-48V Hot-Swap Controllers

8

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Power-Supply Ramping

The MAX5948A/MAX5948B can reside either on the

backplane or the removable circuit board (Figure 6a).

Power is delivered to the load by placing an external

n-channel MOSFET pass transistor in the power-

supply path.

After the circuit board is inserted into the backplane and

voltage and overvoltage tolerance, the MAX5948A/

MAX5948B turn on Q1. The MAX5948A/MAX5948B grad-

ually turn on the external MOSFET by charging the gate

of Q1 with a 45µA current source. Capacitor C2 provides

a feedback signal to accurately limit the inrush current.

The inrush current can be calculated:

Figure 6b shows the inrush current waveform. The cur-

rent through C2 controls the GATE voltage. At the end

of the DRAIN ramp, the GATE voltage is charged to its

final value. The GATE-to-SENSE clamp limits the maxi-

Board Removal

If the card is removed from a live backplane, the output

capacitor on the card may not be immediately dis-

charged. While the output capacitor is discharging, the

MAX5948_ continues to operate as if the input supply

were still connected because the output capacitor tem-

porarily supplies operating current to the IC. If the cir-

cuit is connected as in Figure 7a, the voltage at the UV

pin falls below the UVLO detect threshold, and the

MAX5948_ turns off the external MOSFET. If R4 in the

circuit is connected directly to the -48V return, the

external MOSFET remains on until the capacitor is dis-

charged sufficiently to drop the UV pin voltage to the

UVLO detect threshold.

In either case, when the MOSFET is turned off, the out-

put capacitor continues to discharge by the IC supply

through the substrate diode of the external MOSFET.

and DRAIN terminals through multiple internal ESD-pro-

tection diodes. The protection circuit built into the IC

allows the DRAIN terminal voltage to drop below that of

LOGIC

AND

GATE DRIVE

REFERENCE

GENERATOR

REF

REF

50mV

UV

OV

SENSE

GATE

DRAIN

PWRGD

PWRGD

MAX5948A

MAX5948B

OUTPUT

DRIVE

Block Diagram