L9951 / L9951XP

Application information

Doc ID 14173 Rev 9

19/36

3

Application information

3.1

Dual power supply: VS and VCC

The power supply voltage VS supplies the half bridges and the high side drivers. An internal

charge-pump is used to drive the high side switches. The logic supply voltage VCC

(stabilized 5V) is used for the logic part and the SPI of the device. Due to the independent

logic supply voltage the control and status information will not be lost, if there are temporary

spikes or glitches on the power supply voltage. In case of power-on (VCC increases from

under voltage to VPOR OFF = 4.0V, typical) the circuit is initialized by an internally generated

power-on-reset (POR).

If the voltage VCC decreases under the minimum threshold (VPOR ON =3.6V, typical), the

outputs are switched to tristate (high impedance) and the status registers are cleared.

3.2

Standby - mode

The standby mode of the L9951 is activated by switching the EN input do GND. All latched

data will be cleared and the inputs and outputs are switched to high impedance. In the

standby mode the current at VS (VCC) is less than 3 µA (1µA) for CSN = high (DO in tristate).

If EN is switched to 5V the device will enter the active mode. In the active mode the charge-

pump and the supervisor functions are activated.

3.3

Inductive loads

Each half bridge is built by an internally connected high side and a low side power DMOS

transistor. Due to the built-in reverse diodes of the output transistors, inductive loads can be

driven at the outputs OUT1 to OUT3 without external free-wheeling diodes. The high side

drivers OUT4 to OUT5 are intended to drive resistive loads. Hence only a limited energy

(E<0.5mJ) can be dissipated by the internal ESD-diodes in freewheeling condition. For

inductive loads (L > 50µH) an external free-wheeling diode connected to GND and the

corresponding output is needed.

3.4

Diagnostic functions

All diagnostic functions (over/open-load, power supply over-/undervoltage, temperature

warning and thermal shutdown) are internally filtered and the condition has to be valid for at

least 32µs (open-load: 1ms, respectively) before the corresponding status bit in the status

registers will be set. The filters are used to improve the noise immunity of the device. Open-

load and temperature warning function are intended for information purpose and will not

change the state of the output drivers. On contrary, the over load and thermal shutdown

condition will disable the corresponding driver (over load) or all drivers (thermal shutdown),

respectively. Without setting the over-current recovery bit in the Input Data Register to logic

high, the microcontroller has to clear the over-current status bit to reactivate the

corresponding driver. 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 short recovery time. The

duty cycle in over-current condition can be programmed by the SPI interface (12% or 25%).

With this feature the device can drive loads with start-up currents higher than the over-

current limits (e.g. inrush current of lamps, cold resistance of motors and heaters).