Three-pin hot-swap controller requires one external part

Supertex has broken the eight-pin barrier for hot-swap controllers with the HV100, available in SOT-223 and MLP-3 packages (Picture). The HV100 further shrinks your circuit area by requiring a single external component: a MOSFET pass element. You needn't program the controller or tweak the circuit to accommodate a wide range of n-channel pass elements. The controller automatically adapts to the pass FET's operating characteristics, despite widely differing C_ISS and R_ON.

The HV100 prevents excessive inrush current during module swaps in 48, –48, and 24V applications, including telecomm, datacomm, and industrial control. The SOT-223 version has an undervoltage-lockout threshold of 15V. The MLP-3, one-third smaller than an SOT-23-6, has a 37V undervoltage-lockout threshold. When you first apply power, the controller holds the pass FET's gate off until the IC's internal regulator has settled, the undervoltage-lockout's thresholds have been satisfied, and the power-on-reset interval has completed. The power-on-reset circuit resets a 3-msec timer any time during the reset interval when the undervoltage-lockout threshold detects a low input-supply voltage, such as one that occurs due to contact bounce.

With the supply checking out of the way, the HV100 "learns" about the MOSFET it's controlling. An internal generator develops a 2.5V/msec ramp that a closed-loop current source driving the FET gate mimics. At the end of the 400-µsec sampling interval with the gate voltage still well below the MOSFET's threshold voltage, the controller stores the magnitude of the current the gate requires to match the ramp for use during the remainder of the hot-swap period. With the pass element characterized, the HV100 uses a constant-current drive to take advantage of the FET's gain and gate-drain capacitance to control the pass element's turn-on. The FET's pass current is limited until the controller charges the capacitance. The gate voltage continues to rise until it reaches a clamp voltage set to guarantee that the FET fully turned on.

The HV100 also detects shorted loads during start-up. However, lacking direct current-sensing inputs, the controller is not an electronic circuit breaker, and designs requiring this function need supplementary circuitry. If the controller detects a shorted load, it retries the start-up sequence after 2.5 sec, which gives the pass element time to cool.

The HV100-K5 SOT-223 and HV100-K6 MLP-3x2 cost 84 cents and 91 cents (10,000), respectively.

Supertex, 1-408-744-0100, www.supertex.com.