LT1374C Linear Technology, LT1374C Datasheet - Page 24

• Manufacturer Part Number: LT1374C
• Description: 4.5A/ 500kHz Step-Down Switching Regulator
• Manufacturer: Linear Technology

LT1374
APPLICATIONS
Operating duty cycle:
(This formula uses an average value for switch loss, so it
may be several percent in error.)
With the conditions above:
This duty cycle is close enough to 50% that I
assumed to be 4.5A.
OUTPUT DIVIDER
If the adjustable part is used, the resistor connected to
V
calculated from:
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 16 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
24
OUT
DC
DC
R
1
(R2) should be set to approximately 5k. R1 is
R V
V
5 5 0 3 5 0 5
2
IN
.
OUT
2 42
V
0 3
5 0 5
.
OUT
.
– .
2 42
U
V
.
OUT
V
F
INFORMATION
U
.
V
F
51
%
W
U
P
can be
give arbitrarily low ripple, but small inductors can give
high ripple.
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 4.5A. 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.
Output current where continuous mode is needed:
Minimum inductor discontinuous mode:
Figure 16. Ripple Voltage on Positive-to-Negative Converter
I
L
CONT
MIN
250
200
150
100
2
50
0
0
V
4
OUT
f I
V
IN
P
I
2
OUT
5
V
INDUCTOR SIZE ( H)
OUT
V
IN
5V TO – 5V CONVERTER
OUTPUT CAPACITOR’S
ESR = 0.05
10
V
2
IN
I
P
I
I
LOAD
LOAD
2
V
OUT
15
= 1A
= 0.25A
1374 F16
V
F
20