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MAX798ESE-T
High-Accuracy Step-Down Controller with Synchronous Rectifier for CPU Power
_______________General DescriptionThe MAX798†high-performance, step-down DC-DCconverter provides main CPU power in battery-poweredsystems. This buck controller achieves 96% efficiencyby using synchronous rectification and Maxim’s propri-etary IdleMode™ control scheme to extend battery lifeat full-load (up to 10A) and no-load outputs. TheMAX798’s high accuracy meets the demanding require-ments of the latest-generation CPUs. Excellent dynamicresponse corrects output transients caused by the latestdynamic-clock CPUs within five 300kHz clock cycles.Unique bootstrap circuitry drives inexpensive N-channelMOSFETs, reducing system cost and eliminating the crow-bar switching currents found in some PMOS/NMOS switchdesigns.
The MAX798 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.
________________________ApplicationsNotebook and Subnotebook Computers
PDAs and Mobile Communicators
____________________________Feature96% Efficiency4.5V to 30V Input Range1.6V to 6V Adjustable Precision Output±0.4% Max Total Load-Regulation Error 0.06%/V Max Line-Regulation Error5V Linear-Regulator OutputPrecision 2.505V Reference OutputAutomatic Bootstrap Circuit150kHz/300kHz Fixed-Frequency PWM OperationProgrammable Soft-Start1.2mA TypicalQuiescent Current
(VIN= 12V, VOUT= 2.5V)1μA Typical Shutdown Currentigh-Accuracy Step-Down Controlleith Synchronous Rectifier for CPU PoweIdle Mode is a trademark of Maxim Integrated Products.†U
19-1175; Rev 0; 12/96
__________________Pin ConfigurationBSTGND
REF
SKIP
TOP VIEW
MAX798
PGND
CSLCSH
SHDN
SYNC
PIN-PACKAGETEMP. RANGEPART16 Narrow SO-40°C to +85°CMAX798ESE
______________Ordering Information
__________Typical Operating CircuitBST
PGND
CSH
CSLGND
SHDNONOFF
VOUT
RSENSEVL
5V, 25mA
4.7μF
0.1μF
INPUT
4.5V TO 30V
MAX798
igh-Accuracy Step-Down Controller ith Synchronous Rectifier for CPU Power ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS(V+ = +15V, GND = PGND = 0V, IVL= IREF= 0A, TA= 0°C to +85°C, unless otherwise noted.)
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 toabsolute maximum rating conditions for extended periods may affect device reliability.
V+ to GND.................................................................-0.3V, +36VGND to PGND........................................................................±2VVL to GND...................................................................-0.3V, +7VBST to GND...............................................................-0.3V, +36VDH to LX........................................................-0.3V, (BST + 0.3V)LX to BST.....................................................................-7V, +0.3V
SHDN to GND............................................................-0.3V, +36VSYNC, SS, REF, SKIP, DL to GND...................-0.3V, (VL + 0.3V)CSH, CSL to GND.......................................................-0.3V, +7VVL Short Circuit to GND..............................................Momentary
REF Short Circuit to GND...........................................ContinuousVL Output Current...............................................................50mAContinuous Power Dissipation (TA= +70°C)SO (derate 8.70mW/°C above +70°C)........................696mWOperating Temperature RangeMAX798ESE....................................................-40°C to +85°CStorage Temperature Range.............................-65°C to +160°CLead Temperature (soldering, 10sec).............................+300°C
Rising edge, hysteresis = 25mV
Rising edge, hysteresis = 15mV
SHDN= 2V, 0mA
CSH - CSL, negative
CSH - CSL, positive
6V < V+ < 30V
External resistor divider
CSH - CSL = 40mV, SKIP= VL
0mV < (CSH - CSL) < 80mV, SKIP= VL
CONDITIONS4.24.7VL/CSL Switchover Voltage3.84.0VL Fault Lockout Voltage4.755.25VL Output Voltage2.0SS Fault Sink Current2.04.06.5SS Source Current4.530Input Supply Range
-40-100-160mV80100120Current-Limit Voltage
%/V0.040.05Line Regulation1.66Nominal Adjustable OutputVoltage Range1.5761.61.624Feedback Voltage±0.4Load Regulation
UNITSMINTYPMAXPARAMETER
No external load (Note 1)V2.4632.5052.537Reference Output Voltage
Falling edgeV1.82.3Reference Fault Lockout Voltage
0µA < IREF< 100µAmV20Reference Load Regulation
SHDN= 0V, CSL = 6V, V+ = 0V or 30V, VL = 0VµA0.11CSL Shutdown Leakage Current
SHDN= 0V, V+ = 30V, CSL = 0V or 6VµA15V+ Shutdown Current
FB = CSH = CSL = 6V, VL switched over to CSLµA15V+ Off-State Leakage Current
V+ = 4V, CSL = 0V (Note 2)mW6.610.5Dropout Power Consumption
CSH = CSL = 6VmW6.48.5Quiescent Power Consumption
STEP-DOWN CONTROLLER
INTERNAL REGULATOR AND REFERENCE
igh-Accuracy Step-Down Controlleith Synchronous Rectifier for CPU PoweSHDN, SKIP
SYNC
SYNC = 0V or 5V
SYNC = REF
Guaranteed by design
SYNC = 0V or 5V
SYNC = REF
2.0VVL - 0.5Input High Voltage939691Maximum Duty Cycle
kHz195340Oscillator Sync Range200SYNC Rise/Fall Time200SYNC Low Pulse Width200SYNC High Pulse Width
125150175kHz270300330Oscillator Frequency
DL forced to 2V
SHDN, 0V or 30V1DL Sink/Source Current
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
CSH, CSL, CSH = CSL = 4V, device not shut down
SYNC, SKIP
FB, FB = 1.6V±100nA
CONDITIONSUNITSMINTYPMAXPARAMETER
ELECTRICAL CHARACTERISTICS (continued)
(V+ = +15V, GND = PGND = 0V, IVL= IREF= 0A, TA= 0°C to +85°C, unless otherwise noted.)Input Current
igh-Accuracy Step-Down Controller ith Synchronous Rectifier for CPU Power ELECTRICAL CHARACTERISTICS
(V+ = +15V, GND = PGND = 0V, IVL= IREF= 0A, TA= -40°C to +85°C, unless otherwise noted.) (Note 3)
CONDITIONSUNITSMINTYPMAXPARAMETER
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
4.154.75VL/CSL Switchover Voltage2.4382.5052.562Reference Output Voltage30Reference Load Regulation3.754.05VL Fault Lockout Voltage
SYNC = REF
SYNC = 0V or 5V91
kHz210320Oscillator Sync Range
kHzSYNC = REF
110150190Oscillator Frequency250SYNC High Pulse Width
CSH = CSL = 6V
250SYNC Low Pulse Width
2503003506.49.1Quiescent Power Consumption
High or low, BST - LX = 4.5V
High or low
SYNC = 0V or 5V7DH On-Resistance7DL On-Resistance9296Maximum Duty Cycle
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 quiescentcurrent in dropout will not exceed 16mA.
Note 3:All -40°C to +85°C specifications above are guaranteed by design.5.030Input Supply Range
CSH - CSL = 40mV, SKIP= VLV1.5601.61.640Feedback Voltage
External resistor dividerV1.66.0Nominal Adjustable OutputVoltage Range
6V < V+ < 30V%/ V0.040.06Line Regulation
CSH - CSL, negative-40-100-160
CSH - CSL, positivemV70130Current-Limit Voltage
0mV < (CSH - CSL) < 80mV, SKIP= VL%±1Load Regulation
SHDN= 2V, 0mA < IVL< 25mA, 5.5V < V+ < 30VV4.75.3VL Output Voltage
STEP-DOWN CONTROLLER
INTERNAL REGULATOR AND REFERENCE
OSCILLATOR AND INPUTS/OUTPUTS
igh-Accuracy Step-Down Controlleith Synchronous Rectifier for CPU Powe______________________________________________________________Pin Description
SKIP
Disables pulse-skipping mode when high. Connect to GND for normal use. Don’t leave
SKIPunconnected.
With SKIPgrounded, the device will automaticallychange from pulse-skipping operation to full PWM opera-tion when the load current exceeds approximately 30% of maximum. DHHigh-Side Gate-Drive Output. Normally drives the main buck switch. DH is a floating driver output that swingsfrom LX to BST, riding on the LX switching-node voltage. LXSwitching Node (inductor) Connection. Can swing 2V below ground without hazard.BSTBoost Capacitor Connection for high-side gate drive (0.1µF)DLLow-Side Gate-Drive Output. 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 to GND with 0.33µF minimum.FBFeedback Input. Regulates at FB = 1.6V. Connect FB to a resistor divider to set the output voltage.SHDNShutdown Control Input, active low. Logic threshold is set at approximately 1V (VTHof an internal N-channelMOSFET). Tie SHDNto V+ for automatic start-up.SYNCOscillator Synchronization and Frequency Select. Tie to GND or VL for 150kHz operation; tie to REF for300kHz operation. A high-to-low transition begins a new cycle. Drive SYNC with 0V to 5V logic levels (see theElectrical Characteristicstable for VIHand VILspecifications). SYNC capture range is 195kHz to 340kHz.GNDLow-Noise Analog Ground and Feedback Reference PointPGNDPower GroundVL5V Internal Linear-Regulator Output. VL is also the supply voltage rail for the chip. VL is switched to the out-put 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 alinear regulator that powers VL. CSLCurrent-Sense Input, low sideCSHCurrent-Sense Input, high side. Current-limit level is 100mV referred to CSL.
igh-Accuracy Step-Down Controller ith Synchronous Rectifier for CPU Power REF
Q1, Q2 =
C1, C2 =
C4 =
L1 =
R1 =
R2 =
f =
SILICONIX Si4410DY or IRF7413
10μF/30V SANYO OS-CON (30SA10)
470μF/4V SPRAGUE 594D SERIES (594D477X0004R2T)
SUMIDA CDRH127 120
6.49kΩ, 1%
11.5kΩ, 1%
150kHz
C5
470pF
VOUT, +2.5V @ 4.2A15mΩ
PGND
CSH
CSL
SYNCGND
VIN, 4.5V TO 30V
0.01μF
L1
12μH
0.1μF
C3
47pF
R4
2.2μF
R3
10Ω
0.33μF
4.7μF
BST
SHDN
SKIP
MAX798
OFF
Figure 1. Standard Application Circuit
_______________Detailed Description
The MAX798 is a BiCMOS, switch-mode power-supplycontroller designed primarily for buck-topology regula-tors in battery-powered applications where high accu-racy, high efficiency, and low quiescent supply currentare critical. The MAX798 also works well in othertopologies such as boost, inverting, and CUK due tothe flexibility of its floating high-speed gate driver.Light-load efficiency is enhanced by automatic idle-mode operation—a variable-frequency pulse-skipping
mode that reduces losses due to MOSFET gate charge.The step-down power-switching circuit consists of twoN-channel MOSFETs, a rectifier, and an LC output filter.The output voltage is the average of the AC voltage atthe switching node, which is adjusted and regulated bychanging the duty cycle of the MOSFET switches. Thegate-drive signal to the N-channel high-side MOSFETmust exceed the battery voltage and is provided by aflying capacitor boost circuit that uses a 100nF capaci-tor connected between BST and LX.
