LTC7802EUFDM datasheet(24/34 Pages) AD

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LTC7802

Rev. 0

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returns to a safe level, normal operation automatically

resumes.

A shorted top MOSFET will result in a high current con-

dition which will open the system fuse. The switching

regulator will regulate properly with a leaky top MOSFET

by altering the duty cycle to accommodate the leakage.

Fault Conditions: Overtemperature Protection

At higher temperatures, or in cases where the internal

power dissipation causes excessive self-heating (such as

a short from INTVCC to ground) internal overtemperature

shutdown circuitry will shut down the LTC7802. When

the internal die temperature exceeds 180°C, the INTVCC

LDO and gate drivers are disabled. When the die cools

to 160°C, the LTC7802 enables the INTVCC LDO and

resumes operation beginning with a soft-start startup.

Long-term overstress (TJ > 125°C) should be avoided

as it can degrade the performance or shorten the life of

the part.

Phase-Locked Loop and Frequency Synchronization

The LTC7802 has an internal phase-locked loop (PLL)

which allows the turn-on of the top MOSFET of controller

1 to be synchronized to the rising edge of an external

clock signal applied to the PLLIN/SPREAD pin. The turn

on of controller 2’s top MOSFET is thus 180° out of phase

with the external clock.

Rapid phase-locking can be achieved by using the FREQ

pin to set a free-running frequency near the desired

synchronization frequency. Before synchronization,

the PLL is prebiased to the frequency set by the FREQ

pin. Consequently, the PLL only needs to make minor

adjustments to achieve phase-lock and synchronization.

Although it is not required that the free-running frequency

be near the external clock frequency, doing so will pre-

vent the oscillator from passing through a large range of

frequencies as the PLL locks.

When synchronized to an external clock, the LTC7802

operates in pulse-skipping mode if it is selected by the

MODE pin, or in forced continuous mode otherwise. The

LTC7802 is guaranteed to synchronize to an external

clock applied to the PLLIN/SPREAD pin that swings up

to at least 2.2V and down to 0.5V or less. Note that the

LTC7802 can only be synchronized to an external clock

frequency within the range of 100kHz to 3MHz.

Efficiency Considerations

The percent efficiency of a switching regulator is equal to

the output power divided by the input power times 100%.

It is often useful to analyze individual losses to determine

what is limiting the efficiency and which change would

produce the most improvement. Percent efficiency can

be expressed as:

%Efficiency = 100% – (L1 + L2 + L3 + ...)

where L1, L2, etc. are the individual losses as a percent-

age of input power.

Although all dissipative elements in the circuit produce

losses, four main sources usually account for most of the

losses in LTC7802 circuits: 1) IC VIN current, 2) INTVCC

regulator current, 3) I2R losses, 4) Topside MOSFET tran-

sition losses.

1. The VIN current is the DC supply current given in

the Electrical Characteristics table, which excludes

MOSFET driver and control currents. Other than at very

light loads in burst mode, VIN current typically results

in a small (<0.1%) loss.

2. INTVCC current is the sum of the MOSFET driver and

control currents. The MOSFET driver current results

from switching the gate capacitance of the power

MOSFETs. Each time a MOSFET gate is switched from

low to high to low again, a packet of charge, dQ, moves

from INTVCC to ground. The resulting dQ/dt is a current

out of INTVCC that is typically much larger than the

control circuit current. In continuous mode, IGATECHG

= fSW(QT + QB), where QT and QB are the gate charges

of the top and bottom MOSFETs.

Supplying INTVCC from an output-derived source

through EXTVCC will scale the VIN current required

for the driver and control circuits by a factor of VOUT/

(VIN • Efficiency). For example, in a 20V to 5V applica-

tion, 10mA of INTVCC current results in approximately

2.5mA of VIN current. This reduces the mid-current

loss from 10% or more (if the driver was powered

directly from VIN) to only a few percent.

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