Datasheet

LTC3125
6
3125fa
PIN FUNCTIONS
GND (Pin 1, Exposed Pad Pin 9): Ground. The exposed
pad must be soldered to the PCB ground plane for electrical
connection and for rated thermal performance.
FB (Pin 2): Feedback Input to the Error Amplifi er. Connect
the resistor divider tap to this pin. The top of the divider
connects to V
OUT
and the bottom of the divider connects
to GND. The output voltage can be adjusted from 1.8V
to 5.25V.
PROG (Pin 3): Programming Input for Average Input Cur-
rent. This pin should be connected to ground through an
external resistor (R
PROG
) to set input average current limit
threshold. Refer to the Component Selection section in
Applications Information for details on selecting R
PROG
.
V
IN
(Pin 4): Input Voltage. The device is powered from V
IN
until V
OUT
exceeds V
IN
. Once V
OUT
is greater than (V
IN
+
0.25V), the device is powered from V
OUT
. Place a ceramic
bypass capacitor from V
IN
to GND. A minimum value of
1µF is recommended. Also connects to CS through 60m
internal sense resistor.
CS (Pin 5): Current Sense Resistor Connection Point.
Connect the inductor directly to CS. An internal 60m
sense resistor is connected between CS and V
IN
.
SHDN (Pin 6): Logic Controlled Shutdown Input. Bringing
this pin above 1V enables the part, forcing this pin below
0.35V disables the part.
V
OUT
(Pin 7): Output Voltage Sense and the Output of the
Synchronous Rectifi er. Connect the output fi lter capacitor
from V
OUT
to GND, close to the IC. A minimum value of
150µF is recommended. Due to the output disconnect
feature, V
OUT
is disconnected from V
IN
when SHDN is low.
SW (Pin 8): Switch Pin. Connect an inductor from this
pin to CS. An internal anti-ringing resistor is connected
across SW and CS after the inductor current has dropped
near zero.
TYPICAL PERFORMANCE CHARACTERISTICS
(T
A
= 25°C unless otherwise noted)
V
OUT
and I
IN
During Soft-Start
V
OUT
2V/DIV
SHDN
5V/DIV
INPUT
CURRENT
200mA/DIV
2s/DIVV
IN
= 3.3V
V
OUT
= 4.5V
C
OUT
= 1F
L = 2.7µH
3125 G20
Effi ciency vs V
IN
V
IN
(V)
2
50
EFFICIENCY (%)
60
65
70
75
80
85
3
4
3125 G21
90
95
100
55
5
I
LOAD
= 200mA
V
OUT
= 3.8V