MAX797HESE ,High-Voltage, Step-Down Controller with Synchronous Rectifier for CPU PowerELECTRICAL CHARACTERISTICS(V+ = 15V, GND = PGND = 0V, I = I = 0A, T = 0°C to +85°C, SYNC = 0V, unle ..
MAX797HESE ,High-Voltage, Step-Down Controller with Synchronous Rectifier for CPU PowerFeaturesThe MAX797H high-performance, step-down DC-DC ' 96% Efficiencyconverter provides main CPU p ..
MAX797HESE ,High-Voltage, Step-Down Controller with Synchronous Rectifier for CPU PowerApplicationsPOWER INPUTSUPPLYINPUTNotebook and Subnotebook ComputersIndustrial ControlsV+ VLSHDN___ ..
MAX797HESE+ ,High-Voltage, Step-Down Controller with Synchronous Rectifier for CPU PowerApplicationsPOWER INPUTSUPPLYINPUTNotebook and Subnotebook ComputersIndustrial ControlsV+ VLSHDN___ ..
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MAX798 ,High-Accuracy Step-Down Controller with Synchronous Rectifier for CPU PowerELECTRICAL CHARACTERISTICS(V+ = +15V, GND = PGND = 0V, I = I = 0A, T = 0°C to +85°C, unless otherwi ..
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MAX797HESE
High-Voltage, Step-Down Controller with Synchronous Rectifier for CPU Power
_______________General DescriptionThe 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 IdleMode™ control scheme to
extend battery life at full-load (up to 10A) and no-load
outputs. Excellent dynamic response corrects output tran-
sients 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 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 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 Current
MAX797H
High-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 Configuration
__________Typical Operating Circuit
______________Ordering Information†U.S. and foreign patents pending.
MAX797H
High-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 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 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
SHDNto 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= +70°C)
SO (derate 8.70mW/°C above +70°C)........................696mW
Operating Temperature Range
MAX797HESE.................................................-40°C to +85°C
Storage Temperature Range.............................-65°C to +160°C
Lead Temperature (soldering, 10sec).............................+300°C
MAX797H
High-Voltage, 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, SYNC = 0V, unless otherwise noted.)
MAX797H
High-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.
MAX797H
High-Voltage, Step-Down Controller with
Synchronous Rectifier for CPU Power
______________________________________________________________Pin Description
Dual Mode is a trademark of Maxim Integrated Products.
Rail-to-Rail is a registered trademark of Nippon Motorola Ltd.
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; there-
fore, 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 con-
tains suggestions for providing the 30V maximum V+
supply input for the MAX797H when power input
exceeds 30V.
__________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 sup-
plied by VL, which uses V+ as its input. While the cur-
rent into V+ is minimal, it depends heavily on the type of
external 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 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 1µA.
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 regula-
tor 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).
For most applications, a better choice than Figure 2’s
circuit takes advantage of the MAX797H’s internal lin-
ear 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 mini-
mum input voltage than that of Figure 2’s circuit, but a
much 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 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 higher-
than-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 applica-
tions. 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.
Duty-Factor Limitations for
Low VOUT/VINRatios
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 switch-
ing frequencies without introducing some amount of
frequency instability. The minimum duty factor is deter-
mined by delays through the error comparator, internal
logic, gate drivers, and external MOSFETs. The delay is
typically 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 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 VINis 40V, the lowest VOUTthat 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).
High-Voltage, Step-Down Controller with
Synchronous Rectifier for CPU Power
