ZXLD1371QESTTC datasheet(23/42 Pages) DIODES

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ZXLD1371Q

Document number: DS37116 Rev. 1 - 2

23 of 42

www.diodes.com

September 2015

ZXLD1371Q

Application 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 VF and VDSON depend on the coil

current. The mean coil current, ICOIL depends upon the topology and upon the mean terminal currents as follows:

ICOIL = ILED

for Buck

ICOIL = IIN

for Boost

ICOIL = IIN + ILED

for Buck-Boost

Equation 8

ILED is the target LED current and is already known. IIN 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 POUT is 90% of the input power, PIN, and the coil

current is estimated as follows.

POUT

0.9 PIN

ILED N VLED

0.9 IIN VIN

where N is the number of LEDs connected in series, and VLED is the forward voltage drop of a single LED at ILED.

ILED

VLED

0.9 VI

Equation 9

Equation 9 can now be used to find ICOIL 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:

= 0.5V

IIN × (RS+RCOIL)

= 0.5V

IOUT × (RS+RCOIL)

= 0.5V

VDSON

= 0.1V

(IIN+IOUT)(RS+RCOIL)

= 1.1V

Then Equation 7 becomes

DBUC

VOUT 1

0.4

for Buck

DBOOST

VOUT - VI

VOUT

0.4

for Boost

DBB

VOUT

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 (VREF) 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 ZXLD1371Q operates in Buck mode when VADJ = 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

VADJ, i.e., the Buck mode threshold voltage is 0.65 VADj /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 VREF if the

maximum input voltage is exceeded.

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