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19-1239; Rev 0; 7/97
High-Voltage, Step-Down Controller with Synchronous Rectifier for CPU Power
_______________General Description
The MAX797H high-performance, step-down DC-DC converter provides main CPU power in battery-powered systems. A 40V rating on the power stage's input allows operation with high-cell-count batteries and a wide range of AC adaptors. 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. 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 MAX797H 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. The MAX797H is pin compatible with the popular MAX797, but has a higher input voltage range. The MAX797H comes in a 16-pin narrow SO package.
____________________________Features
o 96% Efficiency o Up to 40V Power Input o 2.5V to 6V Adjustable Output o o o o o o o o Preset 3.3V and 5V Outputs (at up to 10A) 5V Linear-Regulator Output Precision 2.505V Reference Output Automatic Bootstrap Circuit 150kHz/300kHz Fixed-Frequency PWM Operation Programmable Soft-Start 375A Quiescent Current (VIN = 12V, VOUT = 5V) 1A Shutdown Current
MAX797H
______________Ordering Information
PART MAX797HESE TEMP. RANGE -40C to +85C PIN-PACKAGE 16 Narrow SO
U.S. and foreign patents pending.
__________Typical Operating Circuit
+4.5V TO +30V SUPPLY INPUT V+ SHDN DH VL +4V TO +40V POWER INPUT
________________________Applications
Notebook and Subnotebook Computers Industrial Controls
__________________Pin Configuration
TOP VIEW
SS 1 SKIP 2 REF 3 GND 4 SYNC 5 SHDN 6 FB 7 CSH 8 16 DH
MAX797H BST
SS 15 LX 14 BST REF DL PGND SYNC 10 V+ 9 CSL GND SKIP FB CSL CSH LX
+3.3V OUTPUT
MAX797H
13 DL 12 PGND 11 VL
SO
Idle Mode is a trademark of Maxim Integrated Products.
________________________________________________________________ Maxim Integrated Products 1
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800 For small orders, phone 408-737-7600 ext. 3468.
High-Voltage, Step-Down Controller with Synchronous Rectifier for CPU Power MAX797H
ABSOLUTE MAXIMUM RATINGS
V+ to GND ................................................................-0.3V to 36V GND to PGND........................................................................2V VL to GND...................................................................-0.3V to 7V BST to GND ..............................................................-0.3V to 46V DH to LX .....................................................-0.3V to (BST + 0.3V) LX to BST ....................................................................-7V to 0.3V SHDN to GND ...........................................................-0.3V to 36V SYNC, SS, REF, FB, SKIP, DL to GND ...........-0.3V to (VL + 0.3V) CSH, CSL to GND.......................................................-0.3V to 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 MAX797HESE .................................................-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, SYNC = 0V, unless otherwise noted.) PARAMETER +3.3V AND +5V STEP-DOWN CONTROLLERS Input Supply Range V+ High-side MOSFET drain 0mV < (CSH - CSL) < 80mV, FB = VL, 6V < power input < 40V, includes line and load regulation (Note 4) 0mV < (CSH - CSL) < 80mV, FB = 0V, 4.5V < power input < 40V, includes line and load regulation (Note 4) External resistor divider CSH - CSL = 0V 0mV < (CSH - CSL) < 80mV 25mV < (CSH - CSL) < 80mV FB = VL, 6V < power input < 40V (Note 4) FB = 0V, 4.5V < power input < 40V (Note 4) CSH - CSL, positive CSH - CSL, negative 80 -50 2.5 2.0 SHDN = 2V, 0mA < IVL < 25mA, 5.5V < V+ < 30V Rising edge, hysteresis = 15mV Rising edge, hysteresis = 25mV 4.85 5.10 4.5 30 40 5.25 V CONDITIONS MIN TYP MAX UNITS
5V Output Voltage (CSL)
V
3.3V Output Voltage (CSL) Nominal Adjustable Output Voltage Range Feedback Voltage Load Regulation
3.20
3.35
3.46
V
REF 2.43 2.505 2.5 1.5 0.04 0.04 100 -100 4.0
6 2.57
V V %
Line Regulation
0.06 0.06 120 -160 6.5
%/V
Current-Limit Voltage SS Source Current SS Fault Sink Current
mV A mA
INTERNAL REGULATOR AND REFERENCE FLYBACK/PWM CONTROLLER VL Output Voltage VL Fault Lockout Voltage VL/CSL Switchover Voltage 4.7 3.8 4.2 5.3 4.1 4.7 V V V
2
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High-Voltage, Step-Down Controller with Synchronous Rectifier for CPU Power
ELECTRICAL CHARACTERISTICS (continued)
(V+ = 15V, GND = PGND = 0V, IVL = IREF = 0A, TA = 0C to +85C, SYNC = 0V, unless otherwise noted.) PARAMETER 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 Falling edge 0A < IREF < 100A SHDN = 0V, CSL = 6V, V+ = 0V or 30V, VL = 0V SHDN = 0V, V+ = 30V, CSL = 0V or 6V FB = CSH = CSL = 6V, VL switched over to CSL V+ = 4V, CSL = 0V (Note 2) CSH = CSL = 6V SYNC = REF SYNC = 0V or 5V 270 125 200 200 Guaranteed by design 190 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 = REF 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 89 93 VL - 0.5 2.0 0.8 0.5 2.0 1.0 50 100 nA A A A 91 96 200 340 0.1 1 1 4 4.8 300 150 CONDITIONS No external load (Note 1) MIN 2.45 1.8 TYP 2.505 MAX 2.55 2.3 50 1 5 5 8 6.6 330 175 UNITS V V mV A A A mW mW
MAX797H
OSCILLATOR AND INPUTS/OUTPUTS Oscillator Frequency SYNC High Pulse Width SYNC Low Pulse Width SYNC Rise/Fall Time Oscillator Sync Range Maximum Duty Factor kHz ns ns ns kHz %
Input High Voltage
V
Input Low Voltage
V
_______________________________________________________________________________________
3
High-Voltage, Step-Down Controller with Synchronous Rectifier for CPU Power MAX797H
ELECTRICAL CHARACTERISTICS (continued)
(V+ = 15V, GND = PGND = 0V, IVL = IREF = 0A, TA = -40 to +85C, SYNC = 0V, unless otherwise noted.) (Note 3) PARAMETER +3.3V and +5V STEP-DOWN CONTROLLERS Input Supply Range V+ High-side MOSFET drain 0mV < (CSH - CSL) < 80mV, FB = VL, 6V < power input < 40V, includes line and load regulation (Note 4) 0mV < (CSH - CSL) < 80mV, FB = 0V, 4.5V < power input < 40V, includes line and load regulation (Note 4) External resistor divider CSH - CSL = 0V FB = VL, 6V < power input < 40V (Note 4) FB = 0V, 4.5V < power input < 40V (Note 4) 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 250 120 250 250 210 SYNC = REF SYNC = 0V or 5V High or low High or low, BST - LX = 4.5V 89 93 91 96 7 7 320 1 1 4.8 300 150 70 -40 4.7 3.75 4.2 2.43 2.505 -100 4.70 5.10 5.0 30 40 5.40 V CONDITIONS MIN TYP MAX UNITS
5V Output Voltage (CSL)
V
3.3V Output Voltage (CSL) Nominal Adjustable Output Voltage Range Feedback Voltage Line Regulation Current-Limit Voltage
3.10
3.35
3.56
V
REF 2.40 0.04 0.04
6.0 2.60 0.06 0.06 130 -160 5.3 4.15 4.7 2.57 50 10 10 8.4 350 180
V V %/V %/V mV
INTERNAL REGULATOR AND REFERENCE FLYBACK/PWM CONTROLLER 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 Factor 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). Note 3: All -40C to +85C specifications are guaranteed by design. Note 4: The power input is the high-side MOSFET drain.
4
_______________________________________________________________________________________
High-Voltage, Step-Down Controller with Synchronous Rectifier for CPU Power
______________________________________________________________Pin Description
PIN 1 2 3 4 NAME SS SKIP REF GND FUNCTION Soft-Start Timing Capacitor Connection. Ramp time to full current limit is approximately 1ms/nF. Disables pulse-skipping mode when high. Connect SKIP to GND for normal use. Do not leave unconnected. With SKIP grounded, the device automatically changes from pulse-skipping operation to full PWM operation when the load current exceeds approximately 30% of maximum. Reference Voltage Output. Bypass REF to GND with 0.33F minimum. Low-noise Analog Ground and Feedback Reference Point Oscillator Synchronization and Frequency Select. Tie SYNC 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 Electrical Characteristics for VIH and VIL specifications). SYNC capture range is guaranteed to be 190kHz 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 = REF (approximately 2.505V) in adjustable mode. FB is a Dual ModeTM input that also selects the fixed-output voltage settings as follows: * Connect to GND for 3.3V operation. * Connect to VL for 5V operation. * Connect to a resistor divider for adjustable mode. FB can be driven with 5V Rail-to-Rail(R) logic to change the output voltage under system control. 8 9 10 CSH CSL V+ Current-Sense Input, High Side. Current-limit level is 100mV referred to CSL. Current-Sense Input, Low Side. CSL also serves as the feedback input in fixed-output modes. 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. It 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. DL normally drives the synchronous-rectifier MOSFET. Swings 0V to VL. Boost Capacitor Connection for High-Side Gate Drive (0.1F) Switching Node (inductor) Connection. LX can swing 2V below ground without hazard. High-Side Gate-Drive Output. DH normally drives the main buck switch. It is a floating driver output that swings from LX to BST, riding on the LX switching-node voltage.
MAX797H
5
SYNC
6
SHDN
7
FB
11 12 13 14 15 16
VL PGND DL BST LX DH
Dual Mode is a trademark of Maxim Integrated Products. Rail-to-Rail is a registered trademark of Nippon Motorola Ltd.
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5
High-Voltage, Step-Down Controller with Synchronous Rectifier for CPU Power MAX797H
_______________Detailed Description
The MAX797H is functionally identical to the MAX797. The only difference between the two devices is in the BST pin's absolute maximum rating. The MAX797H's rating is 46V; the MAX797's rating is 36V. The higher rating allows the MAX797H to use a power input up to 40V, provided that the V+ pin is powered by a separate supply between 4.5V and 30V. Circuit design and component selection for the MAX797H are identical to those for the MAX797; therefore, such information is not included in this data sheet. Refer to the MAX796/MAX797/MAX799 data sheet for design formulas and applications information. The Applications Information section in this data sheet contains suggestions for providing the 30V maximum V+ supply input for the MAX797H when power input exceeds 30V. For most applications, a better choice than Figure 2's circuit takes advantage of the MAX797H's internal linear regulator. There is no need to provide a regulated supply to V+, provided it is within the +4.5V to +30V V+ input voltage range. In Figure 3, Q1 is used to drop a 40V (max) input to 30V by dividing it by approximately 4/3. This approach results in a somewhat higher minimum input voltage than that of Figure 2's circuit, but a much lower quiescent current than that of a linear regulator. If quiescent current must be minimized, an N-channel MOSFET can be substituted for Q1, and the divider-resistor values can be increased. Powering V+ with a zener diode can be done in many different ways. The simplest is to use a standard shunt regulator to provide a regulated voltage in the 4.5V to 30V range (Figure 4). Resistor R1 must be chosen to allow the maximum required V+ current to be obtained from the minimum power input voltage. If the power input voltage varies appreciably, the result is higherthan-necessary input current from the highest power input voltage. An approach that reduces quiescent current is to use a zener diode as a dropping diode to keep V+ under 30V (Figure 5). This results in a severely restricted minimum range for the power input voltage, which is not a problem for most high-voltage applications. RL must be added to draw current and to ensure that there is sufficient forward drop across the zener diode if the MAX797H can be shut down or bootstrap off its output voltage.
__________Applications Information
Powering the V+ Pin
V+ can be supplied directly if a system supply between 4.5V and 30V is available (see the Typical Operating Circuit). Most of the MAX797H's internal blocks are supplied by VL, which uses V+ as its input. While the current into V+ is minimal, it depends heavily on the type of external MOSFET used and the switching frequency: IGATE = Qg x fSW where Q g is the sum of the high- and low-side MOSFET's total gate charges, and fSW is the switching frequency. Furthermore, if the circuit output voltage on CSL exceeds the VL/CSL switchover voltage, the MAX797H bootstraps itself (it connects VL to CSL and turns off the linear regulator, supplying the IC from the circuit output), and V+ current is reduced to about 1A. If a 5V regulated supply is available, V+ and VL can be connected and fed from that supply (Figure 1). In this mode, the VL regulator is bypassed. Do not use this approach if the output voltage on CSL can exceed the VL/CSL switchover voltage. If a 5V regulated supply is not available, a linear regulator with a sufficient input voltage range can provide it (Figure 2). This approach allows for a very wide input voltage range, which is useful if the circuit must run from several different power sources. The drawback of the linear regulator is the high quiescent current that these devices typically require, in addition to the current used by the feedback divider resistors (R1 and R2).
Duty-Factor Limitations for Low VOUT/VIN Ratios
The MAX797H's output voltage is adjustable down to 2.5V (min). However, the combination of high input and low output voltages may not be possible at high switching frequencies without introducing some amount of frequency instability. The minimum duty factor is determined by delays through the error comparator, internal logic, gate drivers, and external MOSFETs. The delay is typically 425ns. With a switching period of 3.33s (300kHz), the minimum duty factor is 0.425s / 3.33s = 0.13. If VOUT / VIN is less than this value, the IC will properly regulate the output voltage, but may extend the period and switch at 150kHz instead of 300kHz. It may also alternate between these two frequencies. For example, if VIN is 40V, the lowest VOUT that does not require less than the minimum duty factor is 40V x 0.13 = 5.2V. Below this output voltage, select the 150kHz switching frequency (connect SYNC to VL or GND).
6
_______________________________________________________________________________________
High-Voltage, Step-Down Controller with Synchronous Rectifier for CPU Power MAX797H
+5V REGULATED SUPPLY
+4V TO +40V POWER INPUT
V+ SHDN
VL
MAX797H
DH BST +3.3V OUTPUT
SS REF
LX DL PGND
SYNC GND SKIP FB
CSH CSL
Figure 1. Powering V+ and VL from a Regulated +5V supply
Similarly, at 150kHz, the minimum duty factor is 0.425s / 6.67s = 0.064. This means that duty factor is not an issue except at the maximum input voltage and minimum output voltage. For example, if V IN is 40V, the lowest VOUT that does not require less than the
minimum duty factor is 40V x 0.064 = 2.56V. If VOUT / VIN is less than this value, the IC will properly regulate the output voltage, but may extend the period and switch at 75kHz instead of 150kHz. It may also alternate between these two frequencies.
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7
High-Voltage, Step-Down Controller with Synchronous Rectifier for CPU Power MAX797H
OUT ADJ R1 +7V TO +40V INPUT Q1 300k R2 IN 100k +8V TO +40V POWER INPUT
V+ SHDN
VL
V+ SHDN
VL
MAX797H
MAX797H
DH
DH
Figure 2. Powering V+ and VL with a +5V Linear Regulator
Figure 3. Dividing the Power Input to Supply V+
UP TO +40V POWER INPUT 12V R1 R1
+20V TO +40V POWER INPUT
V+ SHDN
VL
V+ SHDN
VL
MAX797H
MAX797H
DH
DH
Figure 4. Powering V+ with a Zener Shunt Regulator
Figure 5. Powering V+ with a Zener Dropping Diode
___________________Chip Information
TRANSISTOR COUNT: 913
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.
8 ___________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 (408) 737-7600 (c) 1997 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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