Datasheet

LTC3458L
7
3458lfa
UU
U
PI FU CTIO S
SW (Pin 1): Switch Pin for Inductor Connection. During
discontinuous conduction mode an antiring resistor con-
nects SW to V
IN
to reduce noise.
V
IN
(Pin 2): Input Supply Pin. Connect this to the input
supply and decouple with 1μF minimum.
SYNC (Pin 3): Oscillator Synchronization Pin. A clock
pulse width of 100ns to 2μs is required to synchronize the
internal oscillator. This pin is disabled when grounded.
SHDN (Pin 4): Shutdown Pin. Grounding this pin shuts
down the IC. Connect to >1.25V to enable.
I
LIM
(Pin 5): Adjustable Peak Current Limit. Connect a
resistor from I
LIM
to GND to program the peak inductor
current according to the following formula:
I
R
LIMIT
ILIM
=
180
where I
LIMIT
is in amps and R
ILIM
is in kΩ.
R
T
(Pin 6): Connect a resistor to ground to program the
oscillator frequency, according to the formula:
f
R
OSC
T
=
+
1
02 0004..•
where f
OSC
is in MHz and R
T
is in kΩ.
Oscillator frequency can be programmed between 400kHz
and 1.5MHz.
FB (Pin 7): Connect Resistor Divider Tap Here. The output
voltage can be adjusted from 2V to 6V. Feedback reference
voltage is typically 1.225V.
COMP (Pin 8): g
m
Error Amp Output. A frequency com-
pensation network is connected from this pin to ground to
compensate the loop. See the section “Compensating the
Feedback Loop” for guidelines.
GND (Pin 9): Signal Ground Pin.
SS (Pin 10): Connect a capacitor between this pin and
ground to set soft-start period. 5μA of current is sourced
from SS during soft-start.
t(msec) = C
SS
(μF )• 200
BURST (Pin 11): Burst Mode Threshold Adjust Pin. A
resistor/capacitor combination from this pin to ground
programs the average load current at which automatic
Burst Mode operation is entered, according to the formula:
R
I
BURST
BURST
=
10
where R
BURST
is in kΩ and I
BURST
is in amps.
C
CV
BURST
OUT OUT
=
•
,10 000
where C
BURST(MIN)
and C
OUT
are in μF.
To force fixed frequency PWM mode, connect BURST to
V
OUT
through a 51k resistor.
V
OUT
(Pin 12): Output of the Synchronous Rectifier and
Internal Gate Drive Source for the Power Switches.
V
R
R
OUT
=+
⎛
⎝
⎜
⎞
⎠
⎟
1 225 1
1
2
.
Exposed Pad (PGND) (Pin 13): Must be soldered to PCB
ground, for electrical contact and optimum thermal
performance.