19-1175; Rev 0; 12/96
High-Accuracy Step-Down Controller
with Synchronous Rectifier for CPU Power
_______________General Description
The 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 Idle Mode™ 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 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.
____________________________Features
o
96% Efficiency
o
4.5V to 30V Input Range
o
1.6V to 6V Adjustable Precision Output
o
o
o
o
o
±0.4% Max Total Load-Regulation Error
0.06%/V Max Line-Regulation Error
5V Linear-Regulator Output
Precision 2.505V Reference Output
Automatic Bootstrap Circuit
MAX798
o
150kHz/300kHz Fixed-Frequency PWM Operation
o
Programmable Soft-Start
o
1.2mA Typical Quiescent Current
(V
IN
= 12V, V
OUT
= 2.5V)
o
1µA Typical Shutdown Current
______________Ordering Information
PART
MAX798ESE
TEMP. RANGE
-40°C to +85°C
PIN-PACKAGE
16 Narrow SO
________________________Applications
Notebook and Subnotebook Computers
PDAs and Mobile Communicators
__________Typical Operating Circuit
INPUT
4.5V TO 30V
__________________Pin Configuration
TOP VIEW
V+
VL
BST
N
5V, 25mA
4.7µF
SS 1
SKIP 2
REF 3
GND 4
SYNC 5
SHDN 6
FB 7
CSH 8
16 DH
15 LX
14 BST
DH
0.1µF
R
SENSE
V
OUT
MAX798
LX
ON
SHDN
DL
PGND
CSH
CSL
N
MAX798
13 DL
12 PGND
11 VL
10 V+
9
CSL
OFF
SO
GND
FB
Idle Mode is a trademark of Maxim Integrated Products.
†
U
________________________________________________________________
Maxim Integrated Products
1
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800
High-Accuracy Step-Down Controller
with Synchronous Rectifier for CPU Power
MAX798
ABSOLUTE MAXIMUM RATINGS
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 (T
A
= +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
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, I
VL
= I
REF
= 0A, T
A
= 0°C to +85°C, unless otherwise noted.)
PARAMETER
STEP-DOWN CONTROLLER
Input Supply Range
Nominal Adjustable Output
Voltage Range
Feedback Voltage
Load Regulation
Line Regulation
Current-Limit Voltage
SS Source Current
SS Fault Sink Current
INTERNAL REGULATOR AND REFERENCE
VL Output Voltage
VL Fault Lockout Voltage
VL/CSL Switchover Voltage
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
SHDN
= 2V, 0mA <I
VL
< 25mA, 5.5V < V+ < 30V
Rising edge, hysteresis = 15mV
Rising edge, hysteresis = 25mV
No external load (Note 1)
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
0.1
1
1
6.6
6.4
4.75
3.8
4.2
2.463
1.8
2.505
5.25
4.0
4.7
2.537
2.3
20
1
5
5
10.5
8.5
V
V
V
V
V
mV
µA
µA
µA
mW
mW
External resistor divider
CSH - CSL = 40mV,
SKIP
= VL
0mV < (CSH - CSL) < 80mV,
SKIP
= VL
6V < V+ < 30V
CSH - CSL, positive
CSH - CSL, negative
80
-40
2.0
2.0
0.04
100
-100
4.0
4.5
1.6
1.576
1.6
30
6
1.624
±0.4
0.05
120
-160
6.5
V
V
V
%
%/V
mV
µA
mA
CONDITIONS
MIN
TYP
MAX
UNITS
2
_______________________________________________________________________________________
High-Accuracy Step-Down Controller
with Synchronous Rectifier for CPU Power
ELECTRICAL CHARACTERISTICS (continued)
(V+ = +15V, GND = PGND = 0V, I
VL
= I
REF
= 0A, T
A
= 0°C to +85°C, unless otherwise noted.)
PARAMETER
Oscillator Frequency
SYNC High Pulse Width
SYNC Low Pulse Width
SYNC Rise/Fall Time
Oscillator Sync Range
Maximum Duty Cycle
Input High Voltage
Input Low Voltage
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 = 1.6V
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
Guaranteed by design
195
89
93
VL - 0.5
2.0
0.8
0.5
2
1
50
±100
nA
A
A
Ω
Ω
91
96
SYNC = REF
SYNC = 0V or 5V
CONDITIONS
MIN
270
125
200
200
200
340
TYP
300
150
MAX
330
175
UNITS
kHz
ns
ns
ns
kHz
%
V
V
µA
µA
MAX798
_______________________________________________________________________________________
3
High-Accuracy Step-Down Controller
with Synchronous Rectifier for CPU Power
MAX798
ELECTRICAL CHARACTERISTICS
(V+ = +15V, GND = PGND = 0V, I
VL
= I
REF
= 0A, T
A
= -40°C to +85°C, unless otherwise noted.) (Note 3)
PARAMETER
STEP-DOWN CONTROLLER
Input Supply Range
Nominal Adjustable Output
Voltage Range
Feedback Voltage
Load Regulation
Line Regulation
Current-Limit Voltage
External resistor divider
CSH - CSL = 40mV,
SKIP
= VL
0mV < (CSH - CSL) < 80mV,
SKIP
= VL
6V < V+ < 30V
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
110
250
250
210
SYNC = REF
SYNC = 0V or 5V
High or low
High or low, BST - LX = 4.5V
88
92
91
96
7
7
320
1
1
6.4
300
150
70
-40
4.7
3.75
4.15
2.438
2.505
-100
0.04
5.0
1.6
1.560
1.6
30
6.0
1.640
±1
0.06
130
-160
5.3
4.05
4.75
2.562
30
10
10
9.1
350
190
V
V
V
%
%/ V
mV
CONDITIONS
MIN
TYP
MAX
UNITS
INTERNAL REGULATOR AND REFERENCE
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 Cycle
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). 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.
4
_______________________________________________________________________________________
High-Accuracy Step-Down Controller
with Synchronous Rectifier for CPU Power
______________________________________________________________Pin Description
PIN
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
NAME
SS
SKIP
REF
GND
SYNC
FUNCTION
Soft-Start Timing Capacitor Connection. Ramp time to full current limit is approximately 1ms/nF.
Disables pulse-skipping mode when high. Connect to GND for normal use.
Don’t leave
SKIP
unconnected.
With
SKIP
grounded, the device will
automatically
change from pulse-skipping operation to full PWM opera-
tion when the load current exceeds approximately 30% of maximum.
Reference Voltage Output. Bypass to GND with 0.33µF minimum.
Low-Noise Analog Ground and Feedback Reference Point
Oscillator Synchronization and Frequency Select. Tie 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 the
Electrical Characteristics
table for V
IH
and V
IL
specifications). SYNC capture range is 195kHz 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 = 1.6V. Connect FB to a resistor divider to set the output voltage.
Current-Sense Input, high side. Current-limit level is 100mV referred to CSL.
Current-Sense Input, low side
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. VL is switched to the out-
put 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. Normally drives the synchronous-rectifier MOSFET. Swings 0V to VL.
Boost Capacitor Connection for high-side gate drive (0.1µF)
Switching Node (inductor) Connection. Can swing 2V below ground without hazard.
High-Side Gate-Drive Output. Normally drives the main buck switch. DH is a floating driver output that swings
from LX to BST, riding on the LX switching-node voltage.
MAX798
SHDN
FB
CSH
CSL
V+
VL
PGND
DL
BST
LX
DH
_______________________________________________________________________________________
5
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