_______________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 ModeTM 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.
________________________Applications
Notebook and Subnotebook Computers
PDAs and Mobile Communicators
____________________________Features
o
96% Efficiency
o
4.5V to 30V Input Range
o
1.6V to 6V Adjustable Precision Output
o
0.4% Max Total Load-Regulation Error
o
0.06%/V Max Line-Regulation Error
o
5V Linear-Regulator Output
o
Precision 2.505V Reference Output
o
Automatic Bootstrap Circuit
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
1A Typical Shutdown Current
MAX798
High-Accuracy Step-Down Controller
with Synchronous Rectifier for CPU Power
________________________________________________________________
Maxim Integrated Products
1
Idle Mode is a trademark of Maxim Integrated Products.
U.S. and foreign patents pending.
19-1175; Rev 0; 12/96
__________________Pin Configuration
16
15
14
13
12
11
10
9
1
2
3
4
5
6
7
8
DH
LX
BST
DL
GND
REF
SKIP
SS
TOP VIEW
MAX798
PGND
VL
V+
CSL
CSH
FB
SHDN
SYNC
SO
PIN-PACKAGE
TEMP. RANGE
PART
16 Narrow SO
-40C to +85C
MAX798ESE
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800
______________Ordering Information
__________Typical Operating Circuit
BST
LX
DL
PGND
CSH
CSL
FB
GND
SHDN
ON
OFF
N
V
OUT
R
SENSE
N
V+
VL
5V, 25mA
4.7
F
0.1
F
INPUT
4.5V TO 30V
DH
MAX798
MAX798
High-Accuracy Step-Down Controller
with Synchronous Rectifier for CPU Power
2
_______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(V+ = +15V, GND = PGND = 0V, I
VL
= I
REF
= 0A, T
A
= 0C to +85C, 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 (T
A
= +70C)
SO (derate 8.70mW/C above +70C) ........................696mW
Operating Temperature Range
MAX798ESE....................................................-40C to +85C
Storage Temperature Range .............................-65C to +160C
Lead Temperature (soldering, 10sec) .............................+300C
Rising edge, hysteresis = 25mV
Rising edge, hysteresis = 15mV
SHDN = 2V, 0mA <I
VL
< 25mA, 5.5V < V+ < 30V
CSH - CSL, negative
CSH - CSL, positive
6V < V+ < 30V
External resistor divider
CSH - CSL = 40mV,
SKIP = VL
0mV < (CSH - CSL) < 80mV,
SKIP = VL
CONDITIONS
V
4.2
4.7
VL/CSL Switchover Voltage
V
3.8
4.0
VL Fault Lockout Voltage
V
4.75
5.25
VL Output Voltage
mA
2.0
SS Fault Sink Current
A
2.0
4.0
6.5
SS Source Current
V
4.5
30
Input Supply Range
-40
-100
-160
mV
80
100
120
Current-Limit Voltage
%/V
0.04
0.05
Line Regulation
V
1.6
6
Nominal Adjustable Output
Voltage Range
V
1.576
1.6
1.624
Feedback Voltage
%
0.4
Load Regulation
UNITS
MIN
TYP
MAX
PARAMETER
No external load (Note 1)
V
2.463
2.505
2.537
Reference Output Voltage
Falling edge
V
1.8
2.3
Reference Fault Lockout Voltage
0A < I
REF
< 100A
mV
20
Reference Load Regulation
SHDN = 0V, CSL = 6V, V+ = 0V or 30V, VL = 0V
A
0.1
1
CSL Shutdown Leakage Current
SHDN = 0V, V+ = 30V, CSL = 0V or 6V
A
1
5
V+ Shutdown Current
FB = CSH = CSL = 6V, VL switched over to CSL
A
1
5
V+ Off-State Leakage Current
V+ = 4V, CSL = 0V (Note 2)
mW
6.6
10.5
Dropout Power Consumption
CSH = CSL = 6V
mW
6.4
8.5
Quiescent Power Consumption
STEP-DOWN CONTROLLER
INTERNAL REGULATOR AND REFERENCE
A
MAX798
High-Accuracy Step-Down Controller
with Synchronous Rectifier for CPU Power
_______________________________________________________________________________________
3
SHDN, SKIP
SYNC
SYNC = 0V or 5V
SYNC = REF
Guaranteed by design
SYNC = 0V or 5V
SYNC = REF
2.0
V
VL - 0.5
Input High Voltage
%
93
96
89
91
Maximum Duty Cycle
kHz
195
340
Oscillator Sync Range
ns
200
SYNC Rise/Fall Time
ns
200
SYNC Low Pulse Width
ns
200
SYNC High Pulse Width
125
150
175
kHz
270
300
330
Oscillator Frequency
DL forced to 2V
SHDN, 0V or 30V
A
1
DL Sink/Source Current
2
SHDN, SKIP
SYNC
0.5
V
0.8
Input Low Voltage
DH forced to 2V, BST - LX = 4.5V
A
1
DH Sink/Source Current
High or low, BST - LX = 4.5V
High or low
7
DH On-Resistance
7
DL On-Resistance
CSH, CSL, CSH = CSL = 4V, device not shut down
SYNC,
SKIP
FB, FB = 1.6V
100
nA
50
1
CONDITIONS
UNITS
MIN
TYP
MAX
PARAMETER
ELECTRICAL CHARACTERISTICS (continued)
(V+ = +15V, GND = PGND = 0V, I
VL
= I
REF
= 0A, T
A
= 0C to +85C, unless otherwise noted.)
A
Input Current
MAX798
High-Accuracy Step-Down Controller
with Synchronous Rectifier for CPU Power
4
_______________________________________________________________________________________
ELECTRICAL CHARACTERISTICS
(V+ = +15V, GND = PGND = 0V, I
VL
= I
REF
= 0A, T
A
= -40C to +85C, unless otherwise noted.) (Note 3)
CONDITIONS
UNITS
MIN
TYP
MAX
PARAMETER
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)
0A < I
REF
< 100A
A
1
10
V+ Off-State Leakage Current
A
1
10
V+ Shutdown Current
V
Rising edge, hysteresis = 15mV
4.15
4.75
VL/CSL Switchover Voltage
V
2.438
2.505
2.562
Reference Output Voltage
mV
30
Reference Load Regulation
V
3.75
4.05
VL Fault Lockout Voltage
SYNC = REF
SYNC = 0V or 5V
88
91
kHz
210
320
Oscillator Sync Range
kHz
SYNC = REF
110
150
190
Oscillator Frequency
ns
250
SYNC High Pulse Width
ns
CSH = CSL = 6V
250
SYNC Low Pulse Width
250
300
350
mW
6.4
9.1
Quiescent Power Consumption
High or low, BST - LX = 4.5V
High or low
SYNC = 0V or 5V
7
DH On-Resistance
7
DL On-Resistance
%
92
96
Maximum 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 quiescent
current in dropout will not exceed 16mA.
Note 3:
All -40C to +85C specifications above are guaranteed by design.
V
5.0
30
Input Supply Range
CSH - CSL = 40mV,
SKIP = VL
V
1.560
1.6
1.640
Feedback Voltage
External resistor divider
V
1.6
6.0
Nominal Adjustable Output
Voltage Range
6V < V+ < 30V
%/ V
0.04
0.06
Line Regulation
CSH - CSL, negative
-40
-100
-160
CSH - CSL, positive
mV
70
130
Current-Limit Voltage
0mV < (CSH - CSL) < 80mV,
SKIP = VL
%
1
Load Regulation
SHDN = 2V, 0mA < I
VL
< 25mA, 5.5V < V+ < 30V
V
4.7
5.3
VL Output Voltage
STEP-DOWN CONTROLLER
INTERNAL REGULATOR AND REFERENCE
OSCILLATOR AND INPUTS/OUTPUTS
MAX798
High-Accuracy Step-Down Controller
with Synchronous Rectifier for CPU Power
_______________________________________________________________________________________
5
______________________________________________________________Pin Description
SKIP
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.
16
DH
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.
15
LX
Switching Node (inductor) Connection. Can swing 2V below ground without hazard.
14
BST
Boost Capacitor Connection for high-side gate drive (0.1F)
13
DL
Low-Side Gate-Drive Output. Normally drives the synchronous-rectifier MOSFET. Swings 0V to VL.
NAME
FUNCTION
1
SS
Soft-Start Timing Capacitor Connection. Ramp time to full current limit is approximately 1ms/nF.
2
PIN
3
REF
Reference Voltage Output. Bypass to GND with 0.33F minimum.
7
FB
Feedback Input. Regulates at FB = 1.6V. Connect FB to a resistor divider to set the output voltage.
6
SHDN
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.
5
SYNC
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.
4
GND
Low-Noise Analog Ground and Feedback Reference Point
12
PGND
Power Ground
11
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 4.7F. VL can supply
up to 5mA for external loads.
10
V+
Battery Voltage Input (4.5V to 30V). Bypass V+ to PGND close to the IC with a 0.1F capacitor. Connects to a
linear regulator that powers VL.
9
CSL
Current-Sense Input, low side
8
CSH
Current-Sense Input, high side. Current-limit level is 100mV referred to CSL.
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