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LTC1703

Calculating RMS Current in CIN

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 0.01

Ω 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 0.0048

Ω. This can be met with three 0.014Ω,

470

µF 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:

RIPPLE

ON QB

OUT

()

TIME

50%

16% 16% 18%

IAVE

5.2

1703 SB1

Figure SB1. Average Current Calculation

APPLICATIO S I FOR ATIO

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