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Short Description: Typical Electronic Circuit Breaker (ECB). Application. Figure 1 shows the LTC4213 in a dual supply ECB ap- plication. An input bypass capacitor is ...
Content Inside: advertisement Electronic Circuit Breaker in Small DFN Package Eliminates Sense Resistor Design Note 402 SH Lim Introduction Overvoltage Protection Traditionally, an Electronic Circuit Breaker (ECB) com- The LTC4213 can provide load overvoltage protection prises a MOSFET, a MOSFET controller and a current sense (OVP) above the bias supply. When VSENSEP > VCC + 0.7V resistor. The LTC®4213 does away with the sense resistor for 65µs, an internal OVP circuit activates with the GATE by using the RDS(ON) of the external MOSFET. The result pin pulling low and the external MOSFET turning off. The is a simple, small solution that offers a signifi cant low OVP circuit protects the system from an incorrect plug-in insertion loss advantage at low operating load voltage. event where the VIN load supply is much higher than the The LTC4213 features two circuit breaking responses to VCC bias voltage. varying overload conditions with three selectable trip thresholds and a high side drive for an external N-chan- Typical Electronic Circuit Breaker (ECB) nel MOSFET switch. Application Overcurrent Protection Figure 1 shows the LTC4213 in a dual supply ECB ap- plication. An input bypass capacitor is recommended to The SENSEP and SENSEN pins monitor the load cur- prevent transient spikes when the VIN supply powers-up rent via the RDS(ON) of the external MOSFET and serve or the ECB responds to overcurrent conditions. Figure 2 as inputs to two internal comparators--SLOWCOMP shows a normal power-up sequence. The LTC4213 exits and FASTCOMP--with trip points at VCB and VCB(FAST), reset mode once the VCC pin is above the internal under respectively. The circuit breaker trips when an overcur- voltage lockout threshold and the ON pin rises above rent fault causes a substantial voltage drop across the 0.8V (see trace 1 in Figure 2). After an internal 60µs de- MOSFET. An overload current exceeding VCB/RDS(ON) bounce cycle, the GATE pin capacitance is charged up causes SLOWCOMP to trip the circuit breaker after a 16µs from ground by an internal 100µA current source (see delay. In the event of a severe overload or short-circuit trace 2). As the GATE pin and the gate of MOSFET charges current exceeding VCB(FAST)/RDS(ON), the FASTCOMP up, the external MOSFET turns on when VGATE exceeds trips the circuit breaker within 1µs, protecting both the the MOSFET's threshold. The circuit breaker is armed MOSFET and the load. when VGATE exceeds VGSARM, a voltage at which the Both of the comparators have a common mode input external MOSFET is deemed fully enhanced and RDS(ON) voltage range from ground to VCC + 0.2V. This allows , LT, LTC, LTM and Burst Mode are registered trademarks of Linear Technology the circuit breaker to operate as the load supply turns Corporation. All other trademarks are the property of their respective owners. on from 0V. Q1 SI4864DY VIN V Flexible Overcurrent Setting OUT 1.25V 1.25V + + C C 3.5A IN LOAD The LTC4213 has an I 100µF 100µF SEL pin to select one of these three overcurrent settings: V SENSEP GATE SENSEN BIAS VCC 3.3V VCC I C1 SEL at GND, VCB = 25mV and VCB(FAST) = 100mV 0.1µF LTC4213 R4 10k ISEL left open, VCB = 50mV and VCB(FAST) = 175mV OFF ON ON READY GND ISEL DN402 F01 ISEL at VCC, VCB = 100mV and VCB(FAST) = 325mV Figure 1. The LTC4213 in an Electronic Circuit Breaker Application 10/06/402