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General Description The MAX796 MAX797 MAX799 high performance step down DC DC converters with single or dual outputs provide main CPU power in battery powered systems These buck controllers achieve efficiency by using synchronous rectification and Maxim's proprietary Idle Mode™ 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 MAX796 MAX799 are specially equipped with a sec ondary feedback input SECFB for transformer based dual output applications This secondary feedback path improves cross regulation of positive MAX796 or nega tive MAX799 auxiliary outputs The MAX797 has a logic controlled and synchronizable fixed frequency pulse width modulating PWM operating mode which reduces noise and RF interference in sensi tive 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 MAX796 MAX797 MAX799 are all available in pin DIP and narrow SO packages See the table below to compare these three converters

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Description

Niveau: Supérieur, Doctorat, Bac+8
_______________General Description The MAX796/MAX797/MAX799 high-performance, step- down DC-DC converters with single or dual outputs provide main CPU power in battery-powered systems. These buck controllers achieve 96% efficiency by using synchronous rectification and Maxim's proprietary Idle Mode™ 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 MAX796/MAX799 are specially equipped with a sec- ondary feedback input (SECFB) for transformer-based dual-output applications. This secondary feedback path improves cross-regulation of positive (MAX796) or nega- tive (MAX799) auxiliary outputs. The MAX797 has a logic-controlled and synchronizable fixed-frequency pulse-width-modulating (PWM) operating mode, which reduces noise and RF interference in sensi- tive 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.

  • low

  • secfb regulation

  • output

  • feedback voltage

  • current-limit voltage

  • down dc-dc

  • sec- ondary feedback

  • line-regulation error

  • current


Subjects

Informations

Published by
Reads 19
Language English

19-0221; Rev 3a; 11/97

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_______________General Description
The MAX796/MAX797/MAX799 high-performance, step-
down DC-DC converters with single or dual outputs
provide main CPU power in battery-powered systems.
These buck controllers achieve 96% efficiency by using
synchronous rectification and Maxim’s proprietary Idle
Mode™ 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 MAX796/MAX799are specially equipped with a sec-
ondary feedback input (SECFB) for transformer-based
dual-output applications. This secondary feedback path
improves cross-regulation of positive (MAX796) or nega-
tive (MAX799) auxiliary outputs.
The MAX797 has a logic-controlled and synchronizable
fixed-frequency pulse-width-modulating (PWM) operating
mode, which reduces noise and RF interference in sensi-
tive mobile-communications and pen-entry applications.
The SKIPoverride 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 MAX796/MAX797/MAX799 are all available in 16-
pin DIP and narrow SO packages. See the table below
to compare these three converters.
PARTMAIN OUTPUTSPECIAL FEATURE
MAX7963.3V/5V or adj.Regulates positive secondary
voltage (such as +12V)
MAX7973.3V/5V or adj.Logic-controlled low-noise mode
MAX7993.3V/5V or adj.Regulates negative secondary
voltage (such as -5V)
________________________Applications
Notebook and Subnotebook Computers
PDAs and Mobile Communicators
Cellular Phones

Idle Mode is a trademark of Maxim Integrated Products.

U.S. and foreign patents pending.

____________________________Features
©
96% Efficiency
©
4.5V to 30V Input Range
©
2.5V to 6V Adjustable Output
©
Preset 3.3V and 5V Outputs (at up to 10A)
©
Multiple Regulated Outputs
©
+5V Linear-Regulator Output
©
Precision 2.505V Reference Output
©
Automatic Bootstrap Circuit
©
150kHz/300kHz Fixed-Frequency PWM Operation
©
Programmable Soft-Start
©
375µA Typ Quiescent Current (V
IN
= 12V, V
OUT
= 5V)
©
1µA Typ Shutdown Current

______________Ordering Information
PART

TEMP. RANGEPIN-PACKAGE
MAX796
CPE0°C to +70°C16 Plastic DIP
MAX796CSE0°C to +70°C16 Narrow SO
MAX796C/D0°C to +70°CDice*
MAX796EPE-40°C to +85°C16 Plastic DIP
MAX796ESE-40°C to +85°C16 Narrow SO
MAX796MJE-55°C to +125°C16 CERDIP
Ordering Information continued at end of data sheet.
*Contact factory for dice specifications.

__________________Pin Configuration
TOP VIEW

SS116DH
(SECFB) SKIP215LX
REF314BST
GND4
MAX796
13DL
SYNC5
MMAAXX779997
12PGND
SHDN611VL
FB710V+
CSH89CSL

DIP/SO
( ) ARE FOR MAX796/ MAX799.

________________________________________________________________Maxim Integrated Products1
For free samples & the latest literature: http:
/
www.maxim-ic.com, or phone 1-800-998-8800.
For sma
l
orders, phone 408-737-7600 ext. 3468.

Step-Down Controllers with
Synchronous Rectifier for CPU Power
ABSOLUTE MAXIMUM RATINGS
V+ to GND.................................................................-0.3V, +36VVL Output Current...............................................................50mA
GND to PGND........................................................................±2VContinuous Power Dissipation (T= +70°C)
AVL to GND...................................................................-0.3V, +7VSO (derate 8.70mW/°C above +70°C)........................696mW
BST to GND...............................................................-0.3V, +36VPlastic DIP (derate 10.53mW/°C above +70°C).........842mW
DH to LX...........................................................-0.3V, BST + 0.3VCERDIP (derate 10.00mW/°C above +70°C)..............800mW
LX to BST.....................................................................-7V, +0.3VOperating Temperature Ranges
SHDNto GND............................................................-0.3V, +36VMAX79_C_ _......................................................0°C to +70°C
SYNC, SS, REF, FB, SECFB, SKIP, DL to GND..-0.3V, VL + 0.3VMAX79_E_ _....................................................-40°C to +85°C
CSH, CSL to GND.......................................................-0.3V, +7VMAX79_MJE.................................................-55°C to +125°C
VL Short Circuit to GND..............................................MomentaryStorage Temperature Range.............................-65°C to +160°C
REF Short Circuit to GND...........................................ContinuousLead Temperature (soldering, 10sec).............................+300°C
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, I= I= 0A, T= 0°C to +70°C for MAX79_C, T= 0°C to +85°C for MAX79_E,
VLREF
AA
T
A
= -55°C to +125°C for MAX79_M, unless otherwise noted.)
PARAMETERCONDITIONSMINTYPMAXUNITS
+3.3V AND +5V STEP-DOWN CONTROLLERS
MAX79_C4.530
Input Supply RangeV
MAX79_E/M5.030
0mV < (CSH-CSL) < 80mV, FB = VL, 6V < V+ < 30V,
5V Output Voltage (CSL)4.855.105.25V
includes line and load regulation
0mV < (CSH-CSL) < 80mV, FB = 0V, 4.5V < V+ < 30V,
3.3V Output Voltage (CSL)3.203.353.46V
includes line and load regulation
Nominal Adjustable Output
External resistor dividerREF6V
Voltage Range
Feedback Voltage(CSH-CSL) = 0V2.432.5052.57V
0mV < (CSH-CSL) < 80mV2.5
Load Regulation%
25mV < (CSH-CSL) < 80mV1.5
Line Regulation6V < V+ < 30V0.040.06%/V
CSH-CSL, positive80100120
Current-Limit VoltagemV
CSH-CSL, negative-50-100-160
SS Source Current2.54.06.5µA
SS Fault Sink Current2.0mA
FLYBACK/PWM CONTROLLER
Falling edge, hysteresis = 15mV (MAX796)2.452.5052.55
SECFB Regulation SetpointV
Falling edge, hysteresis = 20mV (MAX799)-0.0500.05
INTERNAL REGULATOR AND REFERENCE
VL Output VoltageSHDN= 2V, 0mA < I
VL
< 25mA, 5.5V < V+ < 30V4.75.3V
VL Fault Lockout VoltageRising edge, hysteresis = 15mV3.84.1V
VL/CSL Switchover VoltageRising edge, hysteresis = 25mV4.24.7V

2_______________________________________________________________________________________

Step-Down Controllers with
Synchronous Rectifier for CPU Power
ELECTRICAL CHARACTERISTICS (continued)
(V+ = 15V, GND = PGND = 0V, IVL= IREF= 0A, T
A
= 0°C to +70°C for MAX79_C, T
A
= 0°C to +85°C for MAX79_E,
T
A
= -55°C to +125°C for MAX79_M, unless otherwise noted.)
PARAMETERCONDITIONSMINTYPMAXUNITS
Reference Output VoltageNo external load (Note 1)MAX79_C2.462.5052.54V
MAX79_E/M2.452.55
Reference Fault Lockout VoltageFalling edge1.82.3V
Reference Load Regulation0µA < I
REF
< 100µA50mV
CSL Shutdown Leakage CurrentSHDN= 0V, CSL = 6V, V+ = 0V or 30V, VL = 0V0.11µA
V+ Shutdown CurrentSHDN= 0V, V+ = 30V, MAX79_C13µA
CSL = 0V or 6VMAX79_E/M15
FB = CSH = CSL = 6V, MAX79_C13
V+ Off-State Leakage CurrentVL switched over to CSLMAX79_E/M15µA
Dropout Power ConsumptionV+ = 4V, CSL = 0V (Note 2)48mW
Quiescent Power ConsumptionCSH = CSL = 6V4.86.6mW
OSCILLATOR AND INPUTS/OUTPUTS
SYNC = REF270300330
Oscillator FrequencySYNC = 0V or 5V125150175kHz
SYNC High Pulse Width200ns
SYNC Low Pulse Width200ns
SYNC Rise/Fall TimeGuaranteed by design200ns
Oscillator Sync Range190340kHz
SYNC = REF8991
Maximum Duty CycleSYNC = 0V or 5V9396%
Input High VoltageSYNCVL - 0.5V
SHDN, SKIP2.0
SYNC0.8
Input Low VoltageSHDN, SKIP0.5V
SHDN, 0V or 30V2.0
SECFB, 0V or 4V0.1
Input CurrentSYNC, SKIP1.0µA
CSH, CSL, CSH = CSL = 6V, device not shut down50
FB, FB = REF±100nA
DL Sink/Source CurrentDL forced to 2V1A
DH Sink/Source CurrentDH forced to 2V, BST-LX = 4.5V1A
DL On-ResistanceHigh or low7
½
DH On-ResistanceHigh or low, BST-LX = 4.5V7
½
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 may 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). See the Quiescent Supply Current
vs. Supply Voltage graph in the Typical Operating Characteristics.
_______________________________________________________________________________________3

Step-Down Controllers with
Synchronous Rectifier for CPU Power
ELECTRICAL CHARACTERISTICS (continued)
(V+ = 15V, GND = PGND = 0V, IVL= IREF= 0A, T
A
= -40°C to +85°C for MAX79_E, unless otherwise noted.) (Note 3)
PARAMETERCONDITIONSMINTYP
+3.3V and +5V STEP-DOWN CONTROLLERS
Input Supply Range5.0
5V Output Voltage (CSL)i0nmclVu d<e (s CliSnHe -a nCdS lLo) a<d 8re0gmuVla, tiFoBn = VL, 6V < V+ < 30V,4.705.10
3.3V Output Voltage (CSL)0mV < (CSH - CSL) < 80mV, FB = VL, 4.5V < V+ < 30V,3.103.35
includes line and load regulation
Nominal Adjustable OutputExternal resistor dividerREF
Voltage Range
Feedback Voltage(CSH-CSL) = 0V2.40
Line Regulation6V < V+ < 30V0.04
CSH - CSL, positive70
Current-Limit VoltageCSH - CSL, negative-40-100
FLYBACK/PWM CONTROLLER
Falling edge, hysteresis = 15mV (MAX796)2.40
SECFB Regulation SetpointFalling edge, hysteresis = 20mV (MAX799)-0.08
INTERNAL REGULATOR AND REFERENCE
VL Output Voltage
SHDN
= 2V, 0mA < I
VL
< 25mA, 5.5V < V+ < 30V4.7
VL Fault Lockout VoltageRising edge, hysteresis = 15mV3.75
VL/CSL Switchover VoltageRising edge, hysteresis = 25mV4.2
Reference Output VoltageNo external load (Note 1)2.432.505
Reference Load Regulation0µA < I
REF
< 100µA
V+ Shutdown Current
SHDN
= 0V, V+ = 30V, CSL = 0V or 6V1
V+ Off-State Leakage CurrentFB = CSH = CSL = 6V, VL switched over to CSL1
Quiescent Power Consumption4.8
OSCILLATOR AND INPUTS/OUTPUTS
SYNC = REF250300
Oscillator FrequencySYNC = 0V or 5V120150
SYNC High Pulse Width250
SYNC Low Pulse Width250
Oscillator Sync Range210
SYNC = REF8991
Maximum Duty CycleSYNC = 0V or 5V9396
DL On-ResistanceHigh or low
DH On-ResistanceHigh or low, BST - LX = 4.5V
Note 3:
All -40°C to +85°C specifications above are guaranteed by design.

4_______________________________________________________________________________________

XAM0304.565.30.606.2060.031061-06.280.03.550.47.475.20501014.805308102377

STINUVVVVVV/%VmVVVVVVmAµAµWmzHksnnszHk%½½

Step-Down Controllers with
Synchronous Rectifier for CPU Power
__________________________________________________Typical Operating Circuits
TUPNI4.5V TO 30V

TUPNI 6V TO 30V

LV+VNDHSMAX797
HDTSBXLSSLDFERDNGPSYNCCSH
GNDCSL
SKIPFB

V+SECFB
SHDNFB
LVMAX796
HDTSBXLLDSSPGND
REFCSH
GNDCSL
CNYS

OU+T3P.U3TV

OUT+P12UVT
V5+OUTPUT

_______________________________________________________________________________________5

Step-Down Controllers with
Synchronous Rectifier for CPU Power
_____________________________________Typical Operating Circuits (continued)

INPUT 6V TO 30V
V+SECFB
SHDNFB
LVMAX799
DH
TSBXLLDSSPGND
REFCSH
GNDCSL
CNYS

FRROEMF

V5–OUTPUT

V5+OUTPUT

__________________________________________Typical Operating Characteristics
(T= +25°C, unless otherwise noted.)
A

EFFICIENCY vs.EFFICIENCY vs.EFFICIENCY vs.
LOAD CURRENT, 5V/3A CIRCUITLOAD CURRENT, 3.3V/3A CIRCUITLOAD CURRENT, 3.3V/10A CIRCUIT
100100100
V
IN
= 6VV
IN
= 5VSKIP = LOW
909090
V
IN
= 30VV
IN
= 12V
80
8080
V
IN
= 30VSKIP = HIGH
0707070660
STANDARD MAX797 5V/3A
60
STANDARD MAX797 3.3V/3A CSITRACNUDITA,R FDI GMUARXE 7197 3.3V/10A
CIRCUIT, FIGURE 1CIRCUIT, FIGURE 1
50
f = 300kHz
f = 300kHzf = 300kHzV
IN
= 5V
505040
0.0010.010.11100.0010.010.11100.1110
LOAD CURRENT (A)LOAD CURRENT (A)LOAD CURRENT (A)
6_______________________________________________________________________________________

Step-Down Controllers with
Synchronous Rectifier for CPU Power
____________________________Typical Operating Characteristics (continued)
(T
A
= +25°C, unless otherwise noted.)
QUIESCENT SUPPLY CURRENTQUIESCENT SUPPLY CURRENT
vs. SUPPLY VOLTAGE,vs. SUPPLY VOLTAGE,QUIESCENT SUPPLY CURRENT vs.
5V/3A CIRCUIT IN IDLE MODE3.3V/3A CIRCUIT IN IDLE MODESUPPLY VOLTAGE, LOW-NOISE MODE
16m140030
15m1200
m41STANDARD MAX797 APPLICATION 1000SWITCHING20
SCKOINP F=I GLUORWE D FOR 5V800f = 300kHz
800
m
SYNC = REF
600NOT SWITCHINGf = 150kHz
600
m
(FB FORCED TO 3.5V)10
004400
m
CSITRACNUDITA,R FDI GMUARXE 719 7 3.3V/3A STANDARD MAX797 3.3V/3A
200
m
200SKIP = LOW CSIKRIPC =U IHT,I GFIHGURE 1
00SYNC = REF0
048121620242832048121620242832048121620242832
SUPPLY VOLTAGE (V)SUPPLY VOLTAGE (V)SUPPLY VOLTAGE (V)
SHUTDOWN SUPPLY CURRENTDROPOUT VOLTAGE vs.REF LOAD-REGULATION ERROR
vs. SUPPLY VOLTAGELOAD CURRENTvs. LOAD CURRENT
1.6
80020
1.4
700
1.2
600
f = 300kHz
15
1.0
500
0.8
40010
0.6SDHEVDINC E= LCOURWRENT ONLY
300
f = 150kHz
0.4
2005
0.2
100
CSTOANNFIDGAURRDE DM AFXO7R 957 VA PPLICATION
0
0
V
OUT
> 4.8V
0
048121620242832
0.010.11101101001000
SUPPLY VOLTAGE (V)LOAD CURRENT (A)REF LOAD CURRENT (
m
A)
MAX796
VL LOAD-REGULATION ERRORSWITCHING FREQUENCY vs.MAXIMUM SECONDARY CURRENT
vs. LOAD CURRENTLOAD CURRENTvs. SUPPLY VOLTAGE, 5V/15V CIRCUIT
5001000450
SYNC = REF (300kHz)400I
OUT
(MAIN) = 0A
400SKIP = LOW
053100+5V, V
IN
= 7.5V
300300I
OUT
(MAIN) = 3A
05210+5V, V
IN
= 30V200
200150
1+3.3V, V
IN
= 7.5V
100100TCRIRACNUSIFTO ORFM FEIRG U= RTET I151870
50V
SEC
> 12.75V
1.000020406080100
m
1m10m100m1048121620242832
VL LOAD CURRENT (mA)
LOAD CURRENT (A)
SUPPLY VOLTAGE (V)
_______________________________________________________________________________________7

Step-Down Controllers with
Synchronous Rectifier for CPU Power
____________________________Typical Operating Characteristics (continued)
(T
A
= +25°C, unless otherwise noted.)
MAX796
MAXIMUM SECONDARY CURRENT vs.MAX799
SUPPLY VOLTAGE, 3.3V/5V CIRCUITvsM. ASXUIPMPULMY VSOELCTOANGDEA, R±Y5 CV UCRIRRECNUTIT
1050I
OUT
(MAIN) = 2A800I
OUT
(MAIN) = 0A
900700
750600
600I
OUT
(MAIN) = 0A500
004450300I
OUT
(MAIN) = 1A
300200CIRCUIT OF FIGURE 13
150TCRIRACNUSIFTO ORFM FEIRG U= RTED 1K 21.5:1VT
S
R
E
A
C
N
£
S F-O5.R1MVER = TTI5926
V
SEC

³
4.8V100
0003691215182124048121620242832
SUPPLY VOLTAGE (V)SUPPLY VOLTAGE (V)
PULSE-WIDTH-MODULATION MODE WAVEFORMSIDLE-MODE WAVEFORMS

I
LOAD
= 1A, V
IN
= 16V,
CIRCUIT OF FIGURE 1

500ns/div

LX VOLTAGE
10V/div

+5V OUTPUT
VOLTAGE
50mV/div

200
m
s/div
I
LOAD
= 100mA, V
IN
= 10V,
CIRCUIT OF FIGURE 1
+5V LOAD-TRANSIENT RESPONSE
A3LOAD CURRENT
A0

+5V OUTPUT
50mV/div

200
m
s/div
V = 15V, CIRCUIT OF FIGURE 1
NI8_______________________________________________________________________________________

+5V OUTPUT
50mV/div

2V/div

SynchronoSutse pR-eDcotiwfine rC foonrt rCoPllUe rPs owwietrh

______________________________________________________________Pin Description
PINNAMEFUNCTION
1SSSoft-Start timing capacitor connection. Ramp time to full current limit is approximately 1ms/nF.
Secondary winding Feedback input. Normally connected to a resistor divider from an auxiliary output.
SECFB
Don’t leave SECFB unconnected.
(MAX796/
·
MAX796: SECFB regulates at VSECFB = 2.505V. Tie to VL if not used.
MAX799)
·
MAX799: SECFB regulates at VSECFB = 0V. Tie to a negative voltage through a high-value current-limit-
2ing resistor (I
MAX
= 100µA) if not used.
Disables pulse-skipping mode when high. Connect to GND for normal use.
Don’t leave SKIPunconnected.
PIKS(MAX797)With SKIPgrounded, the device will automaticallychange from pulse-skipping operation to full PWM opera-
tion when the load current exceeds approximately 30% of maximum. (See Table 3.)
3REFReference voltage output. Bypass to GND with 0.33µF minimum.
4GNDLow-noise analog Ground and feedback reference point.
Oscillator Synchronization and frequency select. Tie to GND or VL for 150kHz operation; tie to REF for
5SYNC300kHz operation. A high-to-low transition begins a new cycle. Drive SYNC with 0V to 5V logic levels (see the
Electrical Characteristicstable for V
IH
and V
IL
specifications). SYNC capture range is 190kHz to 340kHz
guaranteed.
6SHDNShutdown control input, active low. Logic threshold is set at approximately 1V (V
TH
of an internal N-channel
MOSFET). Tie SHDNto V+ for automatic start-up.
Feedback input. Regulates at FB = REF (approximately 2.505V) in adjustable mode. FB is a Dual-Mode
TM
input that also selects the fixed output voltage settings as follows:
7FB
·
Connect to GND for 3.3V operation.
·
Connect to VL for 5V operation.
·
Connect FB to a resistor divider for adjustable mode. FB can be driven with +5V rail-to-rail logic in order to
change the output voltage under system control.
8CSHCurrent-Sense input, High side. Current-limit level is 100mV referred to CSL.
9CSLCurrent-Sense input, Low side. Also serves as the feedback input in fixed-output modes.
10V+Battery voltage input (4.5V to 30V). Bypass V+ to PGND close to the IC with a 0.1µF 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
11VLvoltage via CSL (V
CSL
> 4.5V) for automatic bootstrapping. Bypass to GND with 4.7µF. VL can
supply up to 5mA for external loads.
12PGNDPower Ground.
13DLLow-side gate-drive output. Normally drives the synchronous-rectifier MOSFET. Swings 0V to VL.
14BSTBoost capacitor connection for high-side gate drive (0.1µF).
15LXSwitching node (inductor) connection. Can swing 2V below ground without hazard.
16DHHigh-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.

Dual Mode is a trademark of Maxim Integrated Products.

_______________________________________________________________________________________9

Step-Down Controllers with
Synchronous Rectifier for CPU Power
______Standard Application Circuit
If the 3A or 5A circuit must be guaranteed to withstand
It is easy to adapt the basic MAX797 single-output 3.3Va continuous output short circuit indefinitely, see the
buck converter (Figure 1) to meet a wide range ofsection MOSFET Switchesunder Selecting Other
applications with inputs up to 28V (limited by choice ofComponents. Don’t change the frequency of these cir-
cuits without first recalculating component values (par-
external MOSFET). Simply substitute the appropriateticularly inductance value at maximum battery voltage).
components from Table 1. These circuits represent a
good set of tradeoffs between cost, size, and efficiency
_______________Detailed Description
while staying within the worst-case specification limitsThe MAX796 is a BiCMOS, switch-mode power-supply
for stress-related parameters such as capacitor ripple
current. Each of these circuits is rated for a continuouscontroller designed primarily for buck-topology regula-
load current at T
A
= +85°C, as shown. The 1A, 2A andtors in battery-powered applications where high effi-
10A applications can withstand a continuous outputciency and low quiescent supply current are critical.
The MAX796 also works well in other topologies such
short-circuit to ground. The 3A and 5A applications canas boost, inverting, and CLK due to the flexibility of its
withstand a short circuit of many seconds duration, butfloating high-speed gate driver. Light-load efficiency is
the synchronous-rectifier MOSFET overheats, exceed-enhanced by automatic idle-mode operation—a vari-
ing the manufacturer’s ratings for junction temperatureable-frequency pulse-skipping mode that reduces
by 50°C or more.

TUPNIC10C.71
m
F
1011D2
4CV+VLCMPSH-34.7
m
F
ON/OFF6SHDNDH16Q1
CONTROLBST14
3CmF1.0LOW-NOISE2SKIP
MAX797
LX15L1
CONTROL
DL13Q2D1
PGND12
1SSCSH8
96C0.01
m
FCSL
(OPTIONAL)GND4
FBSYNCREF3C5
750.33
m
F
NOTE:
KEEP CURRENT-SENSEJ1
TLIONGEEST SHEHRO. RSTE AE NFIDG .C L10OSE1J5U0MkPHEz/R300kHz

1R

Figure 1. Standard 3.3V Application Circuit
10______________________________________________________________________________________

5+5mVA AT

O+3U.T3PVUT
2CGONUDT

R+E2.F5 0O5UVT APTU T100
m
A