LTC1530 LINER [Linear Technology], LTC1530 Datasheet - Page 13

• Manufacturer Part Number: LTC1530
• Description: High Power Synchronous Switching Regulator Controller
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
Inductor Selection
The inductor is often the largest component in an LTC1530
design and must be chosen carefully. Choose the inductor
value and type based on output slew rate requirements
and expected peak current. The required output slew rate
primarily controls the inductor value. The maximum rate
of rise of inductor current is set by the inductor’s value, the
input-to-output voltage differential and the LTC1530’s
maximum duty cycle. In a typical 5V input, 2.8V output
application, the maximum rise time will be:
where L is the inductor value in H. With proper frequency
compensation, the combination of the inductor and output
capacitor values determine the transient recovery time. In
general, a smaller value inductor improves transient
response at the expense of ripple and inductor core
saturation rating. A 2 H inductor has a 0.9A/ s rise time
in this application, resulting in a 5.5 s delay in responding
to a 5A load current step. During this 5.5 s, the difference
between the inductor current and the output current is
made up by the output capacitor. This action causes a
temporary voltage droop at the output. To minimize this
effect, the inductor value should usually be in the 1 H to
5 H range for most 5V input LTC1530 circuits. Different
combinations of input and output voltages and expected
loads may require different values.
Once the required inductor value is selected, choose the
inductor core type based on peak current and efficiency
requirements. Peak current in the inductor is equal to the
maximum output load current plus half of the peak-to-
peak inductor ripple current. Inductor ripple current is set
by the inductor’s value, the input voltage, the output
voltage and the operating frequency. If the efficiency is
high, ripple current is approximately equal to:
where
f
L
OSC
O
DC
I
= Inductor value
RIPPLE
= LTC1530 oscillator frequency
MAX
V
IN
V
IN
f
OSC
L
V
OUT
U
V
OUT
L
O
U
V
V
1 85
IN
OUT
L
W
A
s
U
Solving this equation for a typical 5V to 2.8V application
with a 2 H inductor, ripple current is:
Peak inductor current at 11.2A load:
The ripple current should generally fall between 10% and
40% of the output current. The inductor must be able to
withstand this peak current without saturating, and the
copper resistance in the winding should be kept as low as
possible to minimize resistive power loss. Note that in
circuits not employing the current limit function, the
current in the inductor may rise above this maximum
under short circuit or fault conditions; the inductor should
be sized accordingly to withstand this additional current.
Inductors with gradual saturation characteristics (example:
powdered iron) are often the best choice.
Input and Output Capacitors
A typical LTC1530 design places significant demands on
both the input and the output capacitors. During normal
steady load operation, a buck converter like the LTC1530
draws square waves of current from the input supply at the
switching frequency. The peak current value is equal to the
output load current plus 1/2 the peak-to-peak ripple cur-
rent. Most of this current is supplied by the input bypass
capacitor. The resulting RMS current flow in the input
capacitor heats it and causes premature capacitor failure
in extreme cases. Maximum RMS current occurs with
50% PWM duty cycle, giving an RMS current value equal
to I
ripple current rating must be used to ensure reliable
operation. Note that capacitor manufacturers’ ripple cur-
rent ratings are often based on only 2000 hours (3 months)
lifetime at rated temperature. Further derating of the input
capacitor ripple current beyond the manufacturer’s speci-
fication is recommended to extend the useful life of the
circuit. Lower operating temperature has the largest effect
on capacitor longevity.
11 2
OUT
300
2 2
/2. A low ESR input capacitor with an adequate
A
kHz
V
0 56
2
2
A
2
H
12 2
2
A
A
P-P
LTC1530
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