AN3981 datasheet(46/54 Pages) STMICROELECTRONICS

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Common application information

AN3981

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Doc ID 022217 Rev 1

Equation 19

Assuming a dimming frequency higher than 100 Hz, to use a 16-bit LED driver and to

require a brightness regulation from 0% to 100% in 0.1% steps, the minimum data clock

frequency should be 1.6 MHz.

5.5

Short detection between adjacent pins

When the device is soldered on the application board, undesired short-circuits between

adjacent pins can occur.

A strategy to detect these anomalous conditions (although it works only for OUT pins) is

provided for those devices that can perform faulty LED detection (STP08DP05,

STP16DP05, STP16DPS05, STP16DPPS05 and STP24DP05).

This method is successful if the forward voltage of the LEDs connected to the outputs does

not vary in a wide range.

The detection of the possible short-circuit (in this example a short-circuit between the

OUT10 and OUT11 pins is assumed) is performed in the following steps:

All even outputs are set ON and an error detection is performed. Since OUT10 and

OUT11 are shorted, the current of the generator of the output 10 (the only one turned

on) is shared between LEDs connected to OUT10 and OUT11 (see Figure 56 a),

resulting in a lower voltage drop across those LEDs (and also lower brightness) and

higher voltage drop across both current generators (pins OUT10 and OUT11).

The result of the faulty LED detection is shown below the schematic of Figure 56 a.

The common LED rail voltage (VLED) is progressively increased while performing a

continuous LED error detection, until a new output is detected as faulty (OUT10). In

fact, it is considered as if it were connected to a shorted LED, since the forward voltage

of the LED is lower (half current compared to other channels) and, consequently, the

voltage across the current generator exceeds the short detection threshold.

Considering the error status code, the conclusion is that a short-circuit can be present

between OUT9 and OUT10 or OUT10 and OUT11.

To detect where the short is placed, the previous actions are repeated also for odd

outputs (see Figure 56 b). A new error status code is obtained.

Comparing the two status codes, it is possible to exactly understand where the short-

circuit is placed (see Figure 56 c).

Figure 57

shows a possible implementation of the above described strategy.

LIM

CLK

steps

⋅

≥

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