Datasheet
LT3430/LT3430-1
23
34301fa
APPLICATIONS INFORMATION
LT3430/LT3430-1 will consume their quiescent operating
current of 1.5mA. The V
IN
pin will also source current to
any other components connected to the input line. If this
load is greater than 10mA or the input could be shorted to
ground, a series Schottky diode must be added, as shown
in Figure 12. With these safeguards, the output can be held
at voltages up to the V
IN
absolute maximum rating.
BUCK CONVERTER WITH ADJUSTABLE SOFT-START
Large capacitive loads or high input voltages can cause
high input currents at start-up. Figure 13 shows a circuit
that limits the dv/dt of the output at start-up, controlling
the capacitor charge rate. The buck converter is a typical
confi guration with the addition of R3, R4, C
SS
and Q1. As
the output starts to rise, Q1 turns on, regulating switch
current via the V
C
pin to maintain a constant dv/dt at the
output. Output rise time is controlled by the current through
C
SS
defi ned by R4 and Q1’s V
BE
. Once the output is in
regulation, Q1 turns off and the circuit operates normally.
R3 is transient protection for the base of Q1.
Rise Time
RC V
V
SS OUT
BE
=
()( )( )
4
Using the values shown in Figure 10,
Rise Time ms=
()( )
()
=
47 10 15 10 5
07
5
39
••
.
–
The ramp is linear and rise times in the order of 100ms are
possible. Since the circuit is voltage controlled, the ramp
rate is unaffected by load characteristics and maximum
output current is unchanged. Variants of this circuit can
be used for sequencing multiple regulator outputs.
5V, 2A
REMOVABLE
INPUT
C2
0.68µF
C
F
220pF
54k
D1
30BQ060
3430 F12
C3
4.7µF
R
C
3.3k
C
C
0.022µF
D3
30BQ060
D2
MMSD914TI
33µH
C1
100µF
10V
ALTERNATE
SUPPLY
25k
R1
15.4k
R2
4.99k
BOOST
V
IN
LT3430
SHDN
SYNC
SW
BIAS
FB
V
C
GND
+
OUTPUT
5V
2A
INPUT
40V
3430 F13
C2
0.68µF
C1
100µF
10V
C
SS
15nF
C
F
220pF
D1
30BQ060
OR B250A
C3
4.7µF
50V
CER
D2
MMSD914TI
L1
33µH
R1
15.4k
R3
2k
C
C
0.022µF
R2
4.99k
R4
47k
L1: CDRH104R-220M
Q1
BOOST BIAS
V
IN
LT3430
SHDN
SYNC
SW
FB
V
C
GND
+
R
C
3.3k
Figure 12. Dual Source Supply with 25µA Reverse Leakage
Figure 13. Buck Converter with Adjustable Soft-Start