LTC7813 datasheet(24/38 Pages) LINER

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LTC3895

3895fa

For more information www.linear.com/LTC3895

applicaTions inForMaTion

The junction temperature can be estimated by using the

equations given in Note 2 of the Electrical Characteristics.

For example, if DRVCC is set to 6V, the DRVCC current is

limitedtolessthan49mAfroma40Vsupplywhennotusing

the EXTVCCorNDRVLDOsata70°Cambienttemperature:

TJ = 70°C + (49mA)(40V)(28°C/W) = 125°C

To prevent the maximum junction temperature from being

exceeded, the VIN supply current must be checked while

operating in forced continuous mode (MODE = INTVCC)

at maximum VIN.

When the voltage applied to EXTVCC rises above its swi-

tchover threshold, the VIN and NDRV LDOs are turned off

and the EXTVCC LDO is enabled. The EXTVCC LDO remains

on as long as the voltage applied to EXTVCC remains above

theswitchoverthresholdminusthecomparatorhysteresis.

TheEXTVCCLDOattemptstoregulatetheDRVCCvoltageto

the voltage as programmed by the DRVSET pin, so while

EXTVCC is less than this voltage, the LDO is in dropout

and the DRVCC voltage is approximately equal to EXTVCC.

When EXTVCC is greater than the programmed voltage,

up to an absolute maximum of 14V, DRVCC is regulated

to the programmed voltage.

Using the EXTVCC LDO allows the MOSFET driver and

control power to be derived from the LTC3895’s switch-

ing regulator output (4.7V/7.7V ≤ VOUT ≤ 14V) during

normal operation and from the VIN or NDRV 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 DRVCC pins. In this case, do not

apply more than 10V to the EXTVCC pin and make sure

that EXTVCC ≤ VIN.

Significant efficiency and thermal gains can be realized

by powering DRVCC 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 + (49mA)(8.5V)(28°C/W) = 82°C

However, for 3.3V and other low voltage outputs, addi-

tional circuitry is required to derive DRVCC power from

the output.

The following list summarizes the five possible connec-

tions for EXTVCC:

1. EXTVCC grounded. This will cause DRVCC to be pow-

ered from the internal VIN or NDRV LDO resulting in

an efficiency penalty of up to 10% at high input volt-

ages. If EXTVCC is grounded, the REGSD feature must

be defeated with a pull-up resistor 330k or smaller

between SS and INTVCC.

2. EXTVCCconnecteddirectlytotheregulatoroutput.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.

4. EXTVCC connected to the regulator output through an

external zener diode. If the output voltage is greater

than 14V, a zener diode can be used to drop the

necessary voltage between VOUT and EXTVCC such

that EXTVCC remains below 14V (Figure 10). In this

configuration,abypasscapacitoronEXTVCCofatleast

0.1μF is recommended. An optional resistor between

EXTVCC and GND can be inserted to ensure adequate

bias current through the zener diode.

3895 F10

LTC3895

VOUT > 14V

EXTVCC

GND

EXTVCC < 14V

0.1µF

Figure 10. Using a Zener Diode Between VOUT and EXTVCC

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