High-Accuracy Step-Down Controller
with Synchronous Rectifier for CPU Power
______________________________________________________________Pin Description
PIN
1
2
3
4
5
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
6
7
8
9
10
SHDN
FB
CSH
CSL
V+
11
12
13
14
15
16
VL
PGND
DL
BST
LX
DH
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5
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
