LT1611 View Datasheet(PDF) - Linear Technology
Part Name
Description
Manufacturer
LT1611 Datasheet PDF : 12 Pages
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OPERATIO
Start-Up/Soft-Start
The LT1611, starting from VOUT = 0V, reaches final voltage
in approximately 450µs 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,
LT1611
measured at VIN, is limited to a peak value of 450mA as the
time required to reach final value increases to 700µs. 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.
CURRENT
L1A
VIN
PROBE
22µH
C2
1µF
5V
L1B
22µH
+
C1
VIN
SW
D1
22µF
RSS
33k
LT1611
R1
29.4k
CP
1200pF
VSS
SHDN
NFB
GND
R2
10k
D2
1N4148
CSS
33nF/0.1µF
VOUT
C1: AVX TAJB226M010
C2: TAIYO YUDEN LMK212BJ105MG
C3: TAIYO YUDEN JMK325BJ226MM (1210 SIZE)
D1: MBR0520
L1: SUMIDA CLS62-220 OR 2× MURATA LQH3C220 (UNCOUPLED)
VOUT
–5V
C3
22µF
1611 F14
Figure 14. RSS and CSS at SHDN Pin Provide Soft-Start to LT1611 Inverting Converter
VOUT
2V/DIV
IIN
200mA/DIV
VS
5V/DIV
LOAD = 150mA
500µs/DIV
1611 F15
Figure 15. RSS = 33k, CSS = 33nF; VOUT Reaches
– 5V in 750µs; Input Current Peaks at 450mA
VOUT
2V/DIV
IIN
200mA/DIV
VS
5V/DIV
LOAD = 150mA
500µs/DIV
1611 F16
Figure 16. RSS = 33k, CSS = 0.1µF; VOUT Reaches
– 5V in 2.1ms; Input Current Peaks at 240mA
9
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