UM1557 datasheet(28/111 Pages) STMICROELECTRONICS

28 / 111 page

Analogue current sense

UM1557

28/111

Doc ID 023521 Rev 2

In order to achieve a better current sense accuracy one or both of the strategies listed below

can be adopted:

Current sense calibration (K-factor measurement) of each HSD

VBAT measurement  bulb current compensation by appropriate software

3.5.2

Diagnostics with different load options

In some cases the requirement profile asks for alternative loads driven with one and the

same high-side driver. This can be a bulb lamp with the alternative of an LED (- cluster). In

this case the driver:

–

Has to handle the high inrush current of the bulb load

–

Must provide sufficiently low power dissipation during continuous operation

–

Must not indicate an open load in case an LED (-cluster) is applied instead of a

bulb.

In the case of different load options (bulb/LED), there is the possibility to use two different

(switchable) sense resistors in order to use the current sense band in the appropriate range

matching the different load currents.

An example of a current sense resistor switching circuit can be seen in the Figure 22. The

measured scale can be extended by Rsense1 switched in parallel to Rsense2 by MOSFET Q1.

Table 3.

Paralleling bulbs-overview

5+5 W

OK without calibration

21+21 W

27+27 W

21+5 W

Calibration and VBAT monitoring recommended

21+21+5 W

Calibration and VBAT monitoring necessary

UM1557 Datasheet(PDF) 28 Page - STMicroelectronics