AN2823 datasheet(29/33 Pages) STMICROELECTRONICS

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AN2823

The L5973AD in LED driving applications

Doc ID 15015 Rev 1

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Figure 29.

Spread of the LED current vs. voltage drop (ILED=350 mA)

Figure 27 and 28 are useful to choose the required voltage sense to satisfy the efficiency

specification. From Figure 29, it follows that the circuit topology using the external

operational amplifier becomes attractive in terms of parameter spread when the sense

voltage is lower than 650 mV (LED current is 350 mA). The trade-off between required

current accuracy and efficiency determines the circuit topology to be used in the design. As

discussed in Section 2.1.4 (topology 2), the accuracy of the current is mainly affected by the

voltage offset of the external error amplifier: better results can be achieved by selecting an

error amplifier with a limited offset (i.e. trimmed). Figure 29 shows the same data provided in

Figure 28, but considering a maximum voltage offset of ±0.5 mV (which is also the typical

offset of the TS321 device): the spread of the load current drops from 10.7% (Figure 29) to

7.7% (Figure 28), when VSENSE=300 mV. As a consequence, selecting an external error

amplifier with a better offset voltage decreases the intersection point of the two traces in

Figure 29, making topology II more convenient than topology I, in terms of current accuracy,

for a wider VSENSE range.

AM01379v1

AN2823 Datasheet(PDF) 29 Page - STMicroelectronics