LTC1735IGN-1 datasheet(13/28 Pages) LINER

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LTC1735-1

APPLICATIO S I FOR ATIO

MOSFET from turning on and storing charge during the

dead-time, which could cost as much as 1% in efficiency.

A 3A Schottky is generally a good size for 10A to 12A regu-

lators due to the relatively small average current. Larger

diodes result in additional transition losses due to their

larger junction capacitance. The diode may be omitted if the

efficiency loss can be tolerated.

CIN Selection

In continuous mode, the source current of the top

N-channel MOSFET is a square wave of duty cycle VOUT /

VIN. To prevent large voltage transients, a low ESR input

capacitor sized for the maximum RMS current must be

used. The maximum RMS capacitor current is given by:

RMS

O MAX

OUT

≅









()

–1

This formula has a maximum at VIN = 2VOUT, where IRMS

= IOUT/2. This simple worst-case condition is commonly

used for design because even significant deviations do not

offer much relief. Note that capacitor manufacturer’s ripple

current ratings are often based on only 2000 hours of life.

This makes it advisable to further derate the capacitor, or

to choose a capacitor rated at a higher temperature than

required. Several capacitors may also be paralleled to

meet size or height requirements in the design. Always

consult the manufacturer if there is any question.

COUT Selection

The selection of COUT is primarily determined by the

effective series resistance (ESR) to minimize voltage ripple.

The output ripple (

∆VOUT) in continuous mode is deter-

mined by:

∆∆

I ESR

OUT

≈+









where f = operating frequency, COUT = output capacitance,

and

∆IL= ripple current in the inductor. The output ripple

is highest at maximum input voltage since

∆IL increases

with input voltage. Typically, once the ESR requirement

for COUT has been met, the RMS current rating generally

far exceeds the IRIPPLE(P-P) requirement. With ∆IL =

0.3IOUT(MAX) and allowing for 2/3 of the ripple due to ESR,

the output ripple will be less than 50mV at max VIN

assuming:

COUT required ESR < 2.2 RSENSE

COUT > 1/(8fRSENSE)

The first condition relates to the ripple current into the ESR

of the output capacitance while the second term guaran-

tees that the output voltage does not significantly dis-

charge during the operating frequency period due to ripple

current. The choice of using smaller output capacitance

increases the ripple voltage due to the discharging term

but can be compensated for by using capacitors of very

low ESR to maintain the ripple voltage at or below 50mV.

The ITHpin OPTI-LOOP compensation components can be

optimized to provide stable, high performance transient

response regardless of the output capacitors selected.

The selection of output capacitors for CPU or other appli-

cations with large load current transients is primarily de-

termined by the voltage tolerance specifications of the load.

The resistive component of the capacitor, ESR, multiplied

by the load current change plus any output voltage ripple

must be within the voltage tolerance of the load (CPU).

The required ESR due to a load current step is:

RESR < ∆V/∆I

where

∆I is the change in current from full load to zero load

(or minimum load) and

∆V is the allowed voltage deviation

(not including any droop due to finite capacitance).

The amount of capacitance needed is determined by the

maximum energy stored in the inductor. The capacitance

must be sufficient to absorb the change in inductor current

when a high current to low current transition occurs. The

opposite load current transition is generally determined by

the control loop OPTI-LOOP components, so make sure

not to over compensate and slow down the response. The

minimum capacitance to assure the inductors’ energy is

adequately absorbed is:

OUT

∆

()

∆

()

where

∆I is the change in load current.

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