LTC1702AIGN datasheet(18/36 Pages) LINER

18 / 36 page

LTC1702A

1702afa

APPLICATIONS INFORMATION

Calculating RMS Current in CIN

A buck regulator like the LTC1702A 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

LTC1702A 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 LTC1702A 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

TIME

50%

16% 16% 18%

IAVE

5.2

1702A SB1

Figure SB1. Average Current Calculation

implies a duty cycle of 32%, which means QT is on 32%

of each switching cycle. During QT’s on-time, the current

drawn from the input equals the load current and during

the rest of the cycle, the current drawn from the input is

near zero. This 0A to 10A, 32% duty cycle pulse train adds

up to 4.7ARMS at the input. At 550kHz, switching cycles

last about 1.8

µs—most system logic supplies have no

hope of regulating output current with that kind of speed.

A local input bypass capacitor is required to make up the

difference and prevent the input supply from dropping

drastically when QT kicks on. This capacitor is usually

chosen for RMS ripple current capability and ESR as well

as value.

The input bypass capacitor in an LTC1702A circuit is

common to both channels. Consider our 10A example

case with the other side of the LTC1702A disabled. The

input bypass capacitor gets exercised in three ways: its

ESR must be low enough to keep the initial drop as QT

turns on within reason (100mV or so); its RMS current

capability must be adequate to withstand the 4.6ARMS

ripple current at the input and the capacitance must be

large enough to maintain the input voltage until the input

supply can make up the difference. Generally, a capacitor

that meets the first two parameters will have far more

capacitance than is required to keep capacitance-based

droop under control. In our example, we need 0.01

Ω ESR

to keep the input drop under 100mV with a 10A current

step and 4.6ARMS ripple current capacity to avoid over-

heating the capacitor. These requirements can be met with

multiple low ESR tantalum or electrolytic capacitors in

parallel, or with a large monolithic ceramic capacitor.

The two sides of the LTC1702A run off a single master

clock and are wired 180

° out of phase with each other to

significantly reduce the total capacitance/ESR needed at

the input. Assuming 100mV of ripple and 10A output

current, we needed an ESR of 0.01

Ω and 4.7A ripple

current capability for one side. Now, assume both sides

are running simultaneously with identical loading. If the

two sides switched in phase, all the loading conditions

would double and we’d need enough capacitance for

9.4ARMS and 0.005Ω ESR. With the two sides out of

phase, the input current is 4.8ARMS—barely larger than

the single case (Figure 7)! The peak current deltas are still

only 10A, requiring the same 0.01

Ω ESR rating. As long as

LTC1702AIGN 数据表(PDF) 18 Page - Linear Technology

LTC1702AIGN 数据表(HTML) 18 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

数据表下载