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LT3782A

3782af

For a given VIN and VOUT, we can calculate the duty cycle D

and then derive the output RMS ripple current from Figure

6. After choosing output capacitors with sufﬁcient RMS

ripple current rating, we also need to consider the ESR

requirement if electrolytic caps, tantulum caps, POSCAPs

or SP CAPs are selected. Given the required output ripple

voltage spec ΔVOUT (in RMS value) and the calculated RMS

ripple current ΔIOUT, one can estimate the ESR value of

the output capacitor to be

ESR ≤

ΔVOUT

ΔIOUT

External Regulator to Bias Gate Drivers

For applications with VINhigherthan24V,theICtemperature

may get too high. To reduce heat, an external regulator

between 12V to 14V should be used to override the internal

VGBIAS regulator to supply the current needed for BGATE1

and BGATE2 (see Figure 7).

Efﬁciency Considerations

The efﬁciency of a switching regulator is equal to the out-

put power divided by the input power (¥100%). Percent

efﬁciency can be expressed as:

% Efﬁciency = 100% – (L1 + L2 + L3 + …),

where L1, L2, etc. are the individual loss components

as a percentage of the input power. It is often useful to

analyze individual losses to determine what is limiting

the efﬁciency and which change would produce the most

improvement. Although all dissipative elements in the

circuit produce losses, four main sources usually account

for the majority of the losses in LT3782A application

circuits:

1. The supply current into VIN. The VIN current is the sum

of the DC supply current IQ (given in the Electrical Char-

acteristics) and the MOSFET driver and control currents.

The DC supply current into the VIN pin is typically about

7mA and represents a small power loss (much less

than 1%) that increases with VIN. The driver current

results from switching the gate capacitance of the power

MOSFET; this current is typically much larger than the

DC current. Each time the MOSFET is switched on and

then off, a packet of gate charge QG is transferred from

GBIAS to ground. The resulting dQ/dt is a current that

must be supplied to the GBIAS capacitor through the

VIN pin by an external supply. In normal operation:

IQ(TOT) ≈ IQ = f • QG

PIC = VIN • (IQ + f • QG)

2. Power MOSFET switching and conduction losses:

PFET =

IO(MAX)

1– DMAX

•RDS(ON) •DMAX • T

+ k• VO

2 •

IO(MAX)

1– DMAX

•CRSS •f

Figure 7

3782A F07

2μF

12V

GBIAS

GBIAS1

GBIAS2

LT3782A

APPLICATIONS INFORMATION

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