DC link overcurrent detection is a common requirement in DC/DC converter and motor-drive designs. Although the inductor current of a DC/DC converter or the phase currents of a motor drive are typically sensed for control purposes, phase current monitoring alone is not sufficient to detect all possible overcurrent conditions (such as shoot-through in the power stage, or DC+ and DC– shorts to ground). The most comprehensive way of implementing DC link overcurrent detection is to monitor the current in the DC+ and DC– lines. This detection, as illustrated in Figure 7-1, can be achieved by monitoring the voltage drop across two shunt resistors.

The DC+ load current flowing through the shunt resistor R10 produces a positive voltage with respect to GND1 that is monitored by the AMC23C11. When the voltage drop across R10 exceeds the reference value set by the external resistor R11, the comparator trips and signals the overcurrent event on the open-drain output OUT.

The DC– load current flowing through the shunt resistor R20 produces a negative voltage in respect to GND1 that is monitored by a AMC23C12. When the voltage drop across R20 exceeds the reference value set by the external resistor R21, the comparator trips and signals the overcurrent event on the open-drain output OUT.

The open-drain outputs from both isolated comparators are shorted together to form a single alert signal to the microcontroller unit (MCU). Similarly, both LATCH signals are tied together and can be controlled by a single GPIO pin from the MCU.

The isolated comparator on the DC+ side requires a high-side power supply that is referenced to the DC+ potential. A low-cost solution is based on the push-pull driver SN6501 and a transformer that supports the desired isolation voltage ratings. The integrated low-dropout (LDO) regulator on the high-side of the AMC23C11 allows direct connection of the VDD1 pin to the transformer output and no further preregulation of the transformer output voltage is required.

The isolated comparator on the DC– side requires a high-side power supply that is referenced to the DC– potential. A common solution is to power the isolated comparator from the low-side gate driver supply, as illustrated in Figure 7-1, or any other voltage supply referenced to DC–. The integrated low-dropout (LDO) regulator on the high side of the AMC23C12 supports a wide range of input voltages and greatly simplifies the power-supply design.

The fast response time and high common-mode transient immunity (CMTI) of the AMC23C11 provide reliable and accurate operation even in high-noise environments.

Figure 7-1 Using the AMC23C11 for DC+ Overcurrent Detection