Diodes ZXLD1371 - Manual (Page 24)

ZXLD1371
ZXLD1371
Document number: DS35436 Rev. 1 - 2
24 of 42
www.diodes.com
February 2012
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A Product Line of
Diodes Incorporated
Applications Information (cont.)
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. 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 I
IN
≈
I
LED
N V
LED
0.9 V
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
D
BUCK
≈
V
OUT
+ 1
V
IN
+ 0.4
for Buck
D
BOOST
≈
V
OUT
- V
IN
+ 1
V
OUT
+ 0.4
for Boost
D
BB
≈
V
OUT
+ 1.6
V
OUT
+ V
IN
+ 0.4
for Buck-boost
Equation 7a
Setting the LED Current
The LED current requirement determines the choice of the sense resistor Rs. This also depends on the voltage on the ADJ
pin and the voltage on the GI pin, according to the topology required.
The ADJ pin may be connected directly to the internal 1.25V reference (V
REF
) to define the nominal 100% LED current. The
ADJ pin can also be driven with an external dc voltage between 125mV and 1.25V to adjust the LED current proportionally
between 10% and 100% of the nominal value.
For a divider ratio GI_ADJ greater than 0.65V, the ZXLD1371 operates in Buck mode when V
ADJ
= 1.25V. If GI_ADJ is less
than 0.65V (typical), the device operates in Boost or buck-Boost mode, according to the load connection. This 0.65V
threshold varies in proportion to V
ADJ
, i.e., the Buck mode threshold voltage is 0.65 V
ADj
/1.25 V.
ADJ and GI are high impedance inputs within their normal operating voltage ranges. An internal 1.3V clamp protects the
device against excessive input voltage and limits the maximum output current to approximately 4% above the maximum
current set by V
REF
if the maximum input voltage is exceeded.