Automatic current limiting regulation protects power switch

Some loads require more current during startup than during operation; some loads can limit the current to a lower power level during startup, but require more current during operation. The application circuit described in this article can automatically increase or decrease the overcurrent protection level of the power circuit after the startup is completed.

Installing switches and fuses between the power supply and its load can control and protect the power supply. An improved method is to integrate simple switches and fuses in a single package to achieve the same function without replacing components and maintenance. The MAX5976 hot-swap power supply solution introduced in this article integrates power MOSFETs and driver circuits internally to provide on-off control and protection. The desired overcurrent protection level is set by an external grounding resistor.

Although the specific functional details and features vary, most integrated load switches use the same operating principle. During startup, the driver circuit of the load switch turns on the MOSFET and monitors the load current and the temperature of the MOSFET switch. If the startup current reaches the set overcurrent threshold before the MOSFET is fully turned on, the driver circuit will quickly reduce the gate drive so that the load switch device operates like a constant current source. This operating mode can last for a limited period of time. If the output voltage has not risen to a value close to the input voltage at the end of this time limit, the load switch will turn off and trigger the fault status output to indicate a startup failure. If the output successfully rises to the specified voltage before the startup timing times out, the power-good indication output will be triggered.

If the load current exceeds the programmed overcurrent threshold at any time after startup, the load switch will act as an electronic circuit breaker to turn off the internal power switch. This will protect the upstream power supply from being damaged by output overload or short circuit faults.

Changing the overcurrent threshold after startup

Some load devices draw more current at startup than during operation. For example, a load with a large input bypass capacitor may require a large charging current, but consumes very little operating current after power-up. Similarly, a device with a motor (such as a disk drive) requires a large rotational current at startup, but once the maximum speed is reached, the current consumed by the motor will be much lower.

To provide the best protection in these situations, it is best to set the overcurrent protection threshold close to the lower operating current. However, this will result in a current starvation situation during startup when the load switch (such as the MAX5976) clamps the current, which will not be able to support the output voltage rise. In fact, the output voltage can enter foldback protection in this case.

To solve this problem, a simple inverting method can be used: after startup, a secondary current-limiting configuration resistor is connected in parallel with the power-good open-drain output (PG) of the load switch. This design (Figure 1) reduces the current available to the load after startup. During startup, when the output voltage is lower than the input, the PG output is low and the current limit is determined by RCB1; when the output rises to the specified value and the power-good state is delayed by 16ms, the PG output enters a high-impedance state. This allows the gate of Q1 to rise, connecting the second resistor RCB2 in parallel with RCB1, thereby lowering the overcurrent threshold.

Figure 1: The PG output controls an external transistor to reduce the overcurrent threshold after startup.

The circuit operation is shown in Figure 2, where the MAX5976 is started up with a load of 330μF and 8.9Ω. Initially, the MAX5976 clamps the inrush current at 3A, determined by RCB1 = 17.4kΩ. After reaching VOUT, the current in the load resistor is 1.3A. The PG output goes high 16ms after VOUT rises, connecting RCB2 = 12.1kΩ in parallel to RCB1, reducing the circuit breaker threshold to 1.25A. The MAX5976 circuit breaker comparator allows an additional 4.8ms of overcurrent before shutting down (for larger overload currents, the circuit breaker will be triggered to shut down more quickly).

Other signals can also be used to switch the resistor in or out of the CB (current limit threshold setting) circuit. This flexibility enables a variety of power management possibilities. For example, a power-on reset (POR) management device can be used to extend the startup timing to just beyond the default PG delay. Such a function may be required to allow a disk drive motor to spin up to operating speed.

For a highly configurable overcurrent protection circuit, an integrated load switch and a digital potentiometer (such as the MAX5434) can be used in combination. This configuration allows a microcontroller (or production fixture) to set the overcurrent limit as desired without changing the physical device. Figure 2 shows such an application circuit.

Figure 2: The MAX5976 is combined with the MAX5434 nonvolatile digital potentiometer to achieve a programmable overcurrent limit.

In applications where thermal constraints are critical, a negative temperature coefficient (NTC) thermistor can be used to set the current limit. The NTC thermistor provides a protection threshold that automatically decreases when the load begins to overheat, thus preventing potential faults from developing to unrecoverable levels.

Conclusion

Because the MAX5976 and similar devices use a single resistor to set both soft-start and overcurrent protection levels, simple modifications to the basic application circuit can accommodate complex loads with widely varying startup and running current requirements. We can easily combine the high integration and performance of an integrated load switch with sophisticated, state-dependent overcurrent protection.

Figure 2: The circuit shown in Figure 1 starts a 330μF, 8.9Ω load with RCB1 = 17.4kΩ and RCB2 = 12.1kΩ.

Conversely, some loads must start up slowly to avoid excessive power dissipation in the load switch's internal MOSFET while the output voltage is ramping up. If the MOSFET is fully enhanced, more current can be delivered without excessive losses. In this case, the MAX5976's PG output itself can be used to control the shunt configuration resistors (Figure 3). After startup is complete, the PG output becomes high impedance, disconnecting the shunt resistors and increasing the current available to the load.

Figure 3: Open-drain PG output for increasing overcurrent limit after startup.