LT3436 Просмотр технического описания (PDF) - Linear Technology

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LT3436

3A, 800kHz Step-Up Switching Regulator

Linear Technology 

LT3436

APPLICATIONS INFORMATION

The inductor must have a rating greater than its peak

operating current to prevent saturation resulting in effi-

ciency loss. Peak inductor current is given by:

ILPEAK

=

(VOUT )(IOUT )

VIN • η

+

VIN(VOUT − VIN)

2VOUT (L)(f)

Also, consideration should be given to the DC resistance

of the inductor. Inductor resistance contributes directly to

the efficiency losses in the overall converter.

thermal resistance number and add in worst-case ambient

temperature:

TJ = TA + θJA (PTOT)

If a true die temperature is required, a measurement of

the SYNC to GND pin resistance can be used. The SYNC

pin resistance across temperature must first be cali-

brated, with no device power, in an oven. The same

measurement can then be used in operation to indicate the

die temperature.

THERMAL CALCULATIONS

Power dissipation in the LT3436 chip comes from four

sources: switch DC loss, switch AC loss, drive current, and

input quiescent current. The following formulas show how

to calculate each of these losses. These formulas assume

continuous mode operation, so they should not be used

for calculating efficiency at light load currents.

DC, duty cycle = (VOUT − VIN)

VOUT

ISW

=

(VOUT )(IOUT )

VIN

Switch loss:

( ) ( ) PSW = (DC)(ISW )2(RSW ) + 17n ISW VOUT (f)

VIN loss:

PVIN

=

(VIN)(ISW )(DC)

50

+ 1mA(VIN)

RSW = Switch resistance (≈ 0.16Ω hot)

Example: VIN = 5V, VOUT = 12V and IOUT = 0.8A

FREQUENCY COMPENSATION

Loop frequency compensation is performed on the output

of the error amplifier (VC pin) with a series RC network.

The main pole is formed by the series capacitor and the

output impedance (≈500kΩ) of the error amplifier. The

pole falls in the range of 2Hz to 20Hz. The series resistor

creates a “zero” at 1kHz to 5kHz, which improves loop

stability and transient response. A second capacitor, typi-

cally one-tenth the size of the main compensation capaci-

tor, is sometimes used to reduce the switching frequency

ripple on the VC pin. VC pin ripple is caused by output

voltage ripple attenuated by the output divider and multi-

plied by the error amplifier. Without the second capacitor,

VC pin ripple is:

VC

Pin

Ripple

=

1.2(VRIPPLE)(gm)(RC)

(VOUT)

VRIPPLE = Output ripple (VP–P)

gm = Error amplifier transconductance

(≈850µmho)

RC = Series resistor on VC pin

VOUT = DC output voltage

Total power dissipation = 0.34 + 0.31 + 0.11 + 0.005 =

0.77W

Thermal resistance for LT3436 package is influenced by

the presence of internal or backside planes. With a full

plane under the package, thermal resistance will be about

40°C/W. To calculate die temperature, use the appropriate

To prevent irregular switching, VC pin ripple should be

kept below 50mVP–P. Worst-case VC pin ripple occurs at

maximum output load current and will also be increased if

poor quality (high ESR) output capacitors are used. The

addition of a 150pF capacitor on the VC pin reduces

switching frequency ripple to only a few millivolts. A low

value for RC will also reduce VC pin ripple, but loop phase

margin may be inadequate.

3436fa

11

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