LTC1435ACG datasheet(10/20 Pages) LINER

10 / 20 page

LTC1435A

APPLICATIONS INFORMATION

CIN and COUT 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:

required

RMS

MAX

OUT

≈

−

()

[]12/

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

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

monly 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 capaci-

tor 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.

The selection of COUT is driven by the required effective

series resistance (ESR). Typically, once the ESR require-

ment is satisfied the capacitance is adequate for filtering.

The output ripple (

∆VOUT) is approximated 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. With

∆IL=0.4IOUT(MAX)theoutputripple

will be less than 100mV at max VIN assuming:

COUT required ESR < 2RSENSE

Manufacturers such as Nichicon, United Chemicon and

Sanyo should be considered for high performance through-

hole capacitors. The OS-CON semiconductor dielectric

capacitor available from Sanyo has the lowest ESR(size)

product of any aluminum electrolytic at a somewhat

higher price. Once the ESR requirement for COUT has been

met, the RMS current rating generally far exceeds the

IRIPPLE(P-P) requirement.

In surface mount applications multiple capacitors may have

to be paralleled to meet the ESR or RMS current handling

requirements of the application. Aluminum electrolytic and

dry tantalum capacitors are both available in surface mount

configurations. In the case of tantalum, it is critical that the

capacitors are surge tested for use in switching power

supplies. An excellent choice is the AVX TPS series of

surface mount tantalum, available in case heights ranging

from 2mm to 4mm. Other capacitor types include Sanyo

OS-CON, Nichicon PL series and Sprague 593D and 595D

series. Consult the manufacturer for other specific recom-

mendations.

INTVCC Regulator

An internal P-channel low dropout regulator produces the

5V supply that powers the drivers and internal circuitry

within the LTC1435A. The INTVCC pin can supply up to

15mA and must be bypassed to ground with a minimum

of 2.2

µF tantalum or low ESR electrolytic. Good bypassing

is necessary to supply the high transient currents required

by the MOSFET gate drivers.

High input voltage applications, in which large MOSFETs

are being driven at high frequencies, may cause the maxi-

mum junction temperature rating for the LTC1435A to be

exceeded. The IC supply current is dominated by the gate

charge supply current when not using an output derived

EXTVCC source. The gate charge is dependent on operat-

ing frequency as discussed in the Efficiency Considerations

section. The junction temperature can be estimated by using

the equations given in Note 1 of the Electrical Character-

istics. For example, the LTC1435A is limited to less than

17mA from a 30V supply:

TJ = 70°C + (17mA)(30V)(100°C/W) = 126°C

To prevent maximum junction temperature from being

exceeded, the input supply current must be checked when

operating in continuous mode at maximum VIN.

EXTVCC Connection

The LTC1435A contains an internal P-channel MOSFET

switch connected between the EXTVCCandINTVCCpins.The

switch closes and supplies the INTVCC power whenever the

EXTVCC pin is above 4.8V, and remains closed until EXTVCC

drops below 4.5V. This allows the MOSFET driver and

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