ZXLD1371QESTTC Diodes Inc. / Zetex, ZXLD1371QESTTC Datasheet - Page 24

• Manufacturer Part Number: ZXLD1371QESTTC
• Description: MOSFET LED Controller TSSOP TSSOP-16EP T&R 2.5K
• Manufacturer: Diodes Inc. / Zetex

Specifications of ZXLD1371QESTTC

• Rohs: yes

ZXLD1371
Document number: DS35436 Rev. 1 - 2
Applications Information
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
and V
currents as follows:
I
now from the electrical power efficiency. If the expected efficiency is roughly 90%, the output power P
input power, P
or
where N is the number of LEDs connected in series, and V
So
Equation 9 can now be used to find I
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:
Then Equation 7 becomes
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
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
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 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
LED
is the target LED current and is already known. I
DSON
I
P
I
V
I
I
V
(I
IN
D
D
D
LED
IN ×
OUT ×
I
I
I
IN
COIL
COIL
COIL
OUT
F
DSON
BUCK
BOOST
BB
+I
depend on the coil current. The mean coil current, I
≈
(R
N V
OUT
= I
= I
= I
(R
IN
I
S
LED
+R
REF
, and the coil current is estimated as follows.
LED
0.9 V
S
LED
IN
IN
)(R
≈
≈
≈
+R
N V
COIL
V
+ I
V
V
V
S
if the maximum input voltage is exceeded.
≈ 0.9 P
OUT
OUT
COIL
IN
≈ 0.9 I
+R
OUT
V
LED
IN
V
OUT
LED
)
+ 0.4
OUT
COIL
+ 1
)
- V
+ V
IN
IN
+ 0.4
)
IN
+ 1.6
IN
V
+ 1
IN
+ 0.4
ADJ
= 0.5V
= 0.5V
= 0.5V
= 0.1V
= 1.1V
(cont.)
, i.e., the Buck mode threshold voltage is 0.65 V
COIL
in Equation 8, which can then be used to estimate the small terms in Equation 7.
www.diodes.com
IN
will be calculated with some accuracy later, but can be estimated
24 of 42
LED
is the forward voltage drop of a single LED at I
COIL
for Buck
for Boost
for Buck-boost
for Buck
for Boost
for Buck-boost
depends upon the topology and upon the mean terminal
REF
) to define the nominal 100% LED current. The
Diodes Incorporated
A Product Line of
ADj
/1.25 V.
ADJ
= 1.25V. If GI_ADJ is less
OUT
© Diodes Incorporated
Equation 7a
Equation 9
Equation 8
LED
February 2012
ZXLD1371
is 90% of the
.
F