LT1611 Linear Technology, LT1611 Datasheet - Page 9

• Manufacturer Part Number: LT1611
• Description: Constant-Current/ Constant-Voltage 1.4MHz Step-Up DC/DC Converter
• Manufacturer: Linear Technology

OPERATIO
Start-Up/Soft-Start
The LT1611, starting from V
in approximately 450 s after SHDN is pulled high, with
C
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
can be used to limit inrush current to a lower value. Figure
15 pictures V
load, with R
OUT
200mA/DIV
= 22 F, V
2V/DIV
5V/DIV
Figure 15. R
– 5V in 750 s; Input Current Peaks at 450mA
V
OUT
V
I
IN
S
LOAD = 150mA
SS
OUT
IN
of 33k and C
= 5V and V
U
and input current, starting into a 33
SS
= 33k, C
SS
Figure 14. R
V
SS
and C
OUT
33nF/0.1 F
OUT
SS
500 s/DIV
V
1N4148
5V
= – 5V. Charging the output
= 33nF; V
SS
SS
IN
R
33k
= 0V, reaches final voltage
D2
C
SS
SS
SS
, as shown in Figure 14,
of 33nF. Input current,
V
and C
OUT
OUT
+
SS
CURRENT
C1: AVX TAJB226M010
C2: TAIYO YUDEN LMK212BJ105MG
C3: TAIYO YUDEN JMK325BJ226MM (1210 SIZE)
D1: MBR0520
L1: SUMIDA CLS62-220 OR 2 MURATA LQH3C220 (UNCOUPLED)
PROBE
Reaches
at SHDN Pin Provide Soft-Start to LT1611 Inverting Converter
C1
22 F
V
SHDN
1611 F15
IN
LT1611
22 H
GND
L1A
NFB
SW
measured at V
time required to reach final value increases to 700 s. In
Figure 16, C
peak input current of 240mA with a V
2.1ms. C
ramp, if desired. Diode D2 serves to quickly discharge C
when V
omitted, resulting in a “soft-stop” slow discharge of the
output capacitor.
R1
29.4k
R2
10k
200mA/DIV
1 F
2V/DIV
5V/DIV
SS
C2
Figure 16. R
– 5V in 2.1ms; Input Current Peaks at 240mA
V
SS
OUT
I
V
is driven low to shut down the device. D2 can be
IN
S
LOAD = 150mA
SS
can be increased further for an even slower
C
1200pF
P
IN
is increased to 0.1 F, resulting in a lower
, is limited to a peak value of 450mA as the
SS
D1
= 33k, C
22 H
L1B
SS
500 s/DIV
C3
22 F
= 0.1 F; V
1611 F14
V
–5V
OUT
OUT
OUT
Reaches
ramp time of
LT1611
1611 F16
9
SS