LINEAR dn402 handbook
Contents
1. when Veate exceeds AVesarm a voltage at which the external MOSFET is deemed fully enhanced and Rps on 4J LT LTC LTM and Burst Mode are registered trademarks of Linear Technology Corporation All other trademarks are the property of their respective owners Qi S14864DY Vec SENSEP GATE SENSEN LTC4213 ON GND Isev Figure 1 The LTC4213 in an Electronic Circuit Breaker Application 10 06 402 minimized Then 50us after the circuit breaker is armed the READY pin goes high see trace 3 and signals the system to power up Viy Trace 4 shows the related Vout waveform when Viy powers up In order to not trip the circuit breaker during startup the load current must be lower than Vcp Resense If needed the Ise 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 Rps 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 Rosny varied by operating temperature and under different Vgs bias condition The MOSFET vendors also do not specify the Rps 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 0 1ms DIV DN402 F02 Figure 2 Normal2. Power Up Sequence RSENSE Vin 7mQ 1 9V Qt Si4420BDY Your SENSEP SENSEN GATE LTC4213 Figure 3 Accurate ECB with High Side Sense Resistor Data Sheet Download http www linear com 3 shows a tolerable Rseyse 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 ap plications with low MOSFET insertion loss It includes selectable dual currentleveland dual response time circuit breaker functions The circuit breaker has wide operating input common mode range from ground to Vec Qt Si4420BDY Vout SENSEP SENSEN GATE LTC4213 DN402 F04 Figure 4 High Side Switch for Logic Level N Channel MOSFET Vec gt 3V 70 a 6 5 Vaate Vsensen V a oOo 30 35 40 45 50 55 60 BIAS SUPPLY VOLTAGE V DN402 F05 Figure 5 Gate Drive Voltage vs Bias Supply Voltage For ap
3. advertisement DESIGN NOTES Electronic Circuit Breaker in Small DFN Package Eliminates Sense Resistor Design Note 402 SH Lim Introduction Traditionally an Electronic Circuit Breaker ECB com prises a MOSFET a MOSFET controller and acurrent sense resistor The LTC 4213 does away with the sense resistor by using the Rps 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 chan nel MOSFET switch Overcurrent Protection The SENSEP and SENSEN pins monitor the load cur rent via the Rps on of the external MOSFET and serve as inputs to two internal comparators SLOWCOMP and FASTCOMP with trip points at Vcg and Vcgyrast respectively The circuit breaker trips when an overcur rent fault causes a substantial voltage drop across the MOSFET An overload current exceeding Vcg Rps on causes SLOWCOMP totrip the circuit breaker after a 16s delay In the event of a severe overload or short circuit current exceeding Vegast Roson the FASTCOMP trips the circuit breaker within 1us protecting both the MOSFET and the load Both of the comparators have a common mode input voltage range from ground to Vec 0 2V This allows th
4. e circuit breaker to operate as the load supply turns on from OV Flexible Overcurrent Setting The LTC4213 has an Ige pin to select one of these three overcurrent settings Ise_ at GND Vcg 25mV and Vcg rast 100mV Ise left open Vcg SOmV and Vog rast 175mV Ise at Voc Vcg 100mV and Veg Fast 325mV Overvoltage Protection The LTC4213 can provide load overvoltage protection OVP above the bias supply When Vsensep gt Vcc 0 7V for 65us 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 Viy 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 ap plication An input bypass capacitor is recommended to prevent transient spikes when the Viy 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 60us de bounce cycle the GATE pin capacitance is charged up from ground by an internal 100A current source see trace 2 As the GATE pin and the gate of MOSFET charges up the external MOSFET turns on when Vente exceeds the MOSFET s threshold The circuit breaker is armed
5. plications help call 408 432 1900 Ext 2452 l ngar Taehniolody Corporation 1630 McCarthy Blvd Milpitas CA 95035 7417 408 432 1900 FAX 408 134 0507 www linear com dn402f LT 1006 305K PRINTED IN THE USA LY tiie LINEAR TECHNOLOGY CORPORATION 2006
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LINEAR dn402 handbook