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LTC3780_12 Datasheet, PDF (22/28 Pages) Linear Technology – High Effi ciency, Synchronous, 4-Switch Buck-Boost Controller | |||
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LTC3780
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 efï¬ciency source, such as an output derived boost
network or alternate supply if available.
4. CIN and COUT loss. The input capacitor has the difï¬cult
job of ï¬ltering the large RMS input current to the regula-
tor in buck mode. The output capacitor has the more
difï¬cult job of ï¬ltering 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 sufï¬cient
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 efï¬ciency, the input
current is the best indicator of changes in efï¬ciency. If you
make a change and the input current decreases, then the
efï¬ciency has increased. If there is no change in input
current, then there is no change in efï¬ciency.
Design Example
As a design example, assume VIN = 5V to 18V (12V nominal),
VOUT = 12V (5%), IOUT(MAX) = 5A and f = 400kHz.
Set the PLLFLTR pin at 2.4V for 400kHz operation. The
inductance value is chosen ï¬rst based on a 30% ripple
current assumption. In buck mode, the ripple current is:
ÎIL,BUCK
=
VOUT
f sL
â
s ââ 1n
VOUT
VIN
â
â â
IRIPPLE,BUCK
=
ÎIL,BUCK
IOUT
s 100
%
The highest value of ripple current occurs at the maximum
input voltage. In boost mode, the ripple current is:
ÎIL,BOOST
=
VIN
f sL
s
â
ââ
1n
VIN
VOUT
â
â â
IRIPPLE,BOOST
=
ÎIL,BOOST
IIN
s 100
%
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 speciï¬cation with some
accommodation for tolerances.
RSENSE
=
2
2 s160mV s VIN(MIN)
sIOUT(MAX,BOOST) s VOUT + ÎIL,BOOST
s
VIN(MIN)
Select an RSENSE of 10mΩ.
Output voltage is 12V. Select R1 as 20k. R2 is:
R2 = VOUT ⢠R1 â R1
0.8
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 Ï150°C =
1.5, the power dissipation at VIN = 5V is:
PA,BOOST
=
â
ââ
12
5
s
5ââ â
2
s
15
s
0.009
=
1.94W
3780fe
22
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