19-1239; Rev 0; 7/97
High-Voltage, Step-Down Controller with
Synchronous Rectifier for CPU Power
_______________General Description
The 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 Idle Mode™ 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
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 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.
____________________________Features
o
96% Efficiency
o
Up to 40V Power Input
o
2.5V to 6V Adjustable Output
o
o
o
o
o
o
o
o
Preset 3.3V and 5V Outputs (at up to 10A)
5V Linear-Regulator Output
Precision 2.505V Reference Output
Automatic Bootstrap Circuit
150kHz/300kHz Fixed-Frequency PWM Operation
Programmable Soft-Start
375µA Quiescent Current (V
IN
= 12V, V
OUT
= 5V)
1µA Shutdown Current
MAX797H
______________Ordering Information
PART
†
MAX797HESE
TEMP. RANGE
-40°C to +85°C
PIN-PACKAGE
16 Narrow SO
†
U.S. and foreign patents pending.
__________Typical Operating Circuit
+4.5V TO +30V
SUPPLY
INPUT
V+
SHDN
DH
VL
+4V TO +40V
POWER INPUT
________________________Applications
Notebook and Subnotebook Computers
Industrial Controls
__________________Pin Configuration
TOP VIEW
SS 1
SKIP 2
REF 3
GND 4
SYNC 5
SHDN 6
FB 7
CSH 8
16 DH
MAX797H
BST
SS
15 LX
14 BST
REF
DL
PGND
SYNC
10 V+
9
CSL
GND
SKIP
FB
CSL
CSH
LX
+3.3V
OUTPUT
MAX797H
13 DL
12 PGND
11 VL
SO
Idle Mode is a trademark of Maxim Integrated Products.
________________________________________________________________
Maxim Integrated Products
1
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800
For small orders, phone 408-737-7600 ext. 3468.
High-Voltage, Step-Down Controller with
Synchronous Rectifier for CPU Power
MAX797H
ABSOLUTE MAXIMUM RATINGS
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
SHDN
to 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 (T
A
= +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
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, IVL = IREF = 0A,
T
A
= 0°C to +85°C,
SYNC = 0V, unless otherwise noted.)
PARAMETER
+3.3V AND +5V STEP-DOWN CONTROLLERS
Input Supply Range
V+
High-side MOSFET drain
0mV < (CSH - CSL) < 80mV, FB = VL,
6V < power input < 40V, includes line and load
regulation (Note 4)
0mV < (CSH - CSL) < 80mV, FB = 0V,
4.5V < power input < 40V, includes line and load
regulation (Note 4)
External resistor divider
CSH - CSL = 0V
0mV < (CSH - CSL) < 80mV
25mV < (CSH - CSL) < 80mV
FB = VL, 6V < power input < 40V (Note 4)
FB = 0V, 4.5V < power input < 40V (Note 4)
CSH - CSL, positive
CSH - CSL, negative
80
-50
2.5
2.0
SHDN
= 2V, 0mA < I
VL
< 25mA, 5.5V < V+ < 30V
Rising edge, hysteresis = 15mV
Rising edge, hysteresis = 25mV
4.85
5.10
4.5
30
40
5.25
V
CONDITIONS
MIN
TYP
MAX
UNITS
5V Output Voltage (CSL)
V
3.3V Output Voltage (CSL)
Nominal Adjustable Output
Voltage Range
Feedback Voltage
Load Regulation
3.20
3.35
3.46
V
REF
2.43
2.505
2.5
1.5
0.04
0.04
100
-100
4.0
6
2.57
V
V
%
Line Regulation
0.06
0.06
120
-160
6.5
%/V
Current-Limit Voltage
SS Source Current
SS Fault Sink Current
mV
µA
mA
INTERNAL REGULATOR AND REFERENCE
FLYBACK/PWM CONTROLLER
VL Output Voltage
VL Fault Lockout Voltage
VL/CSL Switchover Voltage
4.7
3.8
4.2
5.3
4.1
4.7
V
V
V
2
_______________________________________________________________________________________
High-Voltage, Step-Down Controller with
Synchronous Rectifier for CPU Power
ELECTRICAL CHARACTERISTICS (continued)
(V+ = 15V, GND = PGND = 0V, IVL = IREF = 0A,
T
A
= 0°C to +85°C,
SYNC = 0V, unless otherwise noted.)
PARAMETER
Reference Output Voltage
Reference Fault Lockout Voltage
Reference Load Regulation
CSL Shutdown Leakage Current
V+ Shutdown Current
V+ Off-State Leakage Current
Dropout Power Consumption
Quiescent Power Consumption
Falling edge
0µA < I
REF
< 100µA
SHDN
= 0V, CSL = 6V, V+ = 0V or 30V, VL = 0V
SHDN
= 0V, V+ = 30V, CSL = 0V or 6V
FB = CSH = CSL = 6V, VL switched over to CSL
V+ = 4V, CSL = 0V (Note 2)
CSH = CSL = 6V
SYNC = REF
SYNC = 0V or 5V
270
125
200
200
Guaranteed by design
190
SYNC = REF
SYNC = 0V or 5V
SYNC
SHDN, SKIP
SYNC
SHDN, SKIP
SHDN,
0V or 30V
Input Current
SYNC,
SKIP
CSH, CSL, CSH = CSL = 4V, device not shut down
FB, FB = REF
DL Sink/Source Current
DH Sink/Source Current
DL On-Resistance
DH On-Resistance
DL forced to 2V
DH forced to 2V, BST - LX = 4.5V
High or low
High or low, BST - LX = 4.5V
1
1
7
7
89
93
VL - 0.5
2.0
0.8
0.5
2.0
1.0
50
±100
nA
A
A
Ω
Ω
µA
91
96
200
340
0.1
1
1
4
4.8
300
150
CONDITIONS
No external load (Note 1)
MIN
2.45
1.8
TYP
2.505
MAX
2.55
2.3
50
1
5
5
8
6.6
330
175
UNITS
V
V
mV
µA
µA
µA
mW
mW
MAX797H
OSCILLATOR AND INPUTS/OUTPUTS
Oscillator Frequency
SYNC High Pulse Width
SYNC Low Pulse Width
SYNC Rise/Fall Time
Oscillator Sync Range
Maximum Duty Factor
kHz
ns
ns
ns
kHz
%
Input High Voltage
V
Input Low Voltage
V
_______________________________________________________________________________________
3
High-Voltage, Step-Down Controller with
Synchronous Rectifier for CPU Power
MAX797H
ELECTRICAL CHARACTERISTICS (continued)
(V+ = 15V, GND = PGND = 0V, IVL = IREF = 0A,
T
A
= -40 to +85°C,
SYNC = 0V, unless otherwise noted.) (Note 3)
PARAMETER
+3.3V and +5V STEP-DOWN CONTROLLERS
Input Supply Range
V+
High-side MOSFET drain
0mV < (CSH - CSL) < 80mV, FB = VL,
6V < power input < 40V, includes line and load
regulation (Note 4)
0mV < (CSH - CSL) < 80mV, FB = 0V,
4.5V < power input < 40V, includes line and load
regulation (Note 4)
External resistor divider
CSH - CSL = 0V
FB = VL, 6V < power input < 40V (Note 4)
FB = 0V, 4.5V < power input < 40V (Note 4)
CSH - CSL, positive
CSH - CSL, negative
SHDN
= 2V, 0mA < I
VL
< 25mA, 5.5V < V+ < 30V
Rising edge, hysteresis = 15mV
Rising edge, hysteresis = 25mV
No external load (Note 1)
0µA < I
REF
< 100µA
SHDN
= 0V, V+ = 30V, CSL = 0V or 6V
FB = CSH = CSL = 6V, VL switched over to CSL
CSH = CSL = 6V
SYNC = REF
SYNC = 0V or 5V
250
120
250
250
210
SYNC = REF
SYNC = 0V or 5V
High or low
High or low, BST - LX = 4.5V
89
93
91
96
7
7
320
1
1
4.8
300
150
70
-40
4.7
3.75
4.2
2.43
2.505
-100
4.70
5.10
5.0
30
40
5.40
V
CONDITIONS
MIN
TYP
MAX
UNITS
5V Output Voltage (CSL)
V
3.3V Output Voltage (CSL)
Nominal Adjustable Output
Voltage Range
Feedback Voltage
Line Regulation
Current-Limit Voltage
3.10
3.35
3.56
V
REF
2.40
0.04
0.04
6.0
2.60
0.06
0.06
130
-160
5.3
4.15
4.7
2.57
50
10
10
8.4
350
180
V
V
%/V
%/V
mV
INTERNAL REGULATOR AND REFERENCE
FLYBACK/PWM CONTROLLER
VL Output Voltage
VL Fault Lockout Voltage
VL/CSL Switchover Voltage
Reference Output Voltage
Reference Load Regulation
V+ Shutdown Current
V+ Off-State Leakage Current
Quiescent Power Consumption
V
V
V
V
mV
µA
µA
mW
OSCILLATOR AND INPUTS/OUTPUTS
Oscillator Frequency
SYNC High Pulse Width
SYNC Low Pulse Width
Oscillator Sync Range
Maximum Duty Factor
DL On-Resistance
DH On-Resistance
kHz
ns
ns
kHz
%
Ω
Ω
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.
4
_______________________________________________________________________________________
High-Voltage, Step-Down Controller with
Synchronous Rectifier for CPU Power
______________________________________________________________Pin Description
PIN
1
2
3
4
NAME
SS
SKIP
REF
GND
FUNCTION
Soft-Start Timing Capacitor Connection. Ramp time to full current limit is approximately 1ms/nF.
Disables pulse-skipping mode when high. Connect
SKIP
to GND for normal use.
Do not leave unconnected.
With
SKIP
grounded, the device
automatically
changes from pulse-skipping operation to full PWM operation
when the load current exceeds approximately 30% of maximum.
Reference Voltage Output. Bypass REF to GND with 0.33µF minimum.
Low-noise Analog Ground and Feedback Reference Point
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 Characteristics
for V
IH
and V
IL
specifications). SYNC capture range is guaranteed to be 190kHz to
340kHz.
Shutdown Control Input, Active Low. Logic threshold is set at approximately 1V (V
TH
of an internal N-channel
MOSFET). Tie
SHDN
to 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:
•
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.
8
9
10
CSH
CSL
V+
Current-Sense Input, High Side. Current-limit level is 100mV referred to CSL.
Current-Sense Input, Low Side. CSL also serves as the feedback input in fixed-output modes.
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. It is switched to the output
voltage via CSL (V
CSL
> 4.5V) for automatic bootstrapping. Bypass to GND with 4.7µF. VL can supply up to
5mA for external loads.
Power Ground
Low-Side Gate-Drive Output. DL normally drives the synchronous-rectifier MOSFET. Swings 0V to VL.
Boost Capacitor Connection for High-Side Gate Drive (0.1µF)
Switching Node (inductor) Connection. LX can swing 2V below ground without hazard.
High-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.
MAX797H
5
SYNC
6
SHDN
7
FB
11
12
13
14
15
16
VL
PGND
DL
BST
LX
DH
Dual Mode is a trademark of Maxim Integrated Products.
Rail-to-Rail is a registered trademark of Nippon Motorola Ltd.
_______________________________________________________________________________________
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