Diodes ZXLD1374 - Manual (Page 19)

ZXLD1374
Document number: DS35032 Rev. 3 - 2
19 of 39
www.diodes.com
September 2012
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ZXLD1374
A Product Line of
Diodes Incorporated
Application Information
(cont.)
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
4 . 0 V V
6 . 1 V
D
IN OUT
OUT
BB
+ +
+
≈ 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 2.5V to adjust the LED current proportionally between 10% and 200% of the nominal
value.
For a divider ratio GI_ADJ greater than 0.65V, the ZXLD1374 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
/1.25V.
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.