ZXLD1370EST16TC datasheet(18/40 Pages) DIODES

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ZXLD1370

Document number: DS32165 Rev. 6 - 2

18 of 40

www.diodes.com

December 2016

ZXLD1370

Application Information (Cont.)

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 ZXLD1370 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 2.6V 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.

Buck Topology

In Buck mode, GI is connected to ADJ as in Figure 25. The LED current depends only

upon RS, VADJ and VREF. From Equation 1 above,



























218

REF

ADJ

LED

SBUCK

Equation 10

If ADJ is directly connected to VREF, this becomes:















218

LED

SBUCK

Figure 25 Setting LED Current in

Buck Configuration

Boost and Buck-Boost Topology

For Boost and Buck-boost topologies, the LED current depends upon the resistors, RS,

RGI1, and RGI2 as in Equations 4 and 2 above. There is more than one degree of

freedom. That is to say, there is not a unique solution. From Equation 4,



























ADJ

225

REF

ADJ

LED

SBOOSTBB

Equation 11

If ADJ is connected to REF, this becomes

ADJ

225

LED

SBOOSTBB















GI_ADJ is given by Equation 2, repeated here for convenience:

















RGI

ADJ

Figure 26 Setting LED Current in Boost

and Buck-Boost Configuration

Note that from considerations of ZXLD1370 input bias current, the recommended limits for RGI1 are:

22kΩ < RGI1 < 100kΩ

Equation 12

ISM

VIN

REF

ADJ

SGND

ISM

VIN

REF

ADJ

SGND

RGI2

RGI1

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