LTC3785_15 Datasheet, PDF (15/20 Pages) Linear Technology – 10V, High Efficiency, Synchronous, No RSENSE Buck-Boost Controller

LTC3785
applications information
Efficiency Considerations
The percentage efficiency of a switching regulator is
equal to the output power divided by the input power
times 100%.
It is often useful to analyze individual losses to determine
what is limiting the efficiency and which change would
produce the most improvement. Although all dissipative
elements in circuits produce losses, four main sources
account for most of the losses in LTC3785 application
circuits:
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:
L
>
(2.7)² •(3.3 – 2.7) •100
500 • 10³ • 3 • 40 • (3.3)²
=
0.67µH
L
>
3.3 •(10 – 3.3) •100
500 • 10³ • 3 • 40 • 10
=
3.7µH
So the worst-case ripple for this application is during buck
mode so a standard inductor value of 3.3µH is chosen.
3785fc
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