L9959S datasheet(24/43 Pages) STMICROELECTRONICS

24 / 43 page

Application information

L9959S, L9959T

24/43

DocID023416 Rev 6

4.2.1

Current feedback

A feedback current signal is provided at pin CF (Current Feedback). This current is

proportional to the current in the H-Bridge, but does not change its direction. It is measured

in the low-side transistor, which is not switched by PWM. This is determined by the input

DIR or the SDIR register respectively. Therefore, the direction of the current can be seen

from this direction signal. Whenever the current direction changes, the current feedback

signal is blanked. In Table 13: Current feedback (CF) the CF behavior over an external

resistor of 5.1k Ohm is specified. The current out of CF consists of a static offset current and

a current proportional to the current in the select low-side transistor. The voltage at pin CF

scales with the resistor at this pin.

Figure 14. Current feedback and current limiting

Figure 14 Current Feedback and Current Limiting shows the current feedback in case the

OUT1 is controlled by PWM (DIR = 0). In this case, the current is measured through low-

side 2. If the direction is inverted, the current is measured through low-side 1.

4.2.2

Current limitation

The H-Bridge output current can be limited to three different values (see Table 14: Current

limiting). If the current reaches the current limiting threshold ICL, the output driver is

switched off after the blanking time tb, and switched on again after the current dropped

below the lower current limit hysteresis threshold (ICL – IHYS). The current limiting

thresholds can be adjusted using the resistor at pin CF. The values in Table 14 refer to a

5.1k Ohm external resistor. The current limiting threshold can be calculated by (4.5V/RCF -

IOFFSET) * (ICLx*5.1k/4.45V) from Table 14 and IOFFSET=10µA (typical). RCF is the resistor

used at pin CF. The overcurrent threshold is not changed by RCF (see Table 15). The

current limitation is active as long as the output driver is switched on. The current limitation

is also active during free-wheeling. The information that the device is in current limitation is

stored in the diagnosis register 2 (DIA_REG2).

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