ZXLD1371QESTTC datasheet(26/42 Pages) DIODES

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ZXLD1371

Document number: DS35436 Rev. 1 - 2

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February 2012

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Applications Information (cont.)

This will give small enough ILED error for most practical purposes. Satisfactory operation will be obtained if VRS is more than

about 80mV. This means GI_ADJ should be greater than (1-DMIN) * 80/225 = (1- DMIN) * 0.355.

There is also a maximum limit on VRS which gives a maximum limit for GI_ADJ. If VRS exceeds approximately 300mV, or

133% of 225mV, the STATUS output may indicate an over-current condition. This will happen for larger DMAX. Therefore,

together with the requirement of Equation 13, the recommended range for GI_ADJ is

0.355 ( 1-DMIN) < GI_ADJ < 1.33 ( 1-DMAX )

Equation 15

An optimum compromise for GI_ADJ has been suggested, i.e.

GI_ADJAUTO = 1 - DMAX

Equation 16

This value has been used for the “Automatic” setting of the web Calculator. If 1-DMAX is less than 0.2, then GI_ADJ is set to

0.2. If 1- DMAX is greater than 0.5 then GI_ADJ is set to 0.5.

Once GI_ADJ has been selected, a value of RGI1 can be selected from Equation 12. Then RGI2 is calculated as follows,

rearranging Equation 2:

RGI2 = RGI1

1-GI_ADJ

GI_ADJ

Equation 17

For example to drive 12 LEDS at a current of 350mA from a 12V supply requires Boost configuration. Each LED has a

forward voltage of 3.2V at 350mA, so Vout = 3.2*12 = 38.4V. From Equation 6, the duty cycle is approximately

VOUT-VIN

VOUT

38.4-12

38.4

= 0.6875

From Equation 16, we set GI_ADJ to 1 – D = 0.3125.

IF RGI1 = 33kΩ, then from Equation 17, RGI2 = 33000 * ( 1 -0.3125 ) / 0.3125 = 72.6kΩ. Let us choose the preferred value

RGI2 = 75kΩ. Now GI_ADJ is adjusted to the new value, using Equation 2.

GI_ADJ =

RGI1

RGI1 +RGI2

33k

33k +75k

=0.305

Now we calculate Rs from Equation 11. Assume ADJ is connected to REF.

RSBoostBB =

0.225

ILED

GI_ADJ

VADJ

VREF

0.225

0.35

* 0.305 = 0.196

Ω

A preferred value of RSBoostBB = 0.2Ω will give the desired LED current with an error of 2% due to the preferred value

selection.

Table 1 shows typical resistor values used to determine the GI_ADJ ratio with E24 series resistors.

Table 1

GI ratio

RGI1

RG2

0.2

30k

120k

0.25

33k

100k

0.3

39k

91k

0.35

30k

56k

0.4

100k

150k

0.45

51k

62k

0.5

30k

This completes the LED current setting.

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