JAJSGQ9A March 2018 – April 2019 DRV8343-Q1
PRODUCTION DATA.
An open load in active mode is disabled by default in the SPI device and can be enabled independently per half-bridge by writing a 1 to the EN_OLA_x bit. In the H/W device, OLA runs if the nDIAG pin is left as unconnected or tied to GND. OLA is detected when the motor gets disconnected from the driver when it is commutating. Figure 40 shows a simplified H-bridge configuration for OLA implementation during high-side current recirculation. When the voltage drop across the body diode of the MOSFET does not exhibit overshoot greater than the VOLA over VM between the time the low-side FET is switched off and the high side FET is switched on during an output PWM cycle. An open load is not detected if the energy stored in the inductor is high enough to cause an overshoot greater than the VOLA over VM caused by the fly-back current flowing through the body diode of the high-side FET.
NOTE
Depending on the operating conditions and on external circuitry, such as the output capacitors, an open load could be reported even though the load is present. This case might occur during a direction change or for small load currents respectively small PWM duty cycles. Therefore, TI recommends evaluating the open load diagnosis only in known suitable operating conditions and to ignore it otherwise.
The device has a failure counter to avoid inadvertent triggering of the open load active diagnosis. Three consecutive occurrences of the internal open load signal must occur, essentially three consecutive PWM pulses without freewheeling detected, before an open load is reported through the nFAULT pin and in the respective SPI register.
In the SPI device, depending on the load configuration and the PWM sequence, OLA on one phase can latch all three OL_PH_x bits high. In that case, the OLP diagnostic can be initiated to determine which phase has the open load condition. The load connections shown in Figure 39 are not supported by OLA.
For OLA to function correctly, place capacitors between the motor phase node and GND. This capacitor is required for BLDC, bi-directional BDC and unidirectional BDC motors at the phase node. If a solenoid load is connected, as shown in Figure 15, the capacitor is not required. Size the capacitors according Equation 5. Make sure that the capacitor (Cphase) is placed on the PCB.
where
The values of Crss and Coss of the MOSFETs should be used for 0-V VDS. Derating of Cphase must be considered when selecting the capacitance.