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LTC3853

3853fc

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is needed to supply 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 LTC3853 to be

exceeded. The INTVCC current, which is dominated by the

gatechargecurrent,maybesuppliedbyeitherthe5Vlinear

regulator or EXTVCC. When the voltage on the EXTVCC pin

is less than 4.7V, the linear regulator 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. For example, the LTC3853

INTVCC current is limited to less than 50mA from a 24V

supplyintheUJpackageandnotusingtheEXTVCCsupply:

TJ = 85°C + (50mA)(24V)(33°C/W) = 125°C

To prevent the maximum junction temperature from being

exceeded, the input supply current must be checked while

operatingincontinuousconductionmode(MODE/PLLIN=

SGND) at maximum VIN. When the voltage applied to EXT-

VCC rises above 4.7V, the INTVCC linear regulator is turned

offandtheEXTVCCisconnectedtotheINTVCC.TheEXTVCC

remainsonaslongasthevoltageappliedtoEXTVCCremains

above 4.5V. Using the EXTVCC allows the MOSFET driver

andcontrolpowertobederivedfromoneoftheLTC3853’s

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 EXTVCCthanisspecified,anexternalSchottky

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 = 85°C + (50mA)(5V)(33°C/W) = 94°C

However,for3.3Vandotherlowvoltageoutputs,additional

circuitryisrequiredtoderiveINTVCCpowerfromtheoutput.

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 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. EXTVCC connected to an external supply. If a 5V

external supply is available, 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 net-

work. 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.

For applications where the main input power is 5V, tie

the VIN and INTVCC pins together and tie the combined

pins to the 5V input with a 1Ω or 2.2Ω resistor as shown

in Figure 8 to minimize the voltage drop caused by the

gate charge current. This will override the INTVCC linear

regulator and will prevent INTVCC from dropping too low

due to the dropout voltage. Make sure the INTVCC voltage

is at or exceeds the RDS(ON) test voltage for the MOSFET

which is typically 4.5V for logic-level devices.

APPLICATIONS INFORMATION

INTVCC

LTC3853

RVIN

1Ω

CIN

3853 F08

CINTVCC

4.7µF

VIN

Figure 8. Setup for a 5V Input

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