LTC7802EUFDM datasheet(23/34 Pages) AD

23 / 34 page

LTC7802

Rev. 0

For more information www.analog.com

Topside MOSFET Driver Supply (CB, DB)

External bootstrap capacitors CB connected to the BOOST

pins supply the gate drive voltages for the topside MOSFETs.

Capacitor CB in the Functional Diagram is charged though

external diode DB from INTVCC when the SW pin is low.

When one of the topside MOSFETs is to be turned on, the

driver places the CB voltage across the gate-source of the

desired MOSFET. This enhances the MOSFET and turns on

the topside switch. 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

topside MOSFET(s). For a typical application, a value of

CB = 0.1μF is generally sufficient.

The external diode DB can be a Schottky diode or sili-

con diode, but in either case it should have low leakage

and fast recovery. The reverse breakdown of the diode

must be greater than VIN(MAX). Pay close attention to the

reverse leakage at high temperatures where it generally

increases substantially.

A leaky diode not only increases the quiescent current

of the regulator, but it can create current path from the

BOOST pin to INTVCC. This will cause INTVCC to rise if

the diode leakage exceeds the current consumption on

INTVCC, which is primarily a concern in Burst Mode oper-

ation where the load on INTVCC can be very small. There

is an internal voltage clamp on INTVCC that prevents the

INTVCC voltage from running away, but this clamp should

be regarded as a failsafe only.

Minimum On-Time Considerations

Minimum on-time tON(MIN) is the smallest time duration

that the LTC7802 is capable of turning on the top MOSFET.

It is determined by internal timing delays and the gate

charge required to turn on the MOSFET. Low duty cycle

applications may approach this minimum on time limit

and care should be taken to ensure that:

tON(MIN) <

VOUT

VIN • fOSC

If the duty cycle falls below what can be accommodated

by the minimum on-time, the controller will begin to

skip cycles. The output voltage will continue to be reg-

ulated, but the ripple voltage and current will increase.

The minimum on-time for the LTC7802 is approximately

40ns. However, as the peak sense voltage decreases the

minimum on-time for gradually increases up to about

60ns. This is of particular concern in forced continuous

applications with low ripple current at light loads. If the

duty cycle drops below the minimum on-time limit in this

situation, a significant amount of cycle skipping can occur

with correspondingly larger current and voltage ripple.

Fault Conditions: Current Limit and Foldback

The LTC7802 includes current foldback to reduce the load

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

voltage falls below 50% of its regulation point, then the

maximum sense voltage is progressively lowered from

100% to 40% of its maximum value. Under short-circuit

conditions with very low duty cycles, the LTC7802 will

begin cycle skipping to limit the short circuit current. In

this situation the bottom MOSFET dissipates most of the

power but less than in normal operation. The short-circuit

ripple current is determined by the minimum on-time,

tON(MIN) ≈ 40ns, the input voltage and inductor value:

ΔIL(SC) = tON(MIN) • VIN/L

The resulting average short-circuit current is:

ISC = 40% • ILIM(MAX) − ΔIL(SC)/2

Fault Conditions: Overvoltage Protection (Crowbar)

The overvoltage crowbar is designed to blow a system

input fuse when the output voltage of the regulator rises

much higher than nominal levels. The crowbar causes

huge currents to flow that blow the fuse to protect against

a shorted top MOSFET if the short occurs while the con-

troller is operating.

If an output voltage rises 10% above the set regulation

point, the top MOSFET is turned off and the bottom

MOSFET is turned on until the overvoltage condition is

cleared. The bottom MOSFET remains on continuously

for as long as the overvoltage condition persists; if VOUT

APPLICATIONS INFORMATION

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