LTC3780EG Datasheet, PDF (23/30 Pages) Linear Technology – High Efficiency, Synchronous,4-Switch Buck-Boost Controller

LTC3780
APPLICATIONS INFORMATION
3. INTV_CC current. This is the sum of the MOSFET driver
and control currents. This loss can be reduced by sup-
plying INTV_CC current through the EXTV_CC pin from a
high efficiency source, such as an output derived boost
network or alternate supply if available.
4. C_IN and C_OUT 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 C_IN and C_OUT 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 V_IN = 5V to 18V (12V nomi-
nal), V_OUT = 12V (5%), I_OUT(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:
∆I_L,BUCK
=
V_OUT
f •L
•


1–
V_OUT
V_IN


I_RIPPLE,BUCK
=
∆I_L,BUCK
I_OUT
• 100
%
The highest value of ripple current occurs at the maximum
input voltage. In boost mode, the ripple current is:
∆I_L,BOOST
=
V_IN
f •L
•


1–
V_IN
V_OUT


I_RIPPLE,BOOST
=
∆I_L,BOOST
I_IN
• 100
%
The highest value of ripple current occurs at V_IN = V_OUT/2.
A 6.8µH inductor will produce 11% ripple in boost mode
(V_IN = 6V) and 29% ripple in buck mode (V_IN = 18V).
The R_SENSE resistor value can be calculated by using the
maximum current sense voltage specification with some
accommodation for tolerances.
R_SENSE
=
2
2 •160mV • V_IN(MIN)
•I_{OUT(MAX,BOOST)} • V_OUT + ∆I_L,BOOST
•
V_IN(MIN)
Select an R_SENSE of 10mΩ.
Output voltage is 12V. Select R1 as 20k. R2 is:
R2 = ^{VOUT • R1} – 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 (R_DS(ON) = 0.009Ω (at V_GS = 6V),
C_RSS = 150pF, θ_JA = 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 T_J = 150°C with ρ_150°C =
1.5, the power dissipation at V_IN = 5V is:
P_A,BOOST
=


12
5
•
5^{}
2
•
1.5
•
0.009
=
1.94W
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