Datasheet

25
LT3431
sn3431 3431fs
APPLICATIO S I FOR ATIO
WUUU
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:
I
I
VV
VVfL
VV
VV VV
MAX
P
IN OUT
OUT IN
OUT IN
OUT IN OUT F
=
+






++
–
()( )
()()()
()(–.)
(–.)()
2
015
015
I
P
= Maximum rated switch current
V
IN
= Minimum input voltage
V
OUT
= Output voltage
V
F
= Catch diode forward voltage
0.15 = Switch voltage drop at 3A
Example: with V
IN(MIN)
= 5.5V, V
OUT
= 12V, L = 3.9µH,
V
F
= 0.52V, I
P
= 3A: I
MAX
= 0.6A.
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:
I
VI
VV VV V
CONT
IN P
IN OUT IN OUT F
>
+++
()()
()( )
22
4
Minimum inductor discontinuous mode:
L
VI
fI
MIN
OUT OUT
P
=
2
2
()()
()( )
Minimum inductor continuous mode:
L
VV
fV V I I
VV
V
MIN
IN OUT
IN OUT P OUT
OUT F
IN
=
++
+














()( )
()( ) –
()
21
For a 12V to –12V converter using the LT3431 with peak
switch current of 3A and a catch diode of 0.52V:
IA
CONT
=
+++
=
()()
()( .)
.
12 3
412121212052
0 742
22
OUTPUT**
–12V, 0.5A
INPUT
12V
3431 F15
C2
0.22µF
C
C
R
C
D1
30BQ060
R1
36.5k
C1
100µF
16V TANT
C3
2.2µF
25V
CER
D2
†
MMSD914TI
L1*
3.9µH
C
F
BOOST
LT3431
V
IN
SW
FB
GND
V
C
R2
4.12k
* INCREASE L1 FOR HIGHER CURRENT APPLICATIONS.
SEE APPLICATIONS INFORMATION
** MAXIMUM LOAD CURRENT DEPENDS ON MINIMUM INPUT VOLTAGE
AND INDUCTOR SIZE. SEE APPLICATIONS INFORMATION
+
Figure 15. Positive-to-Negative Converter