LTC1703 datasheet(18/36 Pages) LINER

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LTC1703

1703fa

The two sides of the LTC1703 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

the capacitor we chose for the single side application can

support the slightly higher 4.8ARMS current, we can add

the second channel without changing the input capacitor

at all. As a general rule, an input bypass capacitor capable

of supporting the larger output current channel can sup-

port both channels running simultaneously (see the

2-Phase Operation section for more information). Details

on how to calculate the maximum RMS input current can

be found in Application Note 77.

Tantalum capacitors are a popular choice as input capaci-

tors for LTC1703 applications, but they deserve a special

caution here. Generic tantalum capacitors have a destruc-

tive failure mechanism when they are subjected to large

RMS currents (like those seen at the input of a LTC1703).

Figure 7. Current Waveforms

At the same time, the input supply needs to supply several

amps of current without excessive voltage drop. The input

supply must have regulation adequate to prevent sudden

load changes from causing the LTC1703 input voltage to

dip. In most typical applications where the LTC1703 is

generating a secondary low voltage logic supply, all of

these input conditions are met by the main system logic

supply when fortified with an input bypass capacitor.

INPUT BYPASS CAPACITOR

A typical LTC1703 circuit running from a 5V logic supply

might provide 1.6V at 10A at one of its outputs. 5V to 1.6V

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 LTC1703 circuit is

common to both channels. Consider our 10A example

case with the other side of the LTC1703 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 4.7ARMS 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 4.7ARMS ripple

current capacity to avoid overheating the capacitor. These

requirements can be met with multiple low ESR tantalum

or electrolytic capacitors in parallel, or with a large mono-

lithic ceramic capacitor.

10A

32%

68%

10A

32% 18%

18%

32%

–3.2A

6.8A

32%

68%

QT CURRENT, SIDE 1 ONLY

(FOR 1-PHASE, 2 SIDES:

MULTIPLY CURRENT BY 2)

CURRENT IN CIN, SIDE 1 ONLY

ICIN = 4.66ARMS, (1-PHASE,

2 SIDES: ICIN = 9.3ARMS)

CURRENT IN CIN,

BOTH SIDES EQUAL LOAD

ICIN = 4.8ARMS

QT1 CURRENT

QT2 CURRENT

BOTH SIDES EQUAL LOAD

2-PHASE OPERATION

–6.4A

3.6A

32%

18%

1703 F07

32%

APPLICATIO S I FOR ATIO

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