LT3431 LINER [Linear Technology], LT3431 Datasheet - Page 25

• Manufacturer Part Number: LT3431
• Description: High Voltage, 3A,500kHz Step-Down Switching Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
POSITIVE-TO-NEGATIVE CONVERTER
The circuit in Figure 15 is a positive-to-negative topology
using a grounded inductor. It differs from the standard
approach in the way the IC chip derives its feedback signal
because the LT3431 accepts only positive feedback sig-
nals. The ground pin must be tied to the regulated negative
output. A resistor divider to the FB pin then provides the
proper feedback voltage for the chip.
The following equation can be used to calculate maximum
load current for the positive-to-negative converter:
Example: with V
V
** MAXIMUM LOAD CURRENT DEPENDS ON MINIMUM INPUT VOLTAGE
F
* INCREASE L1 FOR HIGHER CURRENT APPLICATIONS.
INPUT
I
V
V
V
0.15 = Switch voltage drop at 3A
I
= 0.52V, I
SEE APPLICATIONS INFORMATION
MAX
P
AND INDUCTOR SIZE. SEE APPLICATIONS INFORMATION
12V
IN
OUT
F
2.2 F
= Maximum rated switch current
CER
25V
= Catch diode forward voltage
C3
= Minimum input voltage
= Output voltage
Figure 15. Positive-to-Negative Converter
I
P
P
V
–
= 3A: I
GND
IN
2
IN(MIN)
C
(
LT3431
BOOST
(
F
V
V
OUT
U
OUT
(
V
MAX
IN
V
C
SW
)(
= 5.5V, V
FB
R
U
V
= 0.6A.
C
C
V
V
C
OUT
IN
IN
)( )( )
– . )(
)
f L
0 15
D1
30BQ060
C2
0.22 F
OUT
MMSD914TI
W
(
D2
V
V
= 12V, L = 3.9 H,
3.9 H
†
OUT
OUT
L1*
36.5k
R2
4.12k
R1
)(
+
V
IN
V
U
F
– . )
)
C1
100 F
16V TANT
OUTPUT**
–12V, 0.5A
0 15
3431 F15
Inductor Value
The criteria for choosing the inductor is typically based on
ensuring that peak switch current rating is not exceeded.
This gives the lowest value of inductance that can be used,
but in some cases (lower output load currents) it may give
a value that creates unnecessarily high output ripple
voltage.
The difficulty in calculating the minimum inductor size
needed is that you must first decide whether the switcher
will be in continuous or discontinuous mode at the critical
point where switch current reaches 3A. The first step is to
use the following formula to calculate the load current
above which the switcher must use continuous mode. If
your load current is less than this, use the discontinuous
mode formula to calculate minimum inductor needed. If
load current is higher, use the continuous mode formula.
Output current where continuous mode is needed:
Minimum inductor discontinuous mode:
Minimum inductor continuous mode:
For a 12V to –12V converter using the LT3431 with peak
switch current of 3A and a catch diode of 0.52V:
I
L
I
L
CONT
CONT
MIN
MIN
2
2
( )(
(
V
f V
4
4 12 12 12 12 0 52
OUT
( )( )
(
(
f I
V
IN
IN
P
)(
I
2
OUT
V
V
OUT
( ) ( )
OUT
12 3
)(
(
)
V
IN
2
)
)(
) ( )
(
I
V
2
V
P
IN
IN
2
I
–
P
)(
I
OUT
2
V
V
OUT
OUT
. )
1
)
V
(
V
F
0 742
OUT
)
LT3431
.
V
IN
sn3431 3431fs
A
25
V
F
)