LT3782AIFE(RevC) データシートの表示（PDF） - Linear Technology

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LT3782AIFE
(Rev.:RevC)

2-Phase Step-Up DC/DC Controller

Linear Technology 

LT3782A

APPLICATIONS INFORMATION

Slope Compensation

The LT3782A is designed for high voltage and/or high

current applications, and very often these applications

generate noise spikes that can be picked up by the cur-

rent sensing amplifier and cause switching jitter. To avoid

switching jitter, careful layout is absolutely necessary to

minimize the current sensing noise pickup. Sometimes

increasing slope compensation to overcome the noise

can help to reduce jitter. The built-in slope compensa-

tion can be increased by adding a resistor RSLOPE from

SLOPE pin to ground. Note that smaller RSLOPE increases

slope compensation and the minimum RSLOPE allowed is

RFREQ/2.

Layout Considerations

To prevent EMI, the power MOSFETs and input bypass

capacitor leads should be kept as short as possible. A

ground plane should be used under the switching circuitry

to prevent interplane coupling and to act as a thermal

spreading path. Note that the bottom pad of the package

is the heat sink, as well as the IC signal ground, and must

be soldered to the ground plane.

In a boost converter, the conversion gain (assuming 100%

efficiency) is calculated as (ignoring the forward voltage

drop of the boost diode):

VOUT = 1

VIN 1−D

where D is the duty ratio of the main switch. D can then

be estimated from the input and output voltages:

D = 1−

VIN

VOUT

; DMAX

=

1−

VIN(MIN)

VOUT

The Peak and Average Input Currents

The control circuit in the LT3782A measures the input

current 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) • VOUT

2VIN

= 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)

10

For more information www.linear.com/LT3782A

3782afc

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