LT1374C Linear Technology, LT1374C Datasheet - Page 25

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

Minimum inductor continuous mode:
For the example above, with maximum load current of 1A:
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 varia-
tions in value. This suggests a minimum inductor of 1.3 H
for this application, but looking at the ripple voltage chart
shows that output ripple voltage could be reduced by a fac-
tor of two by using a 15 H inductor. There is no rule of thumb
here to make a final decision. If modest ripple is needed and
the larger inductor does the trick, go for it. If ripple is non-
critical use the smaller inductor. If ripple is extremely criti-
cal, a second filter may have to be added in any case, and
the lower value of inductance can be used. Keep in mind
that the output capacitor is the other critical factor in deter-
mining output ripple voltage. Ripple shown on the graph
(Figure 16) is with two parallel capacitor’s ESR of 0.1 . This
is reasonable for AVX type TPS “D” or “E” size surface mount
solid tantalum capacitors, but the final capacitor chosen
must be looked at carefully for ESR characteristics.
APPLICATIONS
L
I
L
CONT
MIN
MIN
2
500 10
f V
4 5 5 5 5 5 5 0 5
2 5 1
•
IN
.
U
3
V
5 5
OUT
.
4 5
2
INFORMATION
.
U
.
4 5
I
V
P
2
.
IN
2
I
1
OUT
V
OUT
H
W
.
1
V
1 15
OUT
.
V
IN
U
A
V
F
Ripple Current in the Input and Output Capacitors
Positive-to-negative converters have high ripple current in
both the input and output capacitors. 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 discontinu-
ous mode. The exact formulas are very complex and
appear in Application Note 44, pages 30 and 31. 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 10 H. It increases to about 2.0 for smaller inductors at
lower load currents.
ff = Fudge factor (1.2 to 2.0)
Diode Current
Average diode current is equal to load current. Peak diode
current will be considerably higher.
Peak diode current:
Keep in mind that during start-up and output overloads,
average diode current may be much higher than with
normal loads. Care should be used if diodes rated less than
3A are used, especially if continuous overload conditions
must be tolerated.
Capacitor
Continuous
I
Discontinuous
OUT
V
IN
V
I
RMS
IN
V
Mode
OUT
Mode =
ff I
2
OUT
L f V
V
2 I
IN
V
V
OUT
OUT
IN
IN
V
L f
OUT
V
V
OUT
OUT
LT1374
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