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LT3782AIFE

2-Phase Step-Up DC/DC Controller

Linear Technology 

LT3782A

APPLICATIONS INFORMATION

The Peak and Average Input Currents

The control circuit in the LT3782A measures the input cur-

rent by using a sense resistor in each MOSFET source, so

the output current needs to be reflected back to the input

in order to dimension the power MOSFET properly. Based

on the fact that, ideally, the output power is equal to the

input power, the maximum average input current is:

IIN(MAX )

=

IO(MAX )

1– DMAX

The peak current is:

IIN(PEAK )

=

1.2

•

IO(MAX )

1– DMAX

The maximum duty cycle, DMAX, should be calculated at

minimum VIN.

Power Inductor Selection

In a boost circuit, a power inductor should be designed

to carry the maximum input DC current. The inductance

should be small enough to generate enough ripple current

to provide adequate signal to noise ratio to the LT3782A.

An empirical starting of the inductor ripple current (per

phase) is about 40% of maximum DC current, which is

half of the input DC current in a 2-phase circuit:

ΔIL

≅ 40% •

IOUT(MAX) •

2VIN

VOUT

= 20% • IOUT(MAX) •

VIN

VOUT

where VIN, VOUT and IOUT are the DC input voltage, output

voltage and output current, respectively.

And the inductance is estimated to be:

L = VIN • D

fs • ΔIL

where fs is the switching frequency per phase.

The saturation current level of inductor is estimated to

be:

ISAT

≥

ΔIL

2

+ IIN

2

≅ 70% • IOUT(MAX) • VOUT

VIN(MIN)

Sense Resistor Selection

During the switch on-time, the control circuit limits the

maximum voltage drop across the sense resistor to about

63mV. The peak inductor current is therefore limited to

63mV/R. The relationship between the maximum load

current, duty cycle and the sense resistor RSENSE is:

R

≤

VSENSE(MAX )

•

1–

1.2 •

DMAX

IO(MAX )

2

Power MOSFET Selection

Important parameters for the power MOSFET include the

drain-to-source breakdown voltage (BVDSS), the threshold

voltage (VGS(TH)), the on-resistance (RDS(ON)) versus gate-

to-source voltage, the gate-to-source and gate-to-drain

charges (QGS and QGD, respectively), the maximum drain

current (ID(MAX)) and the MOSFET’s thermal resistances

(RTH(JC) and RTH(JA)).

3782af

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