LT1375HV Linear Technology, LT1375HV Datasheet - Page 25

• Manufacturer Part Number: LT1375HV
• Description: 1.5A/ 500kHz Step-Down Switching Regulators
• Manufacturer: Linear Technology

The difficulty in calculating the minimum inductor size
needed is that you must first know whether the switcher
will be in continuous or discontinuous mode at the critical
point where switch current is 1.5A. The first step is to use
the following formula to calculate the load current where
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.
INDUCTOR VALUE
Unlike buck converters, positive-to-negative converters
cannot use large inductor values to reduce output ripple
voltage. At 500kHz, values larger than 25 H make almost
no change in output ripple. The graph in Figure 19 shows
peak-to-peak output ripple voltage for a 5V to – 5V con-
verter versus inductor value. The criteria for choosing the
inductor is therefore 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. A com-
promise value is often chosen that reduces output ripple.
As you can see from the graph, large inductors will not
give arbitrarily low ripple, but small inductors can give
high ripple.
APPLICATIONS
Figure 19. Ripple Voltage on Positive-to-Negative Converter
150
120
90
60
30
0
0
U
5
INDUCTOR SIZE ( H)
INFORMATION
U
10
5V TO –5V CONVERTER
OUTPUT CAPACITOR
ESR = 0.1
I
LOAD
I
15
LOAD
= 0.25A
W
= 0.1A
20
1375/76 F19
25
U
Output current where continuous mode is needed:
Minimum inductor discontinuous mode:
Minimum inductor continuous mode:
For the example above, with maximum load current of
0.25A:
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
3 H for this application, but looking at the ripple voltage
chart shows that output ripple voltage could be reduced by
a factor of two by using a 15 H inductor. There is no rule
I
I
L
L
L
CONT
CONT
MIN
MIN
MIN
2
2
500 10
V
4
f V
4 5 5 5 5 0 5
OUT
2 5 0 25
f I
V
IN
IN
•
P
5 1 5
I
2
OUT
V
3
V
2
.
OUT
OUT
V
1 5
IN
.
2
V
I
2
V
2
P
IN
IN
I
P
LT1375/LT1376
I
2
2 2
OUT
V
V
.
OUT
OUT
0 37
H
1
V
A
F
V
OUT
V
IN
25
V
F