LTC3785 Datasheet, PDF (15/20 Pages) Linear Technology – 10V, High Effi ciency, Synchronous, No RSENSE Buck-Boost Controller

LTC3785
APPLICATIO S I FOR ATIO
1. DC I²R losses. These arise from the resistances of the
MOSFETs, sensing resistor (if used), inductor and PC
board traces and cause the efficiency to drop at high
output currents.
2. Transition loss. This loss arises from the brief voltage
transition time of switch A or switch C. It depends upon
the switch voltage, inductor current, driver strength and
MOSFET capacitance, among other factors.
Transition Loss ~ V_SW2 • I_L • C_RSS • f
where C_RSS is the reverse transfer capacitance.
3. 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.
4. Other losses. Optional Schottky diodes D1 and D2 are
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 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.
5. V_CC regulator loss. In applications where the input
voltage is above 5V, such as two Li-Ion cells, the V_CC
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_CC pin can be driven
directly from a high efficiency external 5V source if
desired to incrementally improve overall efficiency at
lighter loads.
DESIGN EXAMPLE
As a design example, assume V_IN = 2.7V to 10V (3.6V
nominal Li-Ion with 9V adapter), V_OUT = 3.3V (5%),
I_OUT(MAX) = 3A and f = 500kHz.
Determine the Inductor Value
Setting the Inductor Ripple to 40% and using the equations
in the Inductor Selection section gives:
(^2.7)^{2 •} (^{3.3 – 2.7}) ^{• 100}
( ) L >
500 • 10³ • 3 • 40 •
3.3 ²
= 0.67µH
3.3 • ₍10 – 3.3₎ • 100
L>
= 3.7µH
500 • 10³ • 3 • 40 • 10
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:
I_{L(MAX _ AV)}
=
V_OUT • I_OUT
V_IN • η
where η = estimated efficiency in this mode (use 80%).
I_L(MAX
_
AV)
=
3.3 • 3
2.7 • 0.8
=
4.6A
To limit the maximum efficiency loss of the inductor ESR
to below 5% the equation is:
ESR_L(MAX)
~
V_OUT • I_OUT • %Loss
I_{L(MAX _ AV)2} • 100
=
24mΩ
A suitable inductor for this application could be a Coiltronics
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.
3785f
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