ZXLD1374 Ver la hoja de datos (PDF) - Diodes Incorporated.

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componentes Descripción

Fabricante

ZXLD1374

60V HIGH ACCURACY 1.5A BUCK/BOOST/BUCK-BOOST LED DRIVER CONVERTER WITH AEC-Q100

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ZXLD1374

Application Information (cont.)

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

or

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.

So

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:

VF

IIN × (RS+RCOIL)

IOUT × (RS+RCOIL)

VDSON

(IIN+IOUT)(RS+RCOIL)

= 0.5V

= 0.5V

= 0.5V

= 0.1V

= 1.1V

Then Equation 7 becomes

DBUCK

≈

VOUT + 1

VIN + 0.4

DBOOST

≈

VOUT − VIN + 1

VOUT + 0.4

DBB

≈

VOUT + 1.6

VOUT + VIN + 0.4

for Buck

for Boost

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 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 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 /1.25V.

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 VREF if the maximum

input voltage is exceeded.

ZXLD1374

Document number: DS35032 Rev. 3 - 2

19 of 39

www.diodes.com

September 2012

© Diodes Incorporated

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