LTC3829 datasheet(29/40 Pages) LINER

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LTC3829

3829f

applicaTions inForMaTion

INTVCC (LDO) and EXTVCC

TheLTC3829featuresatruePMOSLDOthatsuppliespower

to INTVCC from the VIN supply. INTVCC powers the gate

drivers and much of the LTC3829’s internal circuitry. The

LDO regulates the voltage at the INTVCCpinto5VwhenVIN

is greater than 5.5V. EXTVCC connects to INTVCC through

a P-channel MOSFET and can supply the needed power

when its voltage is higher than 4.7V. Each of these can

supply a peak current of 100mA and must be bypassed

to ground with a minimum of 4.7µF ceramic capacitor or

low ESR electrolytic capacitor. No matter what type of bulk

capacitor is used, an additional 0.1µF 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.

High input voltage applications in which large MOSFETs

are being driven at high frequencies may cause the maxi-

mum junction temperature rating for the LTC3829 to be

exceeded. The INTVCC current, which is dominated by the

gate charge current, may be supplied by either the 5V LDO

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

than 4.7V, the LDO is enabled. Power dissipation for the

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

tables. For example, the LTC3829 INTVCCcurrentislimited

to less than 42mA from a 38V supply in the UHF package

and not using the EXTVCC supply:

TJ = 70°C + (42mA)(38V)(34°C/W) = 125°C

To prevent the maximum junction temperature from be-

ing exceeded, the input supply current must be checked

while operating in continuous conduction mode (MODE

= SGND) at maximum VIN. When the voltage applied to

EXTVCC rises above 4.7V, the INTVCC LDO is turned off

and the EXTVCC is connected to the INTVCC. The EXTVCC

remainsonaslongasthevoltageappliedtoEXTVCCremains

above 4.5V. Using the EXTVCC allows the MOSFET driver

and control power to be derived from one of switching

regulator outputs during normal operation and from the

INTVCC when the output is out of regulation (e.g., start-

up, short circuit). If more current is required through the

EXTVCC than is specified, an external Schottky diode can

be added between the EXTVCC and INTVCC pins. 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).

Tying the EXTVCC pin to a 5V supply reduces the junction

temperature in the previous example from 125°C to:

TJ = 70°C + (42mA)(5V)(34°C/W) = 77°C

However, for low voltage outputs, additional circuitry is

required to derive INTVCC power from the output.

The following list summarizes the four possible connec-

tions for EXTVCC:

1. EXTVCCleftopen(orgrounded).ThiswillcauseINTVCC

to be powered from the internal 5V LDO 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. EXTVCCconnectedtoanexternalsupply.Ifa5Vexternal

supply is available, it may be used to power EXTVCC

providing it is compatible with the MOSFET gate drive

requirements.

LTC3829 Datasheet(PDF) 29 Page - Linear Technology

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