LTC1871-1 LINER [Linear Technology], LTC1871-1 Datasheet - Page 22

• Manufacturer Part Number: LTC1871-1
• Description: High Effi ciency, Synchronous, 4-Switch Buck-Boost Controller
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LTC3780
3. INTV
4. C
5. Other losses. Schottky diode D1 and D2 are respon-
When making adjustments to improve effi ciency, the input
current is the best indicator of changes in effi ciency. If you
make a change and the input current decreases, then the
effi ciency has increased. If there is no change in input
current, then there is no change in effi ciency.
Design Example
As a design example, assume V
V
Set the PLLFLTR pin at 2.4V for 400kHz operation. The
inductance value is chosen fi rst based on a 30% ripple
current assumption. In buck mode, the ripple current is:
22
OUT
and control currents. This loss can be reduced by sup-
plying INTV
high effi ciency source, such as an output derived boost
network or alternate supply if available.
job of fi ltering the large RMS input current to the regula-
tor in buck mode. The output capacitor has the more
diffi cult job of fi ltering the large RMS output current
in boost mode. Both C
low ESR to minimize the AC I
capacitance to prevent the RMS current from causing
additional upstream losses in fuses or batteries.
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.
Δ
I
RIPPLE,BUCK
IN
I
L BUCK
= 12V (5%), I
,
and C
CC
current. This is the sum of the MOSFET driver
=
OUT
V
CC
f L
OUT
=
loss. The input capacitor has the diffi cult
current through the EXTV
ΔI
OUT(MAX)
L,BUCK
⎛
⎝ ⎜
I
1
OUT
IN
V
V
OUT
and C
IN
100
= 5A and f = 400kHz.
IN
⎞
⎠ ⎟
= 5V to 18V (12V nominal),
OUT
%
2
R loss and suffi cient
are required to have
CC
pin from a
R
The highest value of ripple current occurs at the maximum
input voltage. In boost mode, the ripple current is:
The highest value of ripple current occurs at V
A 6.8μH inductor will produce 11% ripple in boost mode
(V
The R
maximum current sense voltage specifi cation with some
accommodation for tolerances.
Select an R
Output voltage is 12V. Select R1 as 20k. R2 is:
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
C
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
1.5, the power dissipation at V
RSS
SENSE
IN
Δ
P
I
R2 =
RIPPLE,BOOST
A BOOST
I
= 6V) and 29% ripple in buck mode (V
L BOOST
,
= 150pF , θ
SENSE
,
=
V
2 I
OUT
SENSE
0.8
resistor value can be calculated by using the
=
=
OUT(MAX,BOOST)
⎛
⎝ ⎜
• R1
f L
12
JA
V
5
=
IN
of 10mΩ.
= 40°C/W).
– R1
ΔI
5
⎞
⎠ ⎟
⎛
⎜
⎝
L,BOOST
1
2
2 160mV V
DS(ON)
I
IN
1 5 0 009 1 94
V
V
OUT
IN
V
IN
100
= 0.009Ω (at V
OUT
⎞
⎟
⎠
J
.
= 5V is:
= 150°C with ρ
%
+ ΔI
IN(MIN)
=
L,BOOST
.
IN
W
IN
= 18V).
GS
= V
V
150°C
= 6V),
OUT
IN(MIN)
3780fe
/2.
=