lt3431efe-trpbf Linear Technology Corporation, lt3431efe-trpbf Datasheet - Page 22

• Manufacturer Part Number: lt3431efe-trpbf
• Description: High Voltage, 3a, 500khz Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

LT3431
APPLICATIO S I FOR ATIO
tantalum output capacitor with a ceramic output capacitor
because of its very low ESR. The zero provided by the
tantalum output capacitor must now be reinserted back
into the loop. Alternatively there may be cases where, even
with the tantalum output capacitor, an additional zero is
required in the loop to increase phase margin for improved
transient response.
A zero can be added into the loop by placing a resistor, R
at the V
or by placing a capacitor, C
FB pin.
When using R
First, the combination of output capacitor ESR and R
stop the loop rolling off altogether. Second, if the loop gain
is not rolled sufficiently at the switching frequency, output
ripple will perturb the V
duty cycle switching similar to subharmonic oscillation. If
needed, an additional capacitor (C
the R
suppress V
With a tantalum output capacitor, the LT3431 already
includes a resistor, R
(see Figures 10 and 11) to compensate the loop over the
entire V
V
be used with a simple adjustment to the resistor R
stable operation. (See Ceramic Capacitors section for
stabilizing LT3431). If additional phase margin is required,
22
IN
-to-V
C
/C
C
IN
OUT
pin in series with the compensation capacitor, C
C
range (to allow for stable pulse skipping for high
C
network from V
ratios 4). A ceramic output capacitor can still
ripple voltage.
Figure 11. Overall Loop Response
–20
–40
C
80
60
40
20
0
, the maximum value has two limitations.
10
V
V
I
C
PHASE
LOAD
IN
OUT
OUT
= 12V
U
= 5V
= 100 F, 10V, 0.1
= 1A
100
C
and filter capacitor, C
GAIN
FREQUENCY (Hz)
C
1k
U
pin enough to cause unstable
FB
C
, between the output and the
10k
pin to ground to further
R
C
C
C
C
F
= 220pF
= 3.3k
= 22nF
F
100k
) can be added across
W
3431 F11
1M
180
150
120
90
60
30
0
F
, at the V
U
C
C
C
may
pin
for
C,
C
a capacitor, C
FB pin but care must be taken for high output voltage
applications. Sudden shorts to the output can create
unacceptably large negative transients on the FB pin.
For V
possible by readjusting the frequency compensation com-
ponents at the V
When checking loop stability, the circuit should be oper-
ated over the application’s full voltage, current and
temperature range. Proper loop compensation may be
obtained by empirical methods as described in detail in
Application Notes 19 and 76.
CONVERTER WITH BACKUP OUTPUT REGULATOR
In systems with a primary and backup supply, for ex-
ample, a battery powered device with a wall adapter input,
the output of the LT3431 can be held up by the backup
supply with the LT3431 input disconnected. In this condi-
tion, the SW pin will source current into the V
SHDN pin is held at ground, only the shut down current of
30 A will be pulled via the SW pin from the second supply.
With the SHDN pin floating, the LT3431 will consume its
quiescent operating current of 1.5mA. The V
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
maximum rating.
REMOVABLE
Figure 12. Dual Source Supply with 25 A Reverse Leakage
INPUT
IN
-to-V
30BQ060
4.7 F
54k
25k
D3
C3
OUT
FB
, can be inserted between the output and
C
ratios <4, higher loop bandwidths are
V
SHDN
SYNC
pin.
GND
0.022 F
IN
LT3431
BOOST
3.3k
C
C
R
C
BIAS
V
SW
C
FB
C
220pF
F
D1
30BQ060
C2
0.22 F
10 H
MMSD914TI
D2
R1
15.4k
R2
4.99k
IN
+
IN
pin will also
IN
5V, 2A
C1
100 F
10V
pin. If the
sn3431 3431fs
absolute
ALTERNATE
SUPPLY
3431 F12