LTC4366 datasheet(22/32 Pages) AD

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LTC7862

Rev 0

For more information www.analog.com

APPLICATIONS INFORMATION

The following list summarizes the four possible connec-

tions for EXTVCC:

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

ered from the internal VIN LDO.

2. EXTVCC connected directly to the regulator output

from 5V to 14V.

3. EXTVCC connected to an external supply. If an exter-

nal 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 nec-

essary voltage between VOUT and EXTVCC such that

EXTVCC remains below 14V (Figure 7). In this con-

figuration, a bypass capacitor on EXTVCC of at least

0.1μF is recommended. An optional resistor between

EXTVCC and GND can be inserted to ensure adequate

bias current through the Zener diode.

Topside MOSFET Driver Supply (CB, DB)

An external bootstrap capacitor CB connected to the

BOOST pin supplies the gate drive voltage for the top-

side MOSFET. Capacitor CB in the Functional Diagram is

charged through the external low leakage diode, DB, from

DRVCC when the SW pin is low. When the topside MOSFET

is to be turned on, the driver places the CB voltage across

the gate-source of the MOSFET. This enhances the top

MOSFET switch and turns it on. The switch node voltage,

SW, rises to VIN and the BOOST pin follows. With the top-

side MOSFET on, the BOOST voltage is above the input

supply: VBOOST = VIN + VDRVCC. The value of the boost

capacitor, CB, needs to be 100 times that of the total input

capacitance of the topside MOSFET(s).

External BOOST Diode Selection (DB)

A Schottky diode should not be used between DRVCC and

BOOST, as the reverse leakage of the Schottky diode at hot

will be more current than the charge pump can provide.

Some example of silicon diodes with low leakage include:

CMHD3595, CMDD3003: Central Semiconductor

ES1DR, PNE20010ER: Nexperia

Fault Conditions: Current Limit

Under short-circuit conditions with very low duty cycles,

the LTC7862 will begin cycle skipping in order to limit the

short-circuit current. In this situation the bottom MOSFET

will be dissipating most of the power. The short-circuit

ripple current is determined by the minimum on-time,

tON(MIN), of the LTC7862 (≈80ns), the input voltage and

inductor value:

ΔIL(SC) = tON(MIN)

VIN

⎛

⎝⎜

⎞

⎠⎟

The resulting average short-circuit current is:

ISC =

VSENSE(MAX)

RSENSE

−

ΔIL(SC)

7862 F07

LTC7862

VOUT > 14V

EXTVCC

GND

EXTVCC < 14V

0.1µF

Figure 7. Using a Zener Diode Between VOUT and EXTVCC

INTVCC Regulator

An additional P-channel LDO supplies power at the INTVCC

pin from the DRVCC pin. Whereas DRVCC powers the gate

drivers, INTVCC powers much of the LTC7862’s internal

circuitry. The INTVCC supply must be bypassed with a

0.1μF ceramic capacitor. INTVCC is also used as a pull-up

to bias other pins, such as FREQ and WARNB.

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