ZXLD1370QESTTC datasheet(18/39 Pages) DIODES

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ZXLD1370Q

Document number: DS37117 Rev. 4 - 2

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www.diodes.com

February 2018

ZXLD1370Q

Application Information (Cont.)

The additional degree of freedom allows us to select GI_ADJ within limits but this may affect overall performance a little. As mentioned above, the

working voltage range at the GI pin is restricted. The permitted range of GI_ADJ in Boost or Buck-Boost configuration is:

0.2 < GI_ADJ < 0.5

Equation 13

The mean voltage across the sense resistor is:

VRS = ICOIL RS

Equation 14

Note that if GI_ADJ is made larger, these equations show that RS is increased and VRS is increased. Therefore, for the same coil current, the

dissipation in RS is increased. So, in some cases, it is better to minimize GI_ADJ. However, consider Equation 5. If ADJ is connected to REF, this

becomes:

















ADJ

225

VRS

This shows that VRS becomes smaller than 225mV if GI_ADJ < 1 - D. If also D is small, VRS can become too small. For example if D = 0.2, and

GI_ADJ is the minimum value of 0.2, then VRS becomes 0.225* 0.2 / 0.8 = 56.25mV. This will increase the LED current error due to small offsets in

the system, such as mV drop in the copper printed wiring circuit, or offset uncertainty in the ZXLD1370Q. If now, GI_ADJ is increased to 0.4 or 0.5,

VRS is increased to a value greater than 100mV. 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 overcurrent 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 is

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











 



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





6875

OUT





















From Equation 16, we set GI_ADJ to 1

– D = 0.3125.

If RGI1

= 33kΩ, then from Equation 17,















 



3125

R 2

Let us choose the preferred value RGI2

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

305

RGI

ADJ























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