LTC7813 datasheet(20/32 Pages) LINER

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LTC7800

7800f

For more information www.linear.com/LTC7800

INTVCC Regulators

The LTC7800 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 pin depending on the connection of the EXTVCC

pin. INTVCC powers the gate drivers and much of the

LTC7800’s internal circuitry. The VIN LDO and the EXTVCC

LDO regulate INTVCC 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

1μFceramiccapacitorplaceddirectlyadjacenttotheINTVCC

and PGND pins is highly recommended. Good bypassing

is needed to supply the high transient currents required

by the MOSFET gate drivers.

HighinputvoltageapplicationsinwhichlargeMOSFETsare

being driven at high frequencies may cause the maximum

junctiontemperatureratingfortheLTC7800tobeexceeded.

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 equa-

tions given in Note 3 of the Electrical Characteristics. For

example, the LTC7800 INTVCC current is limited to less

than 26mA from a 40V supply when not using the EXTVCC

supply at a 70°C ambient temperature:

TJ = 70°C + (26mA)(40V)(52°C/W) = 124°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 LTC7800’s switch-

ing 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 + (26mA)(8.5V)(52°C/W) = 81°C

However,for3.3Vandotherlowvoltageoutputs,additional

circuitryisrequiredtoderiveINTVCCpowerfromtheoutput.

The following list summarizes the three 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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