lt3686a Linear Technology Corporation, lt3686a Datasheet - Page 19

• Manufacturer Part Number: lt3686a
• Description: 37v/1.2a Step-down Regulator In 3mm
• Manufacturer: Linear Technology Corporation

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
1.2nF
Figure 11. LT3686A Confi gured to Track Voltage on SS Pin
SS
SS
500mV/DIV
GND
GND
2V/DIV
V
V
OUT
SS
500mA/DIV
500mA/DIV
10V/DIV
10V/DIV
2V/DIV
2V/DIV
V
V
V
V
OUT
OUT
SW
SW
I
I
L
L
V
V
L = 6.8μH
C
C
V
V
L = 6.8μH
C
C
IN
OUT
OUT
SS
IN
OUT
OUT
SS
1ms/DIV
= 10V
= 10V
= 0
= 1.2nF
= 3.3V
= 22μF
= 3.3V
= 22μF
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.
50μs/DIV
5μs/DIV
3686A F11
3686A F12
LT3686A
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