Datasheet

LTC3780
23
3780ff
For more information www.linear.com/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 I
2
R 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
V
fL
V
V
LBUCK
OUT OUT
IN
,
•
•–1
I
RIPPLE ,BUCK
=
∆I
L ,B UCK
• 100
I
OUT
%
The highest value of ripple current occurs at the maximum
input voltage. In boost mode, the ripple current is:
∆=






I
V
fL
V
V
L BOOST
IN IN
OUT
,
•
•–1
I
RIPPLE ,BOOST
=
∆I
L ,B OOST
• 100
I
IN
%
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•160mV • V
IN(MIN)
2•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 =
V
OUT
•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 (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:
PW
ABOOS T,
••.•..=






=
12
5
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