Datasheet
9
LT1611
OPERATIO
U
Start-Up/Soft-Start
The LT1611, starting from V
OUT
= 0V, reaches final voltage
in approximately 450µs after SHDN is pulled high, with
C
OUT
= 22µF, V
IN
= 5V and V
OUT
= – 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 R
SS
and C
SS
, as shown in Figure 14,
can be used to limit inrush current to a lower value. Figure
15 pictures V
OUT
and input current, starting into a 33Ω
load, with R
SS
of 33kΩ and C
SS
of 33nF. Input current,
V
IN
V
IN
5V
V
SS
V
OUT
V
OUT
–5V
1611 F14
SW
L1A
22µH
CURRENT
PROBE
L1B
22µH
D1
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)
C1
22µF
R
SS
33k
D2
1N4148
C3
22µF
C2
1µF
R2
10k
R1
29.4k
C
P
1200pF
NFB
SHDN
+
C
SS
33nF/0.1µF
Figure 14. R
SS
and C
SS
at SHDN Pin Provide Soft-Start to LT1611 Inverting Converter
measured at V
IN
, is limited to a peak value of 450mA as the
time required to reach final value increases to 700µs. In
Figure 16, C
SS
is increased to 0.1µF, resulting in a lower
peak input current of 240mA with a V
OUT
ramp time of
2.1ms. C
SS
can be increased further for an even slower
ramp, if desired. Diode D2 serves to quickly discharge C
SS
when V
SS
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. R
SS
= 33k, C
SS
= 33nF; V
OUT
Reaches
–5V in 750µs; Input Current Peaks at 450mA
V
OUT
2V/DIV
I
IN
200mA/DIV
LOAD = 150mA 500µs/DIV
1611 F15
V
S
5V/DIV
Figure 16. R
SS
= 33k, C
SS
= 0.1µF; V
OUT
Reaches
–5V in 2.1ms; Input Current Peaks at 240mA
V
OUT
2V/DIV
I
IN
200mA/DIV
LOAD = 150mA 500µs/DIV
1611 F16
V
S
5V/DIV