU Power
ELECTRICAL CHARACTERISTICS (continued)
(V+ = +15V, GND = PGND = 0V, IVL = IREF = 0A, TA = 0C to +85C, unless otherwise noted.) PARAMETER Oscillator Frequency SYNC High Pulse Width SYNC Low Pulse Width SYNC Rise/Fall Time Oscillator Sync Range Maximum Duty Cycle Input High Voltage Input Low Voltage SYNC = REF SYNC = 0V or 5V SYNC SHDN, SKIP SYNC SHDN, SKIP SHDN, 0V or 30V Input Current SYNC, SKIP CSH, CSL, CSH = CSL = 4V, device not shut down FB, FB = 1.6V DL Sink/Source Current DH Sink/Source Current DL On-Resistance DH On-Resistance DL forced to 2V DH forced to 2V, BST - LX = 4.5V High or low High or low, BST - LX = 4.5V 1 1 7 7 Guaranteed by design 195 89 93 VL - 0.5 2.0 0.8 0.5 2 1 50 100 nA A A 91 96 SYNC = REF SYNC = 0V or 5V CONDITIONS MIN 270 125 200 200 200 340 TYP 300 150 MAX 330 175 UNITS kHz ns ns ns kHz % V V A A
MAX798
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High-Accuracy Step-Down Controller with Synchronous Rectifier for CPU Power MAX798
ELECTRICAL CHARACTERISTICS
(V+ = +15V, GND = PGND = 0V, IVL = IREF = 0A, TA = -40C to +85C, unless otherwise noted.) (Note 3) PARAMETER STEP-DOWN CONTROLLER Input Supply Range Nominal Adjustable Output Voltage Range Feedback Voltage Load Regulation Line Regulation Current-Limit Voltage External resistor divider CSH - CSL = 40mV, SKIP = VL 0mV < (CSH - CSL) < 80mV, SKIP = VL 6V < V+ < 30V CSH - CSL, positive CSH - CSL, negative SHDN = 2V, 0mA < IVL < 25mA, 5.5V < V+ < 30V Rising edge, hysteresis = 15mV Rising edge, hysteresis = 25mV No external load (Note 1) 0A < IREF < 100A SHDN = 0V, V+ = 30V, CSL = 0V or 6V FB = CSH = CSL = 6V, VL switched over to CSL CSH = CSL = 6V SYNC = REF SYNC = 0V or 5V 250 110 250 250 210 SYNC = REF SYNC = 0V or 5V High or low High or low, BST - LX = 4.5V 88 92 91 96 7 7 320 1 1 6.4 300 150 70 -40 4.7 3.75 4.15 2.438 2.505 -100 0.04 5.0 1.6 1.560 1.6 30 6.0 1.640 1 0.06 130 -160 5.3 4.05 4.75 2.562 30 10 10 9.1 350 190 V V V % %/ V mV CONDITIONS MIN TYP MAX UNITS
INTERNAL REGULATOR AND REFERENCE VL Output Voltage VL Fault Lockout Voltage VL/CSL Switchover Voltage Reference Output Voltage Reference Load Regulation V+ Shutdown Current V+ Off-State Leakage Current Quiescent Power Consumption V V V V mV A A mW
OSCILLATOR AND INPUTS/OUTPUTS Oscillator Frequency SYNC High Pulse Width SYNC Low Pulse Width Oscillator Sync Range Maximum Duty Cycle DL On-Resistance DH On-Resistance kHz ns ns kHz %
Note 1: Since the reference uses VL as its supply, V+ line-regulation error is insignificant. Note 2: At very low input voltages, quiescent supply current can increase due to excess PNP base current in the VL linear regulator. This occurs only if V+ falls below the preset VL regulation point (5V nominal). The typical maximum quiescent current in dropout will not exceed 16mA. Note 3: All -40C to +85C specifications above are guaranteed by design.
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High-Accuracy Step-Down Controller with Synchronous Rectifier for CPU Power
______________________________________________________________Pin Description
PIN 1 2 3 4 5 NAME SS SKIP REF GND SYNC FUNCTION Soft-Start Timing Capacitor Connection. Ramp time to full current limit is approximately 1ms/nF. Disables pulse-skipping mode when high. Connect to GND for normal use. Don't leave SKIP unconnected. With SKIP grounded, the device will automatically change from pulse-skipping operation to full PWM operation when the load current exceeds approximately 30% of maximum. Reference Voltage Output. Bypass to GND with 0.33F minimum. Low-Noise Analog Ground and Feedback Reference Point Oscillator Synchronization and Frequency Select. Tie to GND or VL for 150kHz operation; tie to REF for 300kHz operation. A high-to-low transition begins a new cycle. Drive SYNC with 0V to 5V logic levels (see the Electrical Characteristics table for VIH and VIL specifications). SYNC capture range is 195kHz to 340kHz. Shutdown Control Input, active low. Logic threshold is set at approximately 1V (VTH of an internal N-channel MOSFET). Tie SHDN to V+ for automatic start-up. Feedback Input. Regulates at FB = 1.6V. Connect FB to a resistor divider to set the output voltage. Current-Sense Input, high side. Current-limit level is 100mV referred to CSL. Current-Sense Input, low side Battery Voltage Input (4.5V to 30V). Bypass V+ to PGND close to the IC with a 0.1F capacitor. Connects to a linear regulator that powers VL. 5V Internal Linear-Regulator Output. VL is also the supply voltage rail for the chip. VL is switched to the output voltage via CSL (VCSL > 4.5V) for automatic bootstrapping. Bypass to GND with 4.7F. VL can supply up to 5mA for external loads. Power Ground Low-Side Gate-Drive Output. Normally drives the synchronous-rectifier MOSFET. Swings 0V to VL. Boost Capacitor Connection for high-side gate drive (0.1F) Switching Node (inductor) Connection. Can swing 2V below ground without hazard. High-Side Gate-Drive Output. Normally drives the main buck switch. DH is a floating driver output that swings from LX to BST, riding on the LX switching-node voltage.
MAX798
6 7 8 9 10
SHDN FB CSH CSL V+
11 12 13 14 15 16
VL PGND DL BST LX DH
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High-Accuracy Step-Down Controller with Synchronous Rectifier for CPU Power MAX798
VIN, 4.5V TO 30V R3 10
2.2F
4.7F
0.33F
V+ REF
SKIP
VL BST DH Q1 0.1F
C1
C2
ON
OFF
SHDN
LX
L1 12H
15m
VOUT, +2.5V @ 4.2A C4
MAX798
DL
C3 47pF Q2
PGND R4 1M 0.01F SS CSH CSL FB SYNC GND
C5 470pF
R1
R2
Q1, Q2 = C1, C2 = C4 = L1 = R1 = R2 = f=
SILICONIX Si4410DY or IRF7413 10F/30V SANYO OS-CON (30SA10) 470F/4V SPRAGUE 594D SERIES (594D477X0004R2T) SUMIDA CDRH127 120 6.49k, 1% 11.5k, 1% 150kHz
Figure 1. Standard Application Circuit
_______________Detailed Description
The MAX798 is a BiCMOS, switch-mode power-supply controller designed primarily for buck-topology regulators in battery-powered applications where high accuracy, high efficiency, and low quiescent supply current are critical. The MAX798 also works well in other topologies such as boost, inverting, and CUK due to the flexibility of its floating high-speed gate driver. Light-load efficiency is enhanced by automatic idlemode operation--a variable-frequency pulse-skipping
mode that reduces losses due to MOSFET gate charge. The step-down power-switching circuit consists of two N-channel MOSFETs, a rectifier, and an LC output filter. The output voltage is the average of the AC voltage at the switching node, which is adjusted and regulated by changing the duty cycle of the MOSFET switches. The gate-drive signal to the N-channel high-side MOSFET must exceed the battery voltage and is provided by a flying capacitor boost circuit that uses a 100nF capacitor connected between BST and LX.
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High-Accuracy Step-Down Controller with Synchronous Rectifier for CPU Power
The MAX798 contains the following seven major circuit blocks, which are shown in Figure 2. PWM Controller Blocks: * Multi-Input PWM Comparator * Current-Sense Circuit * PWM Logic Block * Gate-Driver Outputs
BATTERY VOLTAGE
Bias Generator Blocks: * +5V Linear Regulator * Automatic Bootstrap Switchover Circuit * +2.505V Reference
MAX798
TO CSL
V+
4.5V
+5V LINEAR REGULATOR SHDN
OUT
VL
+5V AT 5mA
BST
SKIP
PWM LOGIC
DH LX MAIN OUTPUT
DL +2.505V REF PWM COMPARATOR REF 1.6V GND 1X PGND
+2.505V AT 100A
CSH CSL
ON/OFF
FB SHDN SYNC
SS
MAX798
Figure 2. Functional Diagram
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High-Accuracy Step-Down Controller with Synchronous Rectifier for CPU Power
These internal IC blocks aren't powered directly from the battery. Instead, a +5V linear regulator steps down the battery voltage to supply both the IC internal rail (VL pin) as well as the gate drivers. The synchronousswitch gate driver is directly powered from +5V VL, while the high-side-switch gate driver is indirectly powered from VL via an external diode-capacitor boost circuit. An automatic bootstrap circuit turns off the +5V linear regulator and powers the IC from its output voltage if the output is above 4.5V.
MAX798
Adjusting the Output Voltage
The output voltage is set by an external resistor divider between the output voltage and GND, with the midpoint connected to FB (Figure 5). The output voltage can be adjusted from 1.6V to 6V, according to the formula in Figure 5. Recommended R2 values range from 5k to 100k. For best noise immunity, place R1 and R2 close to FB. For a 1.6V output, connect the output voltage directly to FB. Remote sensing of the output voltage is easily achieved by connecting the top of R1 and, if desired, the bottom of R2, to the remote sense point.
PWM Controller Block
The heart of the current-mode PWM controller is a multiinput open-loop comparator that sums three signals: output voltage error signal with respect to the reference voltage, current-sense signal, and slope compensation ramp (Figure 3). The PWM controller is a direct summing type, lacking a traditional error amplifier and the phase shift associated with it. This direct-summing configuration approaches the ideal of cycle-by-cycle control over the output voltage. For more comprehensive information on the MAX798 internal blocks, please refer to the MAX796/MAX797/ MAX799 data sheet.
Bypassing and Compensation Components
The MAX798 is designed to deliver a more accurate output voltage than the MAX797. A major source of the MAX797's output error is the decrease in output voltage with increasing load. This error is greatly reduced in the MAX798 by increasing the gain of the voltage-sense signal relative to the current-sense signal. As a result of this increased gain, the MAX798 is slightly more noise sensitive than the MAX797, and requires some small compensation components as well. On the other hand, output capacitor ESR requirements can be greatly relaxed compared to the MAX797, with the limiting factor being the maximum total output voltage ripple that the application can tolerate. To control noise problems, place the bypass capacitors on REF, VL, and V+ as close as possible to the IC, and use a 10 series resistor (R3, Figure 1) on V+ to form a small lowpass filter. Feed-forward components (R4, C3, and C5) are chosen for stable switching at 150kHz with the components shown. For 300kHz switching and a 4.7H inductor, use R4 = 470k and C5 = 220pF. Keep the components that connect to FB (R4, C5, R1, R2) close to the IC's FB pin.
__________Applications Information
Powering VL from a +5V Supply
If the circuit's output voltage is greater than the VL/CSL switchover voltage, the IC automatically bootstraps and runs off its output. Running from the high-efficiency output rather than the VL linear regulator is particularly desirable if the input voltage is high. If the output is not greater than the VL/CSL switchover threshold, efficiency can be improved by powering VL from another efficient system supply (Figure 4). VL can be connected directly to a +5V supply, provided its maximum excursions are within a 4.5V to 6V range and the main output voltage's maximum is less than 4.2V. If the circuit's output voltage is greater than 4.2V, the IC can activate its internal bootstrap switch and connect the circuit output to VL.
Design Procedure
With the exception of the items previously mentioned, follow the design procedure for the MAX797. The MAX796/MAX797/MAX799 data sheet contains all necessary information on component values, component selection, layout, and additional applications.
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High-Accuracy Step-Down Controller with Synchronous Rectifier for CPU Power MAX798
CSH 1X CSL 1.6V FROM FEEDBACK DIVIDER
MAIN PWM COMPARATOR
R Q S LEVEL SHIFT
BST DH LX
SLOPE COMP
OSC
30mV SKIP VL 4A CURRENT LIMIT SHOOTTHROUGH CONTROL
SS
24R
2.5V SHDN N
1R
MAX798
SYNCHRONOUS RECTIFIER CONTROL R -100mV S VL Q LEVEL SHIFT DL PGND
Figure 3. PWM Controller Detailed Block Diagram
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High-Accuracy Step-Down Controller with Synchronous Rectifier for CPU Power MAX798
V+ DH 0.1F V+ VL REMOTE SENSE LINES
5V 10%
VBATT
MAIN OUTPUT
MAX798
DL
MAX798
DH VOUT DL (VOUT (MAX) < 4.2V) GND
CSH CSL FB R2
R1
VOUT = 1.6V
R1 (1 + ---) R2
Figure 4. Powering VL from a Separate +5V Supply
Figure 5. Adjusting the Output Voltage
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High-Accuracy Step-Down Controller with Synchronous Rectifier for CPU Power
___________________Chip Information
TRANSISTOR COUNT: 1008
MAX798
________________________________________________________Package Information
DIM INCHES MAX MIN 0.069 0.053 0.010 0.004 0.019 0.014 0.010 0.007 0.157 0.150 0.050 0.244 0.228 0.050 0.016 MILLIMETERS MIN MAX 1.35 1.75 0.10 0.25 0.35 0.49 0.19 0.25 3.80 4.00 1.27 5.80 6.20 0.40 1.27
D A e B
0.101mm 0.004in.
0-8
A1
C
L
A A1 B C E e H L
E
H
Narrow SO SMALL-OUTLINE PACKAGE (0.150 in.)
DIM PINS D D D 8 14 16
INCHES MILLIMETERS MIN MAX MIN MAX 0.189 0.197 4.80 5.00 0.337 0.344 8.55 8.75 0.386 0.394 9.80 10.00
21-0041A
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High-Accuracy Step-Down Controller with Synchronous Rectifier for CPU Power MAX798
NOTES
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
12 __________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 (408) 737-7600 (c) 1996 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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