LTC3780
APPLICATIONS INFORMATION
3. INTV current. This is the sum of the MOSFET driver
Thehighestvalueofripplecurrentoccursatthemaximum
input voltage. In boost mode, the ripple current is:
CC
and control currents. This loss can be reduced by sup-
plying INTV current through the EXTV pin from a
CC
CC
V
IN
⎛
V
IN
⎞
ΔIL,BOOST
=
s 1n
high efficiency source, such as an output derived boost
⎜
⎟
⎠
f sL ⎝ VOUT
network or alternate supply if available.
ΔIL,BOOST s100
4. C and C
loss. The input capacitor has the difficult
IN
OUT
IRIPPLE,BOOST
=
%
joboffilteringthelargeRMSinputcurrenttotheregula-
tor in buck mode. The output capacitor has the more
difficult job of filtering the large RMS output current
I
IN
The highest value of ripple current occurs at V = V /2.
IN
OUT
in boost mode. Both C and C
are required to have
IN
OUT
A 6.8μH inductor will produce 11% ripple in boost mode
(V = 6V) and 29% ripple in buck mode (V = 18V).
2
low ESR to minimize the AC I R loss and sufficient
capacitance to prevent the RMS current from causing
additional upstream losses in fuses or batteries.
IN
IN
The R
resistor value can be calculated by using the
maximum current sense voltage specification with some
accommodation for tolerances.
SENSE
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.
2s160mV sV
IN(MIN)
RSENSE
=
2sIOUT(MAX,BOOST) sVOUT + ΔIL,BOOST sV
IN(MIN)
Select an R
of 10mΩ.
SENSE
Whenmakingadjustmentstoimproveefficiency, theinput
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.
Output voltage is 12V. Select R1 as 20k. R2 is:
VOUT •R1
R2 =
–R1
0.8
Select R2 as 280k. Both R1 and R2 should have a toler-
ance of no more than 1%.
Design Example
Asadesignexample,assumeV =5Vto18V(12Vnominal),
IN
Next, choose the MOSFET switches. A suitable choice is
V
OUT
= 12V (5%), I
= 5A and f = 400kHz.
OUT(MAX)
the Siliconix Si4840 (R
= 0.009Ω (at V = 6V),
DS(ON)
= 150pF, θ = 40°C/W).
GS
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:
C
RSS
JA
The maximum power dissipation of switch A occurs in
boost mode when switch A stays on all the time. Assum-
⎛
⎞
VOUT
f sL
VOUT
ing a junction temperature of T = 150°C with ρ
=
J
150°C
ΔIL,BUCK
=
s 1n
⎜
⎝
⎟
1.5, the power dissipation at V = 5V is:
V
IN
⎠
IN
2
12
5
⎛
⎞
⎠
ΔIL,BUCK s100
PA,BOOST
=
s5 s1ꢀ5s0.009=1.94W
⎜
⎝
⎟
IRIPPLE,BUCK
=
%
IOUT
3780fe
22
Linear Systems [ Linear Systems ]
