LT8708-1
10
Rev 0
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PIN FUNCTIONS
CLKOUT (Pin 1):Clock Output Pin. Use this pin to syn-
chronize one or more compatible switching regulator ICs.
CLKOUT toggles at the same frequency as the internal
oscillator or as the SYNC pin, but is approximately 180°
out of phase. CLKOUT may also be used as a temperature
monitor since the CLKOUT duty cycle varies linearly with
the part’s junction temperature. The CLKOUT pin can drive
capacitive loads up to 200pF.
SS (Pin 2):Soft-Start Pin. Place a capacitor from this pin
to ground. A capacitor identical to the SS pin capacitor
used on the master LT8708 is recommended. Upon start-
up, this pin will be charged by an internal resistor to 3.3V.
SHDN (Pin 3):Shutdown Pin. Tie high to enable chip.
Ground to shut down and reduce quiescent current to a
minimum. Do not float this pin.
CSN (Pin 4):The (–) Input to the Inductor Current Sense
and DCM Detect Comparator.
CSP (Pin 5):The (+) Input to the Inductor Current Sense
and DCM Detect Comparator. The VCpin voltage and built-
in offsets between CSP and CSN pins, in conjunction with
the RSENSE value, set the inductor current trip threshold.
It is recommended to use the same value RSENSE as the
master LT8708.
ICN (Pin 6):Negative VOUT Current Command Pin. The
voltage on this pin determines the negative VOUT current
for LT8708-1 to regulate to. Connect this pin to the master
LT8708’s ICN pin. See the Applications Information sec-
tion for more information.
DIR (Pin 7):Direction pin when MODE is set for DCM
(discontinuous conduction mode) or HCM (hybrid con-
duction mode) operation. Otherwise this pin is ignored.
Connect the pin to GND to process power from the V
OUT
to V
IN
. Connect the pin to LDO33 to process power from
the V
IN
to V
OUT
. Drive this pin with the same control
signal, or connect this pin to the same voltages as the
master LT8708.
FBIN (Pin 8):VIN Feedback Pin. This pin is connected to
the input of error amplifier EA3. Typically, connect this pin
to LDO33 to disable the EA3.
FBOUT (Pin 9):VOUT Feedback Pin. This pin is connected
to the input of error amplifier EA4. Typically, connect this
pin to GND to disable the EA4.
V
C
(Pin 10):Error Amplifier Output Pin. Tie external com-
pensation network to this pin.
IMON_INP (Pin 11):Positive VIN Current Monitor and
Limit Pin. The current out of this pin is 20µA plus a cur-
rent proportional to the positive average VIN current.
IMON_INP also connects to error amplifier EA5 and can
be used to limit the maximum positive V
IN
current. See the
Applications Information section for more information.
IMON_INN (Pin 12):Negative VIN Current Monitor and
Limit Pin. The current out of this pin is 20µA plus a cur-
rent proportional to the negative average VIN current.
IMON_INN also connects to error amplifier EA1 and can
be used to limit the maximum negative VIN current. See
the Applications Information section for more information.
RT (Pin 13):Timing Resistor Pin. Adjusts the switching
frequency. Place a resistor from this pin to ground to set
the frequency. It is recommended to use the same value
RTresistor as the master LT8708. Do not float this pin.
SYNC (Pin 14):To synchronize the switching frequency
to an outside clock, simply drive this pin with a clock. The
high voltage level of the clock needs to exceed 1.3V, and
the low level should be less than 0.5V. In a two-phase
system, connect this pin to the master LT8708’s CLKOUT
pin to have a 180°phase shift. See the Applications
Information section for more information.
BG1, BG2 (Pin 16, Pin 18):Bottom Gate Drive. Drives the
gate of the bottom N-channel MOSFETs between ground
and GATEVCC.
GATEVCC (Pin 17):Power supply for bottom gate drivers.
Must be connected to the INTV
CC
pin. Do not power from
any other supply. Locally bypass to GND. It is recom-
mended to use the same value bypass cap as the master
LT8708.
