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MAX798ESE
High-Accuracy Step-Down Controller with Synchronous Rectifier for CPU Power
_______________General DescriptionThe MAX798†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 propri-
etary IdleMode™ control scheme to extend battery life
at full-load (up to 10A) and no-load outputs. The
MAX798’s high accuracy meets the demanding require-
ments 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 crow-
bar 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 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.
________________________ApplicationsNotebook and Subnotebook Computers
PDAs and Mobile Communicators
____________________________Features96% 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 Current
MAX798
High-Accuracy Step-Down Controller
with Synchronous Rectifier for CPU PowerIdle Mode is a trademark of Maxim Integrated Products.
†U.S. and foreign patents pending.
19-1175; Rev 0; 12/96
__________________Pin Configuration
______________Ordering Information
__________Typical Operating Circuit
MAX798
High-Accuracy 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, 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 to
absolute maximum rating conditions for extended periods may affect device reliability.
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= +70°C)
SO (derate 8.70mW/°C above +70°C)........................696mW
Operating Temperature Range
MAX798ESE....................................................-40°C to +85°C
Storage Temperature Range.............................-65°C to +160°C
Lead Temperature (soldering, 10sec).............................+300°C
MAX798
High-Accuracy Step-Down Controller
with Synchronous Rectifier for CPU Power
ELECTRICAL CHARACTERISTICS (continued)(V+ = +15V, GND = PGND = 0V, IVL= IREF= 0A, TA= 0°C to +85°C, unless otherwise noted.)
MAX798
High-Accuracy Step-Down Controller
with 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)
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 -40°C to +85°C specifications above are guaranteed by design.
MAX798
High-Accuracy Step-Down Controller
with Synchronous Rectifier for CPU Power
______________________________________________________________Pin DescriptionMAX798
High-Accuracy Step-Down Controller
with Synchronous Rectifier for CPU Power Figure 1. Standard Application Circuit
_______________Detailed DescriptionThe MAX798 is a BiCMOS, switch-mode power-supply
controller designed primarily for buck-topology regula-
tors in battery-powered applications where high accu-
racy, 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 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 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 capaci-
tor connected between BST and LX.
