19
LTC1703
1703fa
Calculating RMS Current in C
IN
A buck regulator like the LTC1703 draws pulses of
current from the input capacitor during normal opera-
tion. The input capacitor sees this as AC current, and
dissipates power proportional to the RMS value of the
input current waveform. To properly specify the capaci-
tor, we need to know the RMS value of the input current.
Calculating the approximate RMS value of a pulse train
with a fixed duty cycle is straightforward, but the LTC1703
complicates matters by running two sides simultaneously
and out of phase, creating a complex waveform at the
input.
To calculate the approximate RMS value of the input
current, we first need to calculate the average DC value
with both sides of the LTC1703 operating at maximum
load. Over a single period, the system will spend some
time with one top switch on and the other off, perhaps
some time with both switches on, and perhaps some
time with both switches off. During the time each top
switch is on, the current will equal that side’s full load
output current. When both switches are on, the total
current will be the sum of the two full load currents, and
when both are off, the current is effectively zero. Multiply
each current value by the percentage of the period that
the current condition lasts, and sum the results—this is
the average DC current value.
As an example, consider a circuit that takes a 5V input
and generates 3.3V at 3A at side 1 and 1.6V at 10A at
side 2. When a cycle starts, TG1 turns on and 3A flows
At some random time after they are turned on, they can
blow up for no apparent reason. The capacitor manufac-
turers are aware of this and sell special “surge tested”
tantalum capacitors specifically designed for use with
switching regulators. When choosing a tantalum input
capacitor, make sure that it is rated to carry the RMS
current that the LTC1703 will draw. If the data sheet
doesn’t give an RMS current rating, chances are the
capacitor isn’t surge tested. Don’t use it!
OUTPUT BYPASS CAPACITOR
The output bypass capacitor has quite different require-
ments from the input capacitor. The ripple current at the
output of a buck regulator like the LTC1703 is much lower
than at the input, due to the fact that the inductor current
is constantly flowing at the output whenever the LTC1703
is operating in continuous mode. The primary concern at
the output is capacitor ESR. Fast load current transitions
at the output will appear as voltage across the ESR of the
output bypass capacitor until the feedback loop in the
LTC1703 can change the inductor current to match the
new load current value. This ESR step at the output is often
the single largest budget item in the load regulation
calculation. As an example, our hypothetical 1.6V, 10A
switcher with a 0.01Ω ESR output capacitor would expe-
rience a 100mV step at the output with a 0 to 10A load
step—a 6.3% output change!
Usually the solution is to parallel several capacitors at the
output. For example, to keep the transient response inside
of 3% with the previous design, we’d need an output ESR
better than 0.0048Ω. This can be met with three 0.014Ω,
470µF tantalum capacitors in parallel.
INDUCTOR
The inductor in a typical LTC1703 circuit is chosen prima-
rily for value and saturation current. The inductor value
sets the ripple current, which is commonly chosen at
around 40% of the anticipated full load current. Ripple
current is set by:
I
tV
L
RIPPLE
ON QB OUT
=
()
()
TIME
0ABCD
50% 16% 16% 18%
I
AVE
0
INPUT CURRENT (A)
5.2
3
10
13
1703 SB1
Figure SB1. Average Current Calculation
APPLICATIO S I FOR ATIO
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