L9953 / L9953XP

Application information

Doc ID 14278 Rev 3

21/38

3.10

Current monitor

The current monitor output sources a current image at the current monitor output which has

a fixed ratio (1/10000) of the instantaneous current of the selected highside driver. The bits

18 and 19 of the Input Data Register 0 control which of the outputs OUT1, OUT4, OUT5,

and OUT8 will be multiplexed to the current monitor output. The current monitor output

allows a more precise analysis of the actual state of the load rather than the detection of an

open- or overload condition. For example this can be used to detect the motor state

(starting, free-running, stalled). Moreover, it is possible to regulate the power of the defroster

more precise by measuring the load current. The current monitor output is bidirectional (c.f.

PWM inputs).

3.11

PWM inputs

Each driver has a corresponding PWM enable bit which can be programmed by the SPI

interface. If the PWM enable bit in Input Data Register 1 is set, the output is controlled by the

logically AND-combination of the PWM signal and the output control bit in Input Data

Register 0. The outputs OUT1-OUT6 and OUT8 are controlled by the PWM1 input and the

output OUT7 is controlled by the bidirectional input CM/PMW2. For example, the two PWM

inputs can be used to dim two lamps independently by external PWM signals.

3.12

Cross-current protection

The six half-bridges of the device are cross-current protected by an internal delay time. If

one driver (LS or HS) is turned-off the activation of the other driver of the same half bridge

will be automatically delayed by the cross-current protection time. After the cross-current

protection time is expired the slew-rate limited switch-off phase of the driver will be changed

to a fast turn-off phase and the opposite driver is turned-on with slew-rate limitation. Due to

this behavior it is always guaranteed that the previously activated driver is totally turned-off

before the opposite driver will start to conduct.

3.13

Programmable softstart function to drive loads with higher

inrush current

Loads with start-up currents higher than the over-current limits (e.g. inrush current of lamps,

start current of motors and cold resistance of heaters) can be driven by using the

programmable softstart function (i.e. overcurrent recovery mode). Each driver has a

corresponding over-current recovery bit. If this bit is set, the device will automatically switch-

on the outputs again after a programmable recovery time. The duty cycle in over-current

condition can be programmed by the SPI interface to be about 12% or 25%. The PWM

modulated current will provide sufficient average current to power up the load (e.g. heat up

the bulb) until the load reaches operating condition. The PWM frequency settles at 3kHz or

6kHz. The device itself cannot distinguish between a real overload and a non linear load like

a light bulb. A real overload condition can only be qualified by time. As an example the

microcontroller can switch on light bulbs by setting the over-current Recovery bit for the first

50ms. After clearing the recovery bit the output will be automatically disabled if the overload

condition still exits.