Datasheet

LT8609/LT8609A/LT8609B
10
Rev. H
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PIN FUNCTIONS
BST: This pin is used to provide a drive voltage, higher
than the input voltage, to the topside power switch. Place
a 0.1µF boost capacitor as close as possible to the IC. Do
not place a resistor in series with this pin.
SW: The SW pin is the output of the internal power
switches. Connect this pin to the inductor and boost
capacitor. This node should be kept small on the PCB for
good performance.
INTV
CC
: Internal 3.5V Regulator Bypass Pin. The inter-
nal power drivers and control circuits are powered from
this voltage. INTV
CC
max output current is 20mA. Voltage
on INTV
CC
will vary between 2.8V and 3.5V. Decouple
this pin to power ground with at least a 1μF low ESR
ceramic capacitor. Do not load the INTV
CC
pin with exter-
nal circuitry.
RT: A resistor is tied between RT and ground to set the
switching frequency.
SYNC: External Clock Synchronization Input. Ground
this pin for low ripple Burst Mode operation at low out-
put loads. Tie to a clock source for synchronization to
an external frequency. Leave floating for pulse-skipping
mode with no spread spectrum modulation. Tie to INTV
CC
or tie to a voltage between 3.2V and 5.0V for pulse-skip-
ping mode with spread spectrum modulation. When in
pulse-skipping mode, the I
Q
will increase to several mA. In
the LT8609B, the SYNC pin is replaced with a no connect;
the LT8609B operates in pulse-skipping mode without
spread spectrum.
FB: The LT8609/LT8609A/LT8609B regulates the FB pin
to 0.782V. Connect the feedback resistor divider tap to
this pin.
TR/SS: Output Tracking and Soft-Start Pin. This pin allows
user control of output voltage ramp rate during start-up.
A TR/SS voltage below 0.782V forces the LT8609/
LT8609A/LT8609B to regulate the FB pin to equal the
TR/SS pin voltage. When TR/SS is above 0.782V, the
tracking function is disabled and the internal reference
resumes control of the error amplifier. An internal 2μA
pull-up current from INTV
CC
on this pin allows a capacitor
to program output voltage slew rate. This pin is pulled
to ground with a 300Ω MOSFET during shutdown and
fault conditions; use a series resistor if driving from a low
impedance output.
PG: The PG pin is the open-drain output of an internal
comparator. PG remains low until the FB pin is within
±8.5% of the final regulation voltage, and there are no
fault conditions. PG is valid when V
IN
is above 3.2V,
regardless of EN/UV pin state.
V
IN
: The V
IN
pin supplies current to the LT8609/LT8609A/
LT8609B internal circuitry and to the internal topside
power switch. This pin must be locally bypassed. Be sure
to place the positive terminal of the input capacitor as
close as possible to the V
IN
pins, and the negative capaci-
tor terminal as close as possible to the GND pins.
EN/UV: The LT8609/LT8609A/LT8609B is shut down
when this pin is low and active when this pin is high. The
hysteretic threshold voltage is 1.05V going up and 1.00V
going down. Tie to V
IN
if the shutdown feature is not used.
An external resistor divider from V
IN
can be used to pro-
gram a V
IN
threshold below which the LT8609/LT8609A/
LT8609B will shut down.
GND: Exposed Pad Pin. The exposed pad must be con
-
nected to the negative terminal of the input capacitor
and soldered to the PCB in order to lower the thermal
resistance.
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