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LT3686

3686fa

ApplicAtions inForMAtion

Soft-Start

The SS pin is used to soft-start the LT3686, 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 LT3686, Add a Capacitor to the SS Pin

Figure 11. LT3686 Configured to Track Voltage on SS Pin

VSS

500mV/DIV

VOUT

2V/DIV

1ms/DIV

3686 F11

VSW

10V/DIV

VOUT

2V/DIV

500mA/DIV

5µs/DIV

VIN = 10V

VOUT = 3.3V

L = 6.8µH

COUT = 22µF

CSS = 0

GND

1.2nF

GND

VSW

10V/DIV

VOUT

2V/DIV

500mA/DIV

50µs/DIV

3686 F12

VIN = 10V

VOUT = 3.3V

L = 6.8µH

COUT = 22µF

CSS = 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.

LT3686 Datasheet(PDF) 19 Page - Linear Technology

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