LTC1439CG datasheet(15/32 Pages) LINER

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LTC1438/LTC1439

APPLICATIONS INFORMATION

The selection of COUT is driven by the required effective

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

ment is satisified the capacitance is adequate for filtering.

The output ripple (

∆VOUT) is approximated by:

∆∆

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) the output

ripple 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 elec-

trolytic 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 tantalums, 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 recommendations.

INTVCC Regulator

An internal P-channel low dropout regulator produces 5V

at the INTVCC pin from the VIN supply pin. INTVCC powers

the drivers and internal circuitry within the LTC1438/

LTC1439. The INTVCC pin regulator can supply 40mA and

must be bypassed to ground with a minimum of 2.2

µF

tantalum or low ESR electrolytic capacitor. Good bypass-

ing 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 LTC1438/LTC1439

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

operating frequency as discussed in the Efficiency Consid-

erations section. The junction temperature can be esti-

mated by using the equations given in Note 1 of the

Electrical Characteristics. For example, the LTC1439 is

limited to less than 21mA from a 30V supply:

TJ = 70°C + (21mA)(30V)(85°C/W) = 124°C

To prevent maximum junction temperature from being

exceeded, the input supply current must be checked while

operating in continuous mode at maximum VIN.

EXTVCC Connection

The LTC1438/LTC1439 contain an internal P-channel

MOSFET switch connected between the EXTVCC and

INTVCC pins. When the voltage applied to EXTVCC rises

above 4.8V, the internal regulator is turned off and an

internal switch closes, connecting the EXTVCC pin to the

INTVCCpin thereby supplying internal power to the IC. The

switch remains closed as long as the voltage applied to

EXTVCC remains above 4.5V. This allows the MOSFET

driver and control power to be derived from the output

during normal operation (4.8V < VOUT < 9V) and from the

internal regulator when the output is out of regulation

(start-up, short circuit). Do not apply greater than 10V to

the EXTVCC pin and ensure that EXTVCC ≤ VIN.

Significant efficiency gains can be realized by powering

INTVCC from the output, since the VIN current resulting

from the driver and control currents will be scaled by a

factor of Duty Cycle/Efficiency. For 5V regulators this

supply means connecting the EXTVCC pin directly to VOUT.

However, for 3.3V and other lower voltage regulators,

additional circuitry is required to derive INTVCC power

from the output.

The following list summarizes the four possible connec-

tions for EXTVCC:

1. EXTVCC left open (or grounded). This will cause INTVCC

to be powered from the internal 5V regulator resulting

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