DESIGN IDEAS 48V Hot Swap Circuit Blocks Reverse Battery Voltage Hot Swap controllers guard against inrush current and short circuits, but reverse battery installation is another matter. In central office applications, OR-ing diodes block reversed input voltages. In systems with a single power source OR-ing diodes become unnecessary; although a single diode could be retained for reverse conditions, its forward drop loss is a significant penalty. The circuit shown in Figure 1 eliminates the loss associated with a blocking diode. The LT1641 and Q1 handle familiar hot swap chores of undervoltage lockout, inrush control, short circuit protection and system reset, while Q2 and Q3 handle reverse input situations. Under positive input conditions, Q2’s body diode is forward biased and power reaches the LT1641 and Q1, allowing them to function in the normal manner. When the LT1641 is commanded to turn on, GATE (pin 6) and R7 slowly charge C1, thereby limiting the inrush current to CLOAD. The LT1641 drives both Q1 and Q2 fully into enhancement mode, minimizing losses and eliminating the drop in Q2’s body diode. Q3 is included as part of the circuitry that blocks reverse inputs, yet it must “get out of the way” when positive inputs are present. With a positive input, Q3’s emitter is pulled up, dragging along its base via D2. Since the base is slightly negative with respect to the emitter, Q3 is off. A small current flows through resistor R9 to ground, but this is of no consequence. If the LT1641 is in the off state, GATE pulls low. Current flows from the forward-biased collector -base junction of Q3 to the LT1641’s GATE pin, but is limited by R7. When the LT1641 turns on, GATE pulls up above the input supply and D3 * 36V–72V INPUT LONG RSNUB 100Ω D5 R7 1M C1, 10nF/160V R9 100k SHORT D4 INTERNATIONAL RECTIFIER IRF530 (310) 322-3331 ON SEMICONDUCTOR MPSA 42 (602) 244-6600 1N5245, 15V 1N4148 DIODES INC. SMAT70A (805) 446-4800 BAV21 BODY DIODE R1 35.7k 1% R6 1k 8 7 6 VCC SENSE GATE PWRGD 1 ON 3 LT1641 R2 1.24k 1% C3 100nF LONG CLOAD 100µF, TYP D1 Q3 D2 Q1, Q2: Q3: D1: D2: D3: D4–D5: * OUTPUT Q1 R5 10Ω R8 10Ω CSNUB 10nF the collector-base junction of Q3 becomes reverse-biased. Q3 is off, so no collector current flows and the LT1641’s GATE output is not loaded by Q3. If a reverse input polarity is applied, Q3 goes to work and ensures that Q2 is held off. Negative inputs pull the emitter of Q3 below ground. Q3 turns on, biased by R9, and the collector effectively shorts Q2’s gate and source. With Q2 in the off state, no current can flow into RS, the LT1641 or Q1. The circuit can take up to –100V in this condition. Of course, some leakage current does flow in Q2; this is absorbed by D3. This circuit can handle instantaneous steps from zero to ±75V. As shown, short circuit protection limits the output to 2.5A. Q1, Q2, and RS can be scaled to handle loads in excess of 1kW. * RS 0.02Ω Q2 by Mitchell Lee FB GND TIMER 4 5 RESET R3 35.7k 1% 2 R4 1.21k 1% C2 1µF 10V RTN Figure 1. Reverse-battery protected LT1641 48V Hot Swap circuit 30 Linear Technology Magazine • February 2001