Zxld1370, Application information – Diodes ZXLD1370 User Manual
Page 17
ZXLD1370
Document number: DS32165 Rev. 5 - 2
17 of 39
September 2012
© Diodes Incorporated
ZXLD1370
A Product Line of
Diodes Incorporated
Application Information
(cont.)
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
Equation 7
(
)(
)
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
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
LED
for
Buck
I
COIL
=
I
IN
for Boost
Equation 8
I
IN
+ I
LED
for Buck-boost
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. If the expected efficiency is roughly 90%, the output power P
OUT
is 90% of the input power, P
IN
, and the coil current is
estimated as follows.
P
OUT
≈ 0.9 P
IN
or
I
LED
N V
LED
≈ 0.9 I
IN
V
IN
where N is the number of LEDs connected in series, and V
LED
is the forward voltage drop of a single LED at I
LED
.
So
V
9
.
0
V
N
I
I
IN
LED
LED
IN
≈
Equation 9
Equation 9 can now be used to find I
COIL
in Equation 8, which can then be used to estimate the small terms in Equation 7. This completes the
calculation of Duty Cycle and the selection of Buck, Boost or Buck-boost topology.
An initial estimate of duty cycle is required before we can choose a coil. In Equation 7, the following approximations are recommended:
V
F
= 0.5V
I
IN
(R
S
+R
COIL
) =
0.5V
I
OUT
(R
S
+R
COIL
)
= 0.5V
V
DSON
=
0.1V
(I
IN
+I
OUT
)(R
S
+R
COIL
)
= 1.1V
Then Equation 7 becomes
4
.
0
V
1
V
D
IN
OUT
BUCK
+
+
≈
for Buck
4
.
0
V
1
V
V
D
OUT
IN
OUT
BOOST
+
+
−
≈
for Boost
Equation 7a
4
.
0
V
V
6
.
1
V
D
IN
OUT
OUT
BB
+
+
+
≈
for Buck-boost