LTC3785-1 Linear Technology, LTC3785-1 Datasheet - Page 15

• Manufacturer Part Number: LTC3785-1
• Description: Buck-Boost Controller
• Manufacturer: Linear Technology

www.DataSheet4U.com
APPLICATIONS INFORMATION
1. DC I
2. Transition loss. This loss arises from the brief voltage
3. C
4. Other losses. Optional Schottky diodes D1 and D2 are
5. V
DESIGN EXAMPLE
As a design example, assume V
nominal Li-Ion with 9V adapter), V
I
OUT(MAX)
MOSFETs, sensing resistor (if used), inductor and PC
board traces and cause the effi ciency to drop at high
output currents.
transition time of switch A or switch C. It depends upon
the switch voltage, inductor current, driver strength and
MOSFET capacitance, among other factors.
where C
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.
responsible for conduction losses during dead time
and light load conduction periods. Core loss is the
predominant inductor loss at light loads. Turning on
switch C causes reverse recovery current loss in boost
mode. 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.
voltage is above 5V, such as two Li-Ion cells, the V
regulator will dissipate some power due the differential
voltage and the average output current to the drive the
gates of the output switches. The V
directly from a high effi ciency external 5V source if
desired to incrementally improve overall effi ciency at
lighter loads.
IN
CC
Transition Loss ~ V
and C
2
regulator loss. In applications where the input
R losses. These arise from the resistances of the
= 3A and f = 500kHz.
RSS
OUT
is the reverse transfer capacitance.
loss. The input capacitor has the diffi cult
IN
SW
and C
2
• I
L
OUT
• C
IN
2
R loss and suffi cient
RSS
= 2.7V to 10V (3.6V
are required to have
CC
OUT
• f
pin can be driven
= 3.3V (5%),
CC
Determine the Inductor Value
Setting the Inductor Ripple to 40% and using the equations
in the Inductor Selection section gives:
So the worst-case ripple for this application is during buck
mode so a standard inductor value of 3.3μH is chosen.
Determine the Proper Inductor Type Selection
The highest inductor current is during boost mode and
is given by:
where η = estimated effi ciency in this mode (use 80%).
To limit the maximum effi ciency loss of the inductor ESR
to below 5% the equation is:
A suitable inductor for this application could be a Coiltron-
ics CD1-3R8 which has a rating DC current of 6A and ESR
of 13mΩ.
Choose a Proper MOSFET Switch
Using the same guidelines for ESR of the inductor, one
suitable MOSFET could be the Siliconix Si7940DP which
is a dual MOSFET in a surface mount package with 25mΩ
at 2.5V and a total gate charge of 12nC.
Checking the power dissipation of each switch will ensure
reliable operation since the thermal resistance of the
package is 60°C/W.
I
I
L >
L >
ESR
L MAX AV
L MAX AV
(
(
500 10
500 10
( ) (
L MAX
3 3 10 3 3 100
2 7
(
. •
_
_
.
•
•
2
(
)
)
)
~
•
=
=
3
3
V
3 3 2 7
2 7 0 8
– .
V
I
• •
• •
3 3 3
OUT
L MAX AV
OUT
. • .
. – .
3 40
3 40 10
(
. •
V
IN
•
)
•
•
I
•
I
OUT
_
OUT
η
•
•
)
=
( )
•
3 3
)
• %
4 6
2
100
.
.
•
=
100
Loss
A
2
3 7
.
=
LTC3785-1
μ μ H
0
=
. .
67
24
μH
m m Ω
15
37851f