ZXLD1371QESTTC datasheet(24/42 Pages) DIODES

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ZXLD1371

Document number: DS35436 Rev. 1 - 2

24 of 42

www.diodes.com

February 2012

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 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.

IIN ≈

ILED N VLED

0.9 VIN

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

DBUCK

≈

VOUT + 1

VIN + 0.4

for Buck

DBOOST ≈

VOUT - VIN + 1

VOUT + 0.4

for Boost

DBB

≈

VOUT + 1.6

VOUT + VIN + 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 ZXLD1371 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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