LTM4607 datasheet(22/28 Pages) LINER

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LTC3780

3780fe

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

Asadesignexample,assumeVIN=5Vto18V(12Vnominal),

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:

Δ=

⎛

⎝⎜

⎞

⎠⎟

LBUCK

OUT

IRIPPLE,BUCK =

ΔI

L,BUCK 100

IOUT

The highest value of ripple current occurs at the maximum

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

Δ=

⎛

⎝⎜

⎞

⎠⎟

L BOOST

OUT

IRIPPLE,BOOST =

ΔI

L,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 +ΔIL,BOOST VIN(MIN)

Select an RSENSE of 10mΩ.

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

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:

A BOOST

= ⎛

⎝⎜

⎞

⎠⎟

1 5 0 009 1 94

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