LT3430 Linear Technology, LT3430 Datasheet - Page 25

• Manufacturer Part Number: LT3430
• Description: High Voltage/ 3A/ 200kHz Step-Down Switching Regulator
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
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:
** MAXIMUM LOAD CURRENT DEPENDS ON MINIMUM INPUT VOLTAGE
5.5V TO
INPUT
I
L
* INCREASE L1 FOR HIGHER CURRENT APPLICATIONS.
†
CONT
MIN
SEE APPLICATIONS INFORMATION
FOR V
PATH OF D2 IS REQUIRED TO ENSURE BOOST PIN MAXIMUM RATING IS
NOT EXCEEDED. SEE APPLICATIONS INFORMATION (BOOST PIN VOLTAGE)
AND INDUCTOR SIZE. SEE APPLICATIONS INFORMATION
48V
4.7 F
100V
CER
†
C3
IN
> 44V AND V
Figure 15. Positive-to-Negative Converter
2
(
V
4
OUT
( )( )
V
(
f I
IN
GND
V
IN
C
OUT
LT3430
P
BOOST
)(
F
U
I
= –12V, ADDITIONAL VOLTAGE DROP IN THE
OUT
2
V
OUT
V
V
C
(
SW
)
FB
V
R
U
C
IN
C
)(
C
) ( )
V
2
IN
I
P
D1
30BQ060
C2
0.68 F
2
MMSD914TI
V
W
OUT
D2
10 H
†
L1*
R2
4.12k
36.5k
R1
V
F
+
)
U
C1
100 F
16V TANT
OUTPUT**
–12V, 0.5A
3430 F15
Minimum inductor continuous mode:
For a 40V to –12V converter using the LT3430 with peak
switch current of 3A and a catch diode of 0.52V:
For a load current of 0.5A, this says that discontinuous
mode can be used and the minimum inductor needed is
found from:
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
10 H 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 for-
mula will give an approximate value for RMS ripple cur-
rent. This formula assumes continuous mode and large
inductor value . Small inductors will give somewhat higher
ripple current, especially in discontinuous mode. The
exact formulas 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 15 H. It increases to
about 2.0 for smaller inductors at lower load currents.
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
L
I
L
Capacitor I
CONT
MIN
MIN
2
(
200 10 3
( )(
f V
4 40 12 40 12 0 52
2 12 0 5
RMS
( )( . )
(
•
IN
3
V
)( )
( )(
OUT
ff I
( ) ( )
40 3
)(
2
OUT
)
2
(
I
V
P
6 7
IN
)
2
.
–
)(
I
V
OUT
V
V
OUT
H
OUT
IN
. )
1
)
(
V
1 148
OUT
LT3430
.
V
IN
sn3430 3430is
25
A
V
F
)