Zxld1374, Application information – Diodes ZXLD1374 User Manual
Page 18
ZXLD1374
Document number: DS35032 Rev. 3 - 2
18 of 39
September 2012
© Diodes Incorporated
ZXLD1374
A Product Line of
Diodes Incorporated
Application Information
(cont.)
Duty Cycle Calculation and Topology Selection
The duty cycle is a function of the input and output voltages. Approximately, the MOSFET switching duty cycle is
V
V
D
IN
OUT
BUCK
≈
for Buck
V
V
V
D
OUT
IN
OUT
BOOST
−
≈
for Boost
V
V
V
D
IN
OUT
OUT
BB
+
≈
for Buck-Boost
Equation 6
Because D must always be a positive number less than 1, these equations show that
V
V
IN
OUT
<
for Buck (voltage step-down)
V
V
IN
OUT
>
for Boost (voltage step-up)
V
or
or
V
IN
OUT
<
=
>
for Buck-boost (voltage step-down or step-up)
This allows us to select the topology for the required voltage range.
More exact equations are used in the web Calculator. These are:
(
)
V
V
V
R
R
I
V
V
D
DSON
F
IN
COIL
S
OUT
F
OUT
BUCK
−
+
+
+
+
=
for Buck
(
)
V
V
V
V
R
R
I
V
V
D
DSON
F
OUT
F
COIL
S
IN
IN
OUT
BOOST
−
+
+
+
+
−
=
for Boost
(
)(
)
V
V
V
V
R
R
I
I
V
V
D
DSON
F
IN
OUT
COIL
S
OUT
IN
F
OUT
BB
−
+
+
+
+
+
+
=
for Buck-Boost
Equation 7
where
V
F
= schottky diode forward voltage, estimated for the expected coil current, I
COIL
V
DSON
= MOSFET drain source voltage in the ON condition (dependent on R
DSON
and drain current = I
COIL
)
R
COIL
= DC winding resistance of L1
The additional terms are relatively small, so the exact equations will only make a significant difference at lower operating voltages at the input
and output, i.e. low input voltage or a small number of LEDs connected in series. The estimates of V
F
and V
DSON
depend on the coil current.
The mean coil current, I
COIL
depends upon the topology and upon the mean terminal currents as follows:
I
COIL
= I
LED
for Buck
I
COIL
= I
IN
for Boost
I
COIL
= I
IN
+ I
LED
for Buck-Boost
Equation 8
I
LED
is the target LED current and is already known. I
IN
will be calculated with some accuracy later, but can be estimated now from the electrical
power efficiency.