LTC1435CS datasheet(11/20 Pages) LINER

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LTC1435

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

in an efficiency penalty of up to 10% at high input

voltages.

2. EXTVCC connected directly to VOUT. This is the normal

connection for a 5V regulator and provides the highest

efficiency.

3. EXTVCC connected to an output-derived boost network.

For 3.3V and other low voltage regulators, efficiency

gains can still be realized by connecting EXTVCC to an

output-derived voltage which has been boosted to

greater than 4.8V. This can be done with either the

inductive boost winding as shown in Figure 4a or the

capacitive charge pump shown in Figure 4b. The charge

pump has the advantage of simple magnetics.

4. EXTVCC connected to an external supply. If an external

supply is available in the 5V to 10V range (EXTVCC ≤

VIN), it may be used to power EXTVCC providing it is

compatible with the MOSFET gate drive requirements.

When driving standard threshold MOSFETs, the exter-

nal supply must always be present during operation to

prevent MOSFET failure due to insufficient gate drive.

APPLICATIONS INFORMATION

INTVCC Regulator

An internal P-channel low dropout regulator produces the

5V supply which powers the drivers and internal circuitry

within the LTC1435. 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

maximum junction temperature rating for the LTC1435 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 LTC1435 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 LTC1435 contains an internal P-channel MOSFET

switch connected between the EXTVCC and INTVCC pins.

The switch closes and supplies the INTVCC power when-

ever the EXTVCC pin is above 4.8V, and remains closed

until EXTVCC drops below 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,

Figure 4a. Secondary Output Loop and EXTVCC Connection

EXTVCC

SFB

SGND

VIN

PGND

LTC1435

N-CH

CIN

VIN

1N4148

µF

COUT

VSEC

1:N

RSENSE

VOUT

OPTIONAL

EXT VCC

CONNECTION

≤ VSEC ≤ 9V

LTC1435 • F04a

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