ZXLD1371QESTTC datasheet(24/42 Pages) DIODES

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ZXLD1371Q

Document number: DS37116 Rev. 1 - 2

24 of 42

www.diodes.com

September 2015

ZXLD1371Q

Application Information (cont.)

Buck Topology

In Buck mode, GI is connected to ADJ as in Figure 5. The LED current

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

SBuck

0.21

ILED

VADJ

V E

Equation 10

If ADJ is directly connected to VREF, this becomes:

SBuck

0.21

ILED

Figure 5 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,

SBoostBB

0.225

ILED

GI ADJ

VADJ

V E

Equation 11

If ADJ is connected to REF, this becomes

SBoostBB

0.225

ILED

GI ADJ

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

GI ADJ

GI1

GI2

Figure 6 Setting LED current in Boost and Buck-

boost configurations

Note that from considerations of ZXLD1371Q input bias current, the recommended limits for RGI1 are:

22kΩ < GI1 < 100kΩ

Equation 12

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

V S 0.225

GI ADJ

1 D

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 = 56.25 mV. 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 ZXLD1371Q. If now,

GI_ADJ is increased to 0.4 or 0.5, VRS is increased to a value greater than 100mV.

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