MAX797HESE+ ,High-Voltage, Step-Down Controller with Synchronous Rectifier for CPU PowerApplicationsPOWER INPUTSUPPLYINPUTNotebook and Subnotebook ComputersIndustrial ControlsV+ VLSHDN___ ..
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MAX797HESE+
High-Voltage, Step-Down Controller with Synchronous Rectifier for CPU Power
_______________General DescriptionThe MAX797H high-performance, step-down DC-DCconverter provides main CPU power in battery-poweredsystems. A 40V rating on the power stage’s input allowsoperation with high-cell-count batteries and a widerange of AC adaptors. This buck controller achieves96% efficiency by using synchronous rectification andMaxim’s proprietary IdleMode™ control scheme toextend battery life at full-load (up to 10A) and no-loadoutputs. Excellent dynamic response corrects output tran-sients caused by the latest dynamic-clock CPUs within five300kHz clock cycles. Unique bootstrap circuitry drivesinexpensive N-channel MOSFETs, reducing system costand eliminating the crowbar switching currents found insome PMOS/NMOS switch designs.
The MAX797H has a logic-controlled and synchronizablefixed-frequency, pulse-width-modulating (PWM) operatingmode, which reduces noise and RF interference in sensi-tive mobile-communications and pen-entry applications.The SKIPoverride input allows automatic switchover toidle-mode operation (for high-efficiency pulse skipping) atlight loads, or forces fixed-frequency mode for lowest noiseat all loads. The MAX797H is pin compatible with the popu-lar MAX797, but has a higher input voltage range.
The MAX797H comes in a 16-pin narrow SO package.
________________________ApplicationsNotebook and Subnotebook Computers
Industrial Controls
____________________________Features96% EfficiencyUp to 40V Power Input2.5V to 6V Adjustable OutputPreset 3.3V and 5V Outputs (at up to 10A)5V Linear-Regulator OutputPrecision 2.505V Reference OutputAutomatic Bootstrap Circuit150kHz/300kHz Fixed-Frequency PWM OperationProgrammable Soft-Start375μA Quiescent Current (VIN= 12V, VOUT= 5V)1μA Shutdown Currentigh-Voltage, Step-Down Controller with
Synchronous Rectifier for CPU PowerIdle Mode is a trademark of Maxim Integrated Products.
19-1239; Rev 0; 7/97
__________________Pin ConfigurationBSTGND
REF
SKIP
TOP VIEW
MAX797H
PGND
CSLCSH
SHDN
SYNC
__________Typical Operating CircuitMAX797H
SHDN
+3.3V
OUTPUT
+4V TO +40V
POWER INPUT
BST
PGND
CSH
CSL
REF
SYNC
GND
SKIPFB
+4.5V TO +30V
SUPPLY
INPUT
PIN-PACKAGETEMP. RANGEPART†
16 Narrow SO-40°C to +85°CMAX797HESE
______________Ordering Informationigh-Voltage, Step-Down Controller with Synchronous Rectifier for CPU Power
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS(V+ = 15V, GND = PGND = 0V, IVL= IREF= 0A, TA
= 0°C to +85°C, SYNC = 0V, unless otherwise noted.)Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functionaloperation 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.
V+ to GND................................................................-0.3V to 36VGND to PGND........................................................................±2V
VL to GND...................................................................-0.3V to 7VBST to GND..............................................................-0.3V to 46V
DH to LX.....................................................-0.3V to (BST + 0.3V)LX to BST....................................................................-7V to 0.3V
SHDNto GND...........................................................-0.3V to 36VSYNC, 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..............................................MomentaryREF Short Circuit to GND...........................................Continuous
VL Output Current...............................................................50mAContinuous Power Dissipation (TA= +70°C)
SO (derate 8.70mW/°C above +70°C)........................696mWOperating Temperature Range
MAX797HESE.................................................-40°C to +85°CStorage Temperature Range.............................-65°C to +160°C
Lead Temperature (soldering, 10sec).............................+300°C
Rising edge, hysteresis = 25mV
Rising edge, hysteresis = 15mV
SHDN= 2V, 0mA < IVL< 25mA, 5.5V < V+ < 30V
CSH - CSL, negative
CSH - CSL, positive
FB = VL, 6V < power input < 40V (Note 4)
25mV < (CSH - CSL) < 80mV
0mV < (CSH - CSL) < 80mV, FB = VL,
6V < power input < 40V, includes line and load
regulation (Note 4)
External resistor divider
CSH - CSL = 0V
0mV < (CSH - CSL) < 80mV
CONDITIONS4.24.7VL/CSL Switchover Voltage3.84.1VL Fault Lockout Voltage4.75.3VL Output Voltage2.0SS Fault Sink Current2.54.06.5SS Source CurrentV4.530Input Supply Range
-50-100-160mV80100120Current-Limit Voltage
1.54.855.105.255V Output Voltage (CSL)REF6Nominal Adjustable Output
Voltage Range2.432.5052.57Feedback Voltage2.5Load Regulation
UNITSMINTYPMAXPARAMETERHigh-side MOSFET drain
0mV < (CSH - CSL) < 80mV, FB = 0V,
4.5V < power input < 40V, includes line and load
regulation (Note 4)3.203.353.463.3V Output Voltage (CSL)
FB = 0V, 4.5V < power input < 40V (Note 4)%/V0.040.06Line Regulation
+3.3V AND +5V STEP-DOWN CONTROLLERS
FLYBACK/PWM CONTROLLERINTERNAL REGULATOR AND REFERENCE0.040.06
igh-Voltage, Step-Down Controller witSynchronous Rectifier for CPU Power
ELECTRICAL CHARACTERISTICS (continued)(V+ = 15V, GND = PGND = 0V, IVL= IREF= 0A, TA
= 0°C to +85°C, SYNC = 0V, unless otherwise noted.)SHDN, 0V or 30V
SHDN, SKIP
SYNC
SYNC = 0V or 5V
No external load (Note 1)
SYNC = REF
Guaranteed by design
CSH = CSL = 6V
V+ = 4V, CSL = 0V (Note 2)
SYNC = 0V or 5V
SHDN= 0V, V+ = 30V, CSL = 0V or 6V
Falling edge
0µA < IREF< 100µA
SYNC = REF
SHDN= 0V, CSL = 6V, V+ = 0V or 30V, VL = 0V
CONDITIONS2.0VVL - 0.5Input High Voltage939691Maximum Duty Factor
kHz190340Oscillator Sync Range200SYNC Rise/Fall Time200SYNC Low Pulse Width200SYNC High Pulse Width
125150175kHz270300330Oscillator Frequency
2.452.5052.55VReference Output Voltage4.86.6Quiescent Power Consumption48Dropout Power Consumption5µAV+ Shutdown Current1.82.3Reference Fault Lockout Voltage50Reference Load Regulation0.11CSL Shutdown Leakage Current
UNITSMINTYPMAXPARAMETERFB = CSH = CSL = 6V, VL switched over to CSL15µAV+ Off-State Leakage Current
DL forced to 2V
FB, FB = REF
CSH, CSL, CSH = CSL = 4V, device not shut down
SYNC, SKIP1DL Sink/Source Current
±100
SHDN, SKIP
SYNC
0.5V0.8Input Low Voltage
DH forced to 2V, BST - LX = 4.5VA1DH Sink/Source Current
High or low, BST - LX = 4.5V
High or low7DH On-Resistance7DL On-Resistance
OSCILLATOR AND INPUTS/OUTPUTSInput CurrentµA
%/V
igh-Voltage, Step-Down Controller with
Synchronous Rectifier for CPU Power
ELECTRICAL CHARACTERISTICS (continued)(V+ = 15V, GND = PGND = 0V, IVL= IREF= 0A, TA
= -40 to +85°C, SYNC = 0V, unless otherwise noted.) (Note 3)
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 -40°C to +85°C specifications are guaranteed by design.
Note 4:The power input is the high-side MOSFET drain.
External resistor divider
0mV < (CSH - CSL) < 80mV, FB = 0V,
4.5V < power input < 40V, includes line and load
regulation (Note 4)
0mV < (CSH - CSL) < 80mV, FB = VL,
6V < power input < 40V, includes line and load
regulation (Note 4)
CONDITIONSREF6.0Nominal Adjustable Output
Voltage Range3.103.353.563.3V Output Voltage (CSL)
5.0304.705.105.405V Output Voltage (CSL)
UNITSMINTYPMAXPARAMETERCSH - CSL, negative
CSH - CSL = 0V
FB = VL, 6V < power input < 40V (Note 4)
CSH - CSL, positive
-40-100-160Current-Limit Voltage2.402.60Feedback Voltage
FB = CSH = CSL = 6V, VL switched over to CSL
SHDN= 0V, V+ = 30V, CSL = 0V or 6V
Rising edge, hysteresis = 25mV
No external load (Note 1)
0µA < IREF< 100µA110V+ Off-State Leakage Current110V+ Shutdown Current
Rising edge, hysteresis = 15mV
SHDN= 2V, 0mA < IVL< 25mA, 5.5V < V+ < 30V
4.24.7VL/CSL Switchover Voltage2.432.5052.57Reference Output Voltage50Reference Load Regulation3.754.15VL Fault Lockout Voltage4.75.3VL Output Voltage
SYNC = REF
SYNC = 0V or 5V91
kHz210320Oscillator Sync Range
kHzSYNC = REF
120150180Oscillator Frequency250SYNC High Pulse Width
CSH = CSL = 6V
250SYNC Low Pulse Width
2503003504.88.4Quiescent Power Consumption
High or low, BST - LX = 4.5V
High or low
SYNC = 0V or 5V7DH On-Resistance7DL On-Resistance9396Maximum Duty Factor
High-side MOSFET drainV40Input Supply Range
FB = 0V, 4.5V < power input < 40V (Note 4)%/V0.040.06Line Regulation
+3.3V and +5V STEP-DOWN CONTROLLERS
FLYBACK/PWM CONTROLLERINTERNAL REGULATOR AND REFERENCE
OSCILLATOR AND INPUTS/OUTPUTS
igh-Voltage, Step-Down Controller witSynchronous Rectifier for CPU Power
______________________________________________________________Pin DescriptionDual Mode is a trademark of Maxim Integrated Products.
Rail-to-Rail is a registered trademark of Nippon Motorola Ltd.
SKIP
Disables pulse-skipping mode when high. Connect SKIPto GND for normal use. Do not leave unconnected.
With SKIPgrounded, the device automaticallychanges from pulse-skipping operation to full PWM operation
when the load current exceeds approximately 30% of maximum.DHHigh-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. LXSwitching Node (inductor) Connection. LX can swing 2V below ground without hazard.BSTBoost Capacitor Connection for High-Side Gate Drive (0.1µF) DLLow-Side Gate-Drive Output. DL normally drives the synchronous-rectifier MOSFET. Swings 0V to VL.
NAMEFUNCTIONSSSoft-Start Timing Capacitor Connection. Ramp time to full current limit is approximately 1ms/nF.
PINREFReference Voltage Output. Bypass REF to GND with 0.33µF minimum.FB
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®logic to change
the output voltage under system control.SHDNShutdown Control Input, Active Low. Logic threshold is set at approximately 1V (VTHof an internal N-channel
MOSFET). Tie SHDNto V+ for automatic start-up.SYNC
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 Characteristicsfor VIHand VILspecifications). SYNC capture range is guaranteed to be 190kHz to
340kHz.GNDLow-noise Analog Ground and Feedback Reference PointPGNDPower GroundVL
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 4.7µF. VL can supply up to
5mA for external loads.V+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. CSLCurrent-Sense Input, Low Side. CSL also serves as the feedback input in fixed-output modes.CSHCurrent-Sense Input, High Side. Current-limit level is 100mV referred to CSL.
_______________Detailed DescriptionThe MAX797H is functionally identical to the MAX797.The only difference between the two devices is in theBST pin’s absolute maximum rating. The MAX797H’srating is 46V; the MAX797’s rating is 36V. The higherrating allows the MAX797H to use a power input up to40V, provided that the V+ pin is powered by a separatesupply between 4.5V and 30V.
Circuit design and component selection for theMAX797H are identical to those for the MAX797; there-fore, such information is not included in this data sheet.Refer to the MAX796/MAX797/MAX799 data sheet fordesign formulas and applications information. TheApplications Information section in this data sheet con-tains suggestions for providing the 30V maximum V+supply input for the MAX797H when power inputexceeds 30V.
__________Applications Information
Powering the V+ PinV+ can be supplied directly if a system supply between4.5V and 30V is available (see the Typical OperatingCircuit). Most of the MAX797H’s internal blocks are sup-plied by VL, which uses V+ as its input. While the cur-rent into V+ is minimal, it depends heavily on the type ofexternal MOSFET used and the switching frequency:
IGATE= Qgx fSW
where Qgis the sum of the high- and low-side MOSFET’s total gate charges, and fSWis the switchingfrequency. Furthermore, if the circuit output voltage onCSL exceeds the VL/CSL switchover voltage, theMAX797H bootstraps itself (it connects VL to CSL andturns off the linear regulator, supplying the IC from thecircuit output), and V+ current is reduced to about 1µA.
If a 5V regulated supply is available, V+ and VL can beconnected and fed from that supply (Figure 1). In thismode, the VL regulator is bypassed. Do not use thisapproach if the output voltage on CSL can exceed theVL/CSL switchover voltage.
If a 5V regulated supply is not available, a linear regula-tor with a sufficient input voltage range can provide it(Figure 2). This approach allows for a very wide inputvoltage range, which is useful if the circuit must run fromseveral different power sources. The drawback of thelinear regulator is the high quiescent current that thesedevices typically require, in addition to the current usedby the feedback divider resistors (R1 and R2).
For most applications, a better choice than Figure 2’scircuit takes advantage of the MAX797H’s internal lin-ear regulator. There is no need to provide a regulatedsupply to V+, provided it is within the +4.5V to +30V V+input voltage range. In Figure 3, Q1 is used to drop a40V (max) input to 30V by dividing it by approximately4/3. This approach results in a somewhat higher mini-mum input voltage than that of Figure 2’s circuit, but amuch lower quiescent current than that of a linear regu-lator. If quiescent current must be minimized, an N-channel MOSFET can be substituted for Q1, and thedivider-resistor values can be increased.
Powering V+ with a zener diode can be done in manydifferent ways. The simplest is to use a standard shuntregulator to provide a regulated voltage in the 4.5V to30V range (Figure 4). Resistor R1 must be chosen toallow the maximum required V+ current to be obtainedfrom the minimum power input voltage. If the powerinput voltage varies appreciably, the result is higher-than-necessary input current from the highest powerinput voltage. An approach that reduces quiescent current is to use a zener diode as a dropping diode tokeep V+ under 30V (Figure 5). This results in a severelyrestricted minimum range for the power input voltage,which is not a problem for most high-voltage applica-tions. RL must be added to draw current and to ensurethat there is sufficient forward drop across the zenerdiode if the MAX797H can be shut down or bootstrapoff its output voltage.
Duty-Factor Limitations for
Low VOUT/VINRatiosThe MAX797H’s output voltage is adjustable down to2.5V (min). However, the combination of high input andlow output voltages may not be possible at high switch-ing frequencies without introducing some amount offrequency instability. The minimum duty factor is deter-mined by delays through the error comparator, internallogic, gate drivers, and external MOSFETs. The delay istypically 425ns. With a switching period of 3.33µs(300kHz), the minimum duty factor is 0.425µs / 3.33µs= 0.13. If VOUT/ VINis less than this value, the IC willproperly regulate the output voltage, but may extendthe period and switch at 150kHz instead of 300kHz. Itmay also alternate between these two frequencies. Forexample, if VINis 40V, the lowest VOUTthat does notrequire less than the minimum duty factor is 40V x 0.13= 5.2V. Below this output voltage, select the 150kHzswitching frequency (connect SYNC to VL or GND).
igh-Voltage, Step-Down Controller with
Synchronous Rectifier for CPU Power
