LT3430/LT3430-1
25
34301fa
APPLICATIONS INFORMATION
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 diffi culty in calculating the minimum inductor size
needed is that you must fi rst decide whether the switcher
will be in continuous or discontinuous mode at the critical
point where switch current reaches 3A. The fi rst 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 40V to –12V converter using the LT3430/LT3430-
1 with peak switch current of 3A and a catch diode of
0.52V:
IA
CONT
=
+++
=
()()
()( .)
.
40 3
440124012052
1 148
22
For a load current of 0.5A, this says that discontinuous
mode can be used and the minimum inductor needed is
found from:
LH
MIN
==µ
212 05
200 10 3
67
32
()(.)
(•)()
.
In practice, the inductor should be increased by about
30% over the calculated minimum to handle losses and
variations in value. This suggests a minimum inductor of
10µH for this application.
Ripple Current in the Input and Output Capacitors
Positive-to-negative converters have high ripple current in
the input capacitor. For long capacitor lifetime, the RMS
value of this current must be less than the high frequency
ripple current rating of the capacitor. The following formula
will give an approximate value for RMS ripple current. This
formula assumes continuous mode and large inductor
value. Small inductors will give somewhat higher ripple
current, especially in discontinuous mode. The exact for-
mulas are very complex and appear in Application Note
44, pages 29 and 30. For our purposes here I have simply
added a fudge factor (ff). The value for ff is about 1.2 for
higher load currents and L ≥15µH. It increases to about
2.0 for smaller inductors at lower load currents.
Capacitor I ff I
V
V
RMS OUT
OUT
IN
= ()( )
ff = 1.2 to 2.0
The output capacitor ripple current for the positive-to-
negative converter is similar to that for a typical buck
regulator—it is a triangular waveform with peak-to-peak
OUTPUT**
–12V, 0.5A
INPUT
5.5V TO
44V
3430 F15
C2
0.68µF
C
C
R
C
D1
30BQ060
R1
36.5k
C1
100µF
16V TANT
C3
4.7µF
100V
CER
D2
†
MMSD914TI
L1*
10µH
C
F
BOOST
LT3430
V
IN
V
SW
FB
GND
V
C
D3
30BQ015
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
+
D4
7V
Figure 15. Positive-to-Negative Converter
