LTC4261/LTC4261-2
9
42612fd
For more information www.linear.com/LTC4261
PGI (Pin 1/Pin 22): Power Good Input. This pin along with
the PGI check timer serves as a watchdog to monitor the
power-up of the DC/DC converter. The PGI pin must be low
before the PGI check timer expires, otherwise the GATE
pin pulls down and stays latched and a power bad fault
is logged into the FAULT register. The PGI timer is started
after the second power good is latched and its delay is
equal to four times the start-up debounce delay. Connect
to V
EE
if unused.
PGIO (Pin 28/Pin 21): General Purpose Input/Output.
Open-drain logic output and logic input. Defaults to pull
low a timer delay after the PG pin goes low to indicate a
second power good output. Configure according to Table 6.
RAMP (Pin 18/Pin 12): Inrush Current Ramp Control
Pin. The inrush current is set by placing a capacitor (C
R
)
between the RAMP pin and the drain terminal of the FET.
At start-up, the GATE pin is pulled up by I
GATE(UP)
until the
pass transistor begins to turn on. A current, I
RAMP
, then
flows through C
R
to ramp down the output voltage V
OUT
.
The value of I
RAMP
is controlled by the SS pin voltage.
When the SS pin reaches its clamp voltage (2.56V), I
RAMP
= 20µA. The ramp rate of V
OUT
and the load capacitor C
L
set the inrush current: I
INRUSH
= (C
L
/C
R
) • I
RAMP
.
SCL (Pin 6/Pin 3): Serial Bus Clock Input. Data at the
SDAI pin is shifted in and data at the SDAO pin is shifted
out on rising edges of SCL. This is a high impedance pin
that is generally connected to the output of the incoming
optoisolator driven by the SCL port of the master controller.
An external pull-up resistor or current source is required.
Pull up to INTV
CC
if unused.
SDAI (Pin 5/Pin 2): Serial Bus Data Input. This is a high
impedance input pin used for shifting in command bits,
data bits and SDAO acknowledge bits. An external pull-up
resistor or current source is required. Normally connected
to the output of the incoming optoisolator that is driven
by the SDA port of the master controller. If the master
controller separates SDAI and SDAO, data read at SDAO
needs to be echoed back to SDAI for proper I
2
C commu-
nication. Pull up to INTV
CC
if unused.
SDAO (Pin 4/Pin 1): Serial Bus Data Output. Open-drain
output used for sending data back to the master controller
or acknowledging a write operation. An external pull-up
resistor or current source is required. Normally connected
to the input of the outgoing optoisolator that outputs to
the SDA port of the master controller. In the single-wire
broadcast mode, the SDAO pin sends out selected data
that is encoded with an internal clock.
SENSE (Pin 14/Pin 9): Current Limit Sense Input. Load
current through the external sense resistor (R
S
) is moni-
tored and controlled by an active current limit amplifier
to 50mV/R
S
. Once V
SENSE
reaches 50mV, a circuit breaker
timer starts and turns off the pass transistor after 530µs. In
the event of a catastrophic short circuit, if V
SENSE
crosses
250mV, a fast response comparator immediately pulls the
GATE pin down to control the current of the N-channel FET.
SS (Pin 19/Pin 13): Soft-Start Input. Connect a capaci
-
tor to this pin to control the rate of rise of inrush current
(dI/dt) during start-up. An internal 10µA current source
charging the external soft-start capacitor (C
SS
) creates
a voltage ramp. This voltage is converted to a current to
charge the GATE pin up and to ramp the output voltage
down. The SS pin is internally clamped to 2.56V limiting
I
GATE(UP)
to 11.5µA and I
RAMP
to 20µA. If the SS capacitor
is absent, the SS pin ramps from 0V to 2.56V in 220µs.
TMR (Pin 20/Pin 14): Delay Timer Input. Connect a capaci
-
tor (C
TMR
) to this pin to create timing delays at start-up,
when power good outputs pull down, during PGI check
and when auto-retrying after faults (except overvoltage
fault). Internal pull-up currents of 10µA and 5µA and
pull-down currents of 5µA and 12mA configure the delay
periods as multiples of a nominal delay of 256ms • C
TMR
/
µF. Delays for start-up and auto-retry following undervolt-
age or power bad fault are the same as the nominal delay.
Delays for sequenced power good outputs are twice of the
nominal delay
. Delays for
PGI check and auto-retry fol
-
lowing overcurrent fault are four times the nominal delay.
(SSOP/QFN)
PIN FUNCTIONS
