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LTC3826

3826fc

APPLICATIONS INFORMATION

gate charge current, may be supplied by either the 5.25V

VIN LDO or the 7.5V EXTVCC LDO. When the voltage on

the EXTVCC pin is less than 4.7V, the VIN LDO is enabled.

Power dissipation for the IC in this case is highest and is

equal to VIN • INTVCC. The gate charge current is depen-

dent on operating frequency as discussed in the Efﬁciency

Considerations section. The junction temperature can be

estimated by using the equations given in Note 2 of the

Electrical Characteristics. For example, the LTC3826 INTVCC

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

in the G package and not using the EXTVCC supply:

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

To prevent the maximum junction temperature from being

exceeded, the input supply current must be checked while

operating in continuous conduction mode (PLLIN/MODE

= INTVCC) at maximum VIN.

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

VIN LDO is turned off and the EXTVCC LDO is enabled. The

EXTVCC LDO remains on as long as the voltage applied to

EXTVCC remains above 4.5V. The EXTVCC LDO attempts

to regulate the INTVCC voltage to 7.5V, so while EXTVCC

is less than 7.5V, the LDO is in dropout and the INTVCC

voltage is approximately equal to EXTVCC. When EXTVCC

is greater than 7.5V up to an absolute maximum of 10V,

INTVCC is regulated to 7.5V.

Using the EXTVCC LDO allows the MOSFET driver and

control power to be derived from one of the LTC3826’s

switching regulator outputs (4.7V ≤ VOUT ≤ 10V) during

normal operation and from the VIN LDO when the output

is out of regulation (e.g., startup, short-circuit). If more

current is required through the EXTVCC LDO than is spec-

iﬁed, an external Schottky diode can be added between the

EXTVCC and INTVCC pins. Do not apply more than 10V to

the EXTVCC pin and make sure than EXTVCC ≤ VIN.

Signiﬁcant efﬁciency and thermal gains can be realized

by powering INTVCC from the output, since the VIN cur-

rent resulting from the driver and control currents will be

scaled by a factor of (Duty Cycle)/(Switcher Efﬁciency). For

5V to 10V regulator outputs, this means connecting the

EXTVCC pin directly to VOUT. Tying the EXTVCC pin to a 5V

supply reduces the junction temperature in the previous

example from 125°C to:

TJ = 70°C + (24mA)(5V)(95°C/W) = 81°C

However, for 3.3V and other low voltage outputs, 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 5.25V 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 to 10V regulator and provides the

highest efﬁciency.

3. EXTVCC Connected to an External supply. If an external

supply is available in the 5V to 10V range, it may be

used to power EXTVCC providing it is compatible with

the MOSFET gate drive requirements.

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 that has been boosted to greater

than 4.7V. This can be done with the capacitive charge

pump shown in Figure 8.

EXTVCC

VIN

TG1

BG1

PGND

LTC3826

RSENSE

VOUT

VN2222LL

COUT

3826 F08

N-CH

CIN

1μF

VIN

BAT85

0.22μF

Figure 8. Capacitive Charge Pump for EXTVCC

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