LTM4607 datasheet(23/30 Pages) LINER

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LTC3780

3780ff

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applicaTions inForMaTion

3. INTVCC current. This is the sum of the MOSFET driver

and control currents. This loss can be reduced by sup-

plying INTVCC current through the EXTVCC pin from a

high efficiency source, such as an output derived boost

network or alternate supply if available.

4. CIN and COUT loss. The input capacitor has the difficult

job of filtering the large RMS input current to the regula-

tor in buck mode. The output capacitor has the more

difficult job of filtering the large RMS output current in

boost mode. Both CIN and COUT are required to have

low ESR to minimize the AC I2R loss and sufficient

capacitance to prevent the RMS current from causing

additional upstream losses in fuses or batteries.

5. Other losses. Schottky diode D1 and D2 are respon-

sible for conduction losses during dead time and light

load conduction periods. Inductor core loss occurs

predominately at light loads. Switch C causes reverse

recovery current loss in boost mode.

When making adjustments to improve efficiency, the input

current is the best indicator of changes in efficiency. If you

make a change and the input current decreases, then the

efficiency has increased. If there is no change in input

current, then there is no change in efficiency.

Design Example

As a design example, assume VIN = 5V to 18V (12V nomi-

nal), VOUT = 12V (5%), IOUT(MAX) = 5A and f = 400kHz.

Set the PLLFLTR pin at 2.4V for 400kHz operation. The

inductance value is chosen first based on a 30% ripple

current assumption. In buck mode, the ripple current is:

∆=









LBUCK

OUT

•

•–

IRIPPLE,BUCK =

∆IL,BUCK •100

IOUT

The highest value of ripple current occurs at the maximum

input voltage. In boost mode, the ripple current is:

∆=









L BOOST

OUT

•

•–

IRIPPLE,BOOST =

∆IL,BOOST •100

IIN

The highest value of ripple current occurs at VIN = VOUT/2.

A 6.8µH inductor will produce 11% ripple in boost mode

(VIN = 6V) and 29% ripple in buck mode (VIN = 18V).

The RSENSE resistor value can be calculated by using the

maximum current sense voltage specification with some

accommodation for tolerances.

RSENSE =

2•160mV • VIN(MIN)

2•IOUT(MAX,BOOST) • VOUT + ∆IL,BOOST • VIN(MIN)

Select an RSENSE of 10mΩ.

Output voltage is 12V. Select R1 as 20k. R2 is:

R2 =

VOUT •R1

0.8

– R1

Select R2 as 280k. Both R1 and R2 should have a toler-

ance of no more than 1%.

Next, choose the MOSFET switches. A suitable choice is

the Siliconix Si4840 (RDS(ON) = 0.009Ω (at VGS = 6V),

CRSS = 150pF, θJA = 40°C/W).

The maximum power dissipation of switch A occurs in

boost mode when switch A stays on all the time. Assum-

ing a junction temperature of TJ = 150°C with ρ150°C =

1.5, the power dissipation at VIN = 5V is:

ABOOST

•• .• ..

= 







51 50 009 194

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