Datasheet

LT3686A
19
3686afa
ApplicAtions inForMAtion
Soft-Start
The SS pin is used to soft-start the LT3686A, eliminating
input current surge during start-up. It can also be used to
track another voltage in the system (Figure 11).
An internal 2µA current source charges an external soft-
start capacitor to generate a voltage ramp. FB voltage is
Figure 12. To Soft Start the LT3686A, Add a Capacitor to the SS Pin
Figure 11. LT3686A Configured to Track Voltage on SS Pin
V
SS
500mV/DIV
V
OUT
2V/DIV
1ms/DIV
3686A F11
V
SW
10V/DIV
V
OUT
2V/DIV
I
L
500mA/DIV
5µs/DIV
V
IN
= 10V
V
OUT
= 3.3V
L = 6.8µH
C
OUT
= 22µF
C
SS
= 0
SS
GND
SS
1.2nF
GND
V
SW
10V/DIV
V
OUT
2V/DIV
I
L
500mA/DIV
50µs/DIV
3686A F12
V
IN
= 10V
V
OUT
= 3.3V
L = 6.8µH
C
OUT
= 22µF
C
SS
= 1.2nF
regulated to the voltage at the SS pin until it exceeds 0.8V,
FB is then regulated to the reference 0.8V. Soft-start also
reduces the oscillator frequency to avoid hitting current
limit during start-up. Figure 12 shows the start-up wave-
forms with and without the soft-start circuit.