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LTC1629/LTC1629-PG

High input voltage applications in which large MOSFETs

are being driven at high frequencies may cause the maxi-

mum junction temperature rating for the LTC1629 to be

exceeded. The supply current is dominated by the gate

charge supply current, in addition to the current drawn

from the differential amplifier output. The gate charge is

dependent on operating frequency as discussed in the

Efficiency Considerations section. The supply current can

either be supplied by the internal 5V regulator or via the

EXTVCC pin. When the voltage applied to the EXTVCC pin

is less than 4.7V, all of the INTVCC load current is supplied

by the internal 5V linear regulator. Power dissipation for

the IC is higher in this case by (IIN)(VIN – INTVCC) and

efficiency is lowered. The junction temperature can be

estimated by using the equations given in Note 1 of the

Electrical Characteristics. For example, the LTC1629 VIN

current is limited to less than 24mA from a 24V supply:

TJ = 70°C + (24mA)(24V)(95°C/W) = 125°C

Use of the EXTVCC pin reduces the junction temperature

to:

TJ = 70°C + (24mA)(5V)(95°C/W) = 81.4°C

The input supply current should be measured while the

controller is operating in continuous mode at maximum

VIN and the power dissipation calculated in order to pre-

vent the maximum junction temperature from being ex-

ceeded.

EXTVCC Connection

The LTC1629 contains an internal P-channel MOSFET

switch connected between the EXTVCC and INTVCC pins.

When the voltage applied to EXTVCC rises above 4.7V, the

internal regulator is turned off and the switch closes,

connecting the EXTVCC pin to the INTVCC pin thereby

supplying internal and MOSFET gate driving power. 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 (4.7V < VEXTVCC < 7V) and from

the internal regulator when the output is out of regulation

(start-up, short-circuit). Do not apply greater than 7V to

the EXTVCC pin and ensure that EXTVCC < VIN + 0.3V when

using the application circuits shown. If an external voltage

source is applied to the EXTVCC pin when the VIN supply is

not present, a diode can be placed in series with the

LTC1629’s VIN pin and a Schottky diode between the

EXTVCCandtheVINpin,topreventcurrentfrombackfeeding

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 the

ratio: (Duty Factor)/(Efficiency). For 5V regulators this

means connecting the EXTVCC pin directly to VOUT. How-

ever, for 3.3V and other lower voltage regulators, addi-

tional 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

a significant efficiency penalty 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 external supply. If an external

supply is available in the 5V to 7V range, it may be used to

power EXTVCC providing it is compatible with the MOSFET

gate drive requirements. VIN must be greater than or equal

to the voltage applied to the EXTVCC pin.

4. 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

4.7V but less than 7V. This can be done with either the

inductive boost winding as shown in Figure 5a or the

capacitive charge pump shown in Figure 5b. The charge

pump has the advantage of simple magnetics.

Topside MOSFET Driver Supply (CB,DB) (Refer to

Functional Diagram)

External bootstrap capacitors CB1 and CB2 connected to

the BOOST1 and BOOST2 pins supply the gate drive

voltages for the topside MOSFETs. Capacitor CB in the

Functional Diagram is charged though diode DB from

INTVCC when the SW pin is low. When the topside MOSFET

turns on, the driver places the CB voltage across the gate-

APPLICATIO S I FOR ATIO

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