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LTC3807

3807fa

For more information www.linear.com/LTC3807

INTVCC Regulators

The LTC3807 features two separate internal P-channel low

dropout linear regulators (LDO) that supply power at the

INTVCC pin from either the VIN supply pin or the EXTVCC

pindependingontheconnectionoftheEXTVCCpin.INTVCC

powersthegatedriversandmuchoftheLTC3807’sinternal

circuitry.TheVINLDOandtheEXTVCCLDOregulateINTVCC

to 5.1V. Each of these can supply a peak current of at least

50mA and must be bypassed to ground with a minimum

of 2.2μF ceramic capacitor. No matter what type of bulk

capacitor is used, an additional 1μF ceramic capacitor

placed directly adjacent to the INTVCC and PGND pins is

highlyrecommended.Goodbypassingisneededtosupply

the high transient currents required by the MOSFET gate

drivers and to prevent interaction between the channels.

HighinputvoltageapplicationsinwhichlargeMOSFETsare

being driven at high frequencies may cause the maximum

junctiontemperatureratingfortheLTC3807tobeexceeded.

TheINTVCCcurrent,whichisdominatedbythegatecharge

current, may be supplied by either the VIN LDO or the

EXTVCC LDO. When the voltage on the EXTVCC pin is less

than4.7V,theVINLDOisenabled.Powerdissipationforthe

IC in this case is highest and is equal to VIN • IINTVCC. The

gate charge current is dependent on operating frequency

as discussed in the Efficiency Considerations section.

The junction temperature can be estimated by using the

equations given in Note 3 of the Electrical Characteristics.

For example, the LTC3807 INTVCC current is limited to less

than 32mA from a 40V supply when not using the EXTVCC

supply at a 70°C ambient temperature:

TJ = 70°C + (32mA)(40V)(43°C/W for QFN) = 125°C

To prevent the maximum junction temperature from be-

ing exceeded, the input supply current must be checked

while operating in forced continuous 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 5.1V, so while EXTVCC

is less than 5.1V, the LDO is in dropout and the INTVCC

voltage is approximately equal to EXTVCC. When EXTVCC

is greater than 5.1V, up to an absolute maximum of 14V,

INTVCC is regulated to 5.1V.

Using the EXTVCC LDO allows the MOSFET driver and

control power to be derived from the LTC3807’s switch-

ing regulator output (4.7V ≤ VOUT ≤ 14V) during normal

operation and from the VIN LDO when the output is out

of regulation (e.g., start-up, short-circuit). If more current

is required through the EXTVCC LDO than is specified, an

external Schottky diode can be added between the EXTVCC

and INTVCC pins. In this case, do not apply more than 6V

to the EXTVCC pin and make sure that 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 14V regulator outputs, this means connecting

the EXTVCC pin directly to VOUT. Tying the EXTVCC pin to

an 8.5V supply reduces the junction temperature in the

previous example from 125°C to:

TJ = 70°C + (32mA)(8.5V)(43°C/W) = 82°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. EXTVCCGrounded.ThiswillcauseINTVCCtobepowered

fromtheinternal5.1Vregulatorresultinginanefficiency

penalty of up to 10% at high input voltages.

2. EXTVCC Connected Directly to VOUT. This is the normal

connection for a 5V to 14V regulator and provides the

highest efficiency.

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

supply is available in the 5V to 14V range, it may be

used to power EXTVCC providing it is compatible with

the MOSFET gate drive requirements. Ensure that

EXTVCC < VIN.

APPLICATIONS INFORMATION

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