Circuit & System Protection


Small DFN electronic circuit breaker eliminates sense resistor

2 November 2005 Circuit & System Protection

Traditionally, an electronic circuit breaker (ECB) comprises a MOSFET, a MOSFET controller and a current sense resistor. The LTC4213 from Linear Technology is a new electronic circuit breaker that does away with the sense resistor by instead using the RDS(ON) of the external MOSFET.

The result is a simple, small solution that offers significant low insertion loss advantage at low operating load voltage. The LTC4213 features two circuit breaking responses to varying overload conditions with three selectable trip thresholds and a high-side drive for an external N-channel MOSFET switch.

Overcurrent protection

The SENSEP and SENSEN pins monitor the load current via the RDS(ON) of the external MOSFET, and serve as inputs to two internal comparators - SLOWCOMP and FASTCOMP - with trip points at VCB and VCB(FAST), respectively. The circuit breaker trips when an overcurrent fault causes a substantial voltage drop across the MOSFET. An overload current exceeding VCB/RDS(ON) causes SLOWCOMP to trip the circuit breaker after a 16 μs delay. In the event of a severe overload or short circuit current exceeding VCB(FAST)/RDS(ON), the FASTCOMP trips the circuit breaker within 1 μs, protecting both the MOSFET and the load.

When the circuit breaker trips, the GATE pin is pulled down immediately to disconnect the load from the supply. In order to reset the circuit breaker fault, either the ON pin must be taken below 0,4 V for at least 80 µs or the bias VCC must be taken below 1,97 V for at least 80 μs. Both of the comparators have a common mode input voltage range from ground to VCC + 0,2 V. This allows the circuit breaker to operate even under severe output short circuit conditions where the load supply voltage collapses.

Flexible overcurrent setting

The LTC4213 has an ISEL pin to select one of these three over-current settings:

* ISEL at GND, VCB = 25 mV and VCB(FAST) = 100 mV.

* ISEL left open, VCB = 50 mV and VCB(FAST) = 175 mV.

* ISEL at VCC, VCB = 100 mV and VCB(FAST) = 325 mV.

ISEL can be stepped dynamically. For example, a higher over-current threshold can be set at start-up and a lower threshold can be selected after the supply current has stabilised.

Overvoltage protection

The LTC4213 can provide load overvoltage protection (OVP) above the bias supply. When VSENSEP > VCC + 0,7 V for 65 μs, an internal OVP circuit activates with the GATE pin pulling low and the external MOSFET turning off. The OVP circuit protects the system from an incorrect plug-in event where the VIN load supply is much higher than the VCC bias voltage. The OVP circuit also cuts off the load from the supply during any prolonged overvoltage conditions. The 65 μs delay prevents the OVP circuit from triggering due fast transient noise. Nevertheless, if fast overvoltage spikes are threats to the system, an external input bypass capacitor and/or transient suppressor should be installed.

Typical ECB application

Figure 1 shows the LTC4213 in a dual supply electronic circuit breaker (ECB) application. An input bypass capacitor is recommended to prevent transient spikes when the VIN supply powers-up or the ECB responds to overcurrent conditions. Figure 2 shows a normal power-up sequence. The LTC4213 exits reset mode once the VCC pin is above the internal under-voltage lockout threshold and the ON pin rises above 0,8 V (see trace 1 in Figure 2). After an internal 60 μs de-bounce cycle, the GATE pin capacitance is charged up from ground by an internal 100 μA current source (see trace 2). As the GATE pin and the gate of MOSFET charges up, the external MOSFET turns on when VGATE exceeds the MOSFET's threshold. The circuit breaker is armed when VGATE exceeds ΔVGATE, a voltage at which the external MOSFET is deemed fully enhanced, and RDS(ON) minimised.

Figure 1. The LTC4213 in an electronic circuit breaker application
Figure 1. The LTC4213 in an electronic circuit breaker application

Figure 2. Normal power-up sequence
Figure 2. Normal power-up sequence

Then, 50 μs after the circuit breaker is armed and the READY pin goes high (see trace 3), the VIN supply starts to power-up. To prevent power-up failures, the VIN supply should rise with a ramp-rate that keeps the inrush current below the ECB trip level. Trace 4 shows the VOUT waveform during the VIN supply power-up. The gate voltage finally peaks at ΔVGSMAX + VSENSEN. The MOSFET gate overdrive voltage is ΔVGSMAX which is higher than the ΔVGSARM. This ensures that the external MOSFET is fully enhanced and the RDS(ON) is further reduced. Choose the MOSFET with the required RDS(ON) at VGS approximately equal to ΔVGSMAX. The LTC4213 monitors the load current when the gate overdrive voltage exceeds ΔVGSARM.

Typical hot swap application

Figure 3 shows the LTC4213 in a single supply hot swap application where the load can be kept in shutdown mode until the hot swap action is completed. Large input bypass capacitors should be avoided in hot swap applications as they cause large inrush currents. Instead, a transient voltage suppressor should be employed to clip and protect against fast transient spikes.

Figure 3. The LTC4213 in a single supply hot swap application
Figure 3. The LTC4213 in a single supply hot swap application

When the PCB long trace makes contact, the ON pin is held below 0,4 V by the D1 Schottky diode. This keeps the LTC4213 in reset mode. The VIN supply is connected to the card when the short trace makes contact. The VCC pin is biased via the R1-C1 filter and VOUT is pre-charged by resistor R5. To power-up successfully, the R5 resistor should provide sufficient initial start up current for the shutdown load circuit and the 280 μA sinking current source at SENSEN pin. On the other hand, the R5 resistor value should limit the load surge current during board insertions and fault conditions.

When ON pin voltage exceeds 0,8 V, the GATE pin ramps up. The GATE voltage finally peaks and the external MOSFET is fully turned on to reduce the voltage drop between VIN and VOUT. The LTC4213 monitors the load current when the gate overdrive voltage exceeds ΔVGSARM.

Conclusion

The LTC4213 is a small package, No RSENSE electronic circuit breaker that is ideally suited for low voltage applications with low MOSFET insertion loss. It includes selectable dual current level and dual response time circuit breaker functions. The circuit breaker has wide operating input common-mode-range from ground to VCC.



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