LTC3370 datasheet(29/44 Pages) LINER

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LTC3372

Rev. A

For more information www.analog.com

APPLICATIONS INFORMATION

bypassing is needed to supply the high transient currents

required by the MOSFET gate drivers and to prevent in-

teraction between the channels.

The INTVCC load current (IINTVCC) is dominated by the

gate charge current and may be supplied by either the

VIN LDO or the VOUT/EXTVCC pin. The gate charge current

is dependent on operating frequency and NMOS size as

discussed in the Efficiency Considerations section.

When the VOUT/EXTVCC regulation is set to 3.3V, the VIN

LDO is always enabled as the voltage on the VOUT/EXTVCC

pinisneverabovethe4.7Vthreshold.Powerdissipationfor

the IC in this case is highest and is equal to VIN • IINTVCC.

High input voltage applications in which large power

MOSFETs are being driven at high frequencies may cause

the maximum junction temperature rating for the IC to be

exceeded. For example, a 30mA INTVCC current from a

48V input supply will result in a junction temperature (TJ)

increase (rise) of:

∆TJ = (30mA)(48V)(34°C/W for QFN) = 48°C

The power dissipation due to IINTVCC from VIN should be

checked at the maximum VIN of the application operating

in forced continuous mode.

When the VOUT regulation is set to 5V and VOUT rises

above 4.7V, INTVCC is connected to VOUT through an

internal switch. Significant efficiency and thermal gains

can be realized by powering INTVCC from the output, since

the input supply current resulting from INTVCC current is

scaled by a factor of (VOUT/VIN)/(efficiency). In this case,

power dissipation in the LTC3372 due to the INTVCC cur-

rent is VINTVCC • IINTVCC, and the junction temperature (TJ)

increase (rise) of:

∆TJ = (30mA)(5V)(34°C/W for QFN) = 5°C

The LTC3372 junction temperature (TJ) can be estimated

fromambienttemperature(TA)andtotalpowerdissipation

(PD) using the equations given in Note 2 of the Electrical

Characteristics. To prevent the maximum junction tem-

perature from being exceeded, all power dissipations from

the HV controller’s INTVCC current and the LV buck power

stages must be considered.

Topside MOSFET Driver Supply (CB, DB)

An external bootstrap capacitor, CB, connected to the

BOOST pin supplies the gate drive voltage for the topside

MOSFET. Capacitor CB in the Block Diagram is charged

though external diode DB from INTVCC 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 topside MOSFET on, the

boost voltage is above the input supply: VBOOST = VIN +

VINTVCC. The value of the boost capacitor, CB, needs to be

100 times that of the total input capacitance of the top-

side MOSFET(s). The reverse breakdown of the external

Schottky diode must be greater than VIN(MAX).

Fault Conditions: Current Limit and Current Foldback

The HV controller includes current foldback to help limit

load current when the output is shorted to ground. If the

output voltage falls below 70% of its nominal output level,

then the maximum sense voltage is progressively lowered

from 100% to 45% of its maximum selected value. Under

short-circuit conditions with very low duty cycles, cycle

skippingwillbegininordertolimittheshort-circuitcurrent.

In this situation the bottom MOSFET will be dissipating

most of the power. The short-circuit ripple current is de-

termined by the minimum on-time, tON(MIN)(seeElectrical

Characteristics), the input voltage and inductor value:

ΔIL(SC) = tON(MIN)

VIN

⎛

⎝

⎜

⎞

⎠

⎟

The resulting average short-circuit current is:

ISC = 45% •ILIM(MAX) −

ΔIL(SC)

Fault Conditions: Overvoltage Protection (Crowbar)

The overvoltage crowbar is designed to blow a system

input fuse when the output voltage of the regulator rises

muchhigherthannominallevels.Thecrowbarcauseshuge

currents to flow, that blow the fuse to protect against a

shorted top MOSFET if the short occurs while the control-

ler is operating.

High Voltage Buck Controller

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