advertisement Electronic Circuit Breaker in Small DFN Package Eliminates Sense Resistor – Design Note 402 SH Lim Introduction Traditionally, an Electronic Circuit Breaker (ECB) comprises a MOSFET, a MOSFET controller and a current sense resistor. The LTC® 4213 does away with the sense resistor by using the RDS(ON) of the external MOSFET. The result is a simple, small solution that offers a 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 16µs delay. In the event of a severe overload or short-circuit current exceeding VCB(FAST)/RDS(ON), the FASTCOMP trips the circuit breaker within 1µs, protecting both the MOSFET and the load. Both of the comparators have a common mode input voltage range from ground to VCC + 0.2V. This allows the circuit breaker to operate as the load supply turns on from 0V. Flexible Overcurrent Setting The LTC4213 has an ISEL pin to select one of these three overcurrent settings: ISEL at GND, VCB = 25mV and VCB(FAST) = 100mV ISEL left open, VCB = 50mV and VCB(FAST) = 175mV ISEL at VCC, VCB = 100mV and VCB(FAST) = 325mV Overvoltage Protection The LTC4213 can provide load overvoltage protection (OVP) above the bias supply. When VSENSEP > VCC + 0.7V for 65µs, 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. Typical Electronic Circuit Breaker (ECB) Application Figure 1 shows the LTC4213 in a dual supply 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 0.8V (see trace 1 in Figure 2). After an internal 60µs debounce cycle, the GATE pin capacitance is charged up from ground by an internal 100µA 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 ΔVGSARM, a voltage at which the external MOSFET is deemed fully enhanced and RDS(ON) , LT, LTC, LTM and Burst Mode are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. Q1 SI4864DY VIN 1.25V CIN 100µF VBIAS 3.3V OFF ON + CLOAD 100µF VCC SENSEP GATE SENSEN C1 0.1µF LTC4213 ON GND ISEL + VOUT 1.25V 3.5A VCC R4 10k READY DN402 F01 Figure 1. The LTC4213 in an Electronic Circuit Breaker Application 10/06/402 minimized. Then, 50µs after the circuit breaker is armed, the READY pin goes high (see trace 3) and signals the system to power up VIN. Trace 4 shows the related VOUT waveform when VIN powers up. In order to not trip the circuit breaker during startup, the load current must be lower than VCB/RSENSE. If needed, the ISEL pin can be stepped dynamically for a higher overcurrent threshold at startup and a lower threshold when the load current has stabilized. Accurate ECB with Sense Resistor The RDS(ON) voltage drop sensing method trades the circuit breaker accuracy for system simplicity. The majority of sensing inaccuracy is due to the external MOSFET’s RDS(ON) varied by operating temperature and under different VGS bias condition. The MOSFET vendors also do not specify the RDS(ON) distribution tightly due to manufacturing variation. If an external tight tolerance resistor is employed for current sensing instead, the LTC4213 reveals its ±10% circuit breaker accuracy. Figure (1) VON 1V/DIV 3 shows a tolerable RSENSE resistor voltage drop and the LTC4213 is used for accurate ECB applications. High Side Switch for N-Channel Logic Level MOSFET Logic level N-Channel MOSFET applications usually requires a minimum gate drive voltage of 4.5V. Figure 4 shows the LTC4213 in a High Side Switch application. The LTC4213’s internal charge pump boosts the GATE above the logic level gate drive requirement and ensures the MOSFET is fully enhanced for VCC ≥ 3V. The typical gate drive versus bias supply voltage curve is shown in Figure 5. 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. VIN 3V TO 6V Q1 Si4420BDY + 10µF (2) VGATE 5V/DIV R4 10k VOUT 3V TO 6V 5A + CLOAD VCC SENSEP SENSEN GATE READY LTC4213 (3) VREADY 2V/DIV OFF ON ON GND ISEL DN402 F04 (4) VOUT ≈ VIN 1V/DIV VIN POWERS UP 0.1ms/DIV DN402 F02 Figure 4. High Side Switch for Logic Level N-Channel MOSFET, VCC > 3V Figure 2. Normal Power-Up Sequence 8.0 VIN 1.9V + Q1 Si4420BDY VOUT 1.9V 5A + CIN 100µF SENSEP VBIAS 5V VCC OFF ON ON CLOAD 100µF GATE SENSEN VIN R4 10k LTC4213 7.5 VGATE – VSENSEN (V) RSENSE 7mΩ 7.0 6.5 6.0 5.5 5.0 4.5 READY GND ISEL DN402 F03 4.0 3.0 3.5 4.0 5.0 5.5 4.5 BIAS SUPPLY VOLTAGE (V) 6.0 DN402 F05 Figure 3. Accurate ECB with High Side Sense Resistor Figure 5. Gate Drive Voltage vs Bias Supply Voltage Data Sheet Download http://www.linear.com For applications help, call (408) 432-1900, Ext. 2452 Linear Technology Corporation dn402f LT 1006 305K • PRINTED IN THE USA FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2006 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ●