LT1611CS5 datasheet(9/12 Pages) LINER

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LT1611

OPERATIO

Start-Up/Soft-Start

The LT1611, starting from VOUT = 0V, reaches final voltage

in approximately 450

µs after SHDN is pulled high, with

COUT = 22µF, VIN = 5V and VOUT = –5V. Charging the output

capacitor at this speed requires an inrush current of over

1A. If a longer start-up time is acceptable, a soft-start

circuit consisting of RSS and CSS, as shown in Figure 14,

can be used to limit inrush current to a lower value. Figure

15 pictures VOUT and input current, starting into a 33Ω

load, with RSS of 33kΩ and CSS of 33nF. Input current,

VIN

VSS

VOUT

–5V

1611 F14

L1A

µH

CURRENT

PROBE

L1B

µH

GND

LT1611

C1: AVX TAJB226M010

C2: TAIYO YUDEN LMK212BJ105MG

C3: TAIYO YUDEN JMK325BJ226MM (1210 SIZE)

D1: MBR0520

L1: SUMIDA CLS62-220 OR 2

× MURATA LQH3C220 (UNCOUPLED)

µF

RSS

33k

1N4148

µF

10k

29.4k

1200pF

NFB

SHDN

CSS

33nF/0.1

µF

Figure 14. RSS and CSS at SHDN Pin Provide Soft-Start to LT1611 Inverting Converter

measured at VIN, is limited to a peak value of 450mA as the

time required to reach final value increases to 700

µs. In

Figure 16, CSS is increased to 0.1µF, resulting in a lower

peak input current of 240mA with a VOUT ramp time of

2.1ms. CSS can be increased further for an even slower

ramp, if desired. Diode D2 serves to quickly discharge CSS

when VSS is driven low to shut down the device. D2 can be

omitted, resulting in a “soft-stop” slow discharge of the

output capacitor.

Figure 15. RSS = 33k, CSS = 33nF; VOUT Reaches

– 5V in 750

µs; Input Current Peaks at 450mA

VOUT

2V/DIV

IIN

200mA/DIV

LOAD = 150mA

500

µs/DIV

1611 F15

5V/DIV

Figure 16. RSS = 33k, CSS = 0.1µF; VOUT Reaches

– 5V in 2.1ms; Input Current Peaks at 240mA

VOUT

2V/DIV

IIN

200mA/DIV

LOAD = 150mA

500

µs/DIV

1611 F16

5V/DIV

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