Positive High-Voltage, Hot-Swap Controllers

10

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Applications Information

Hot-Circuit Insertion

When circuit boards are inserted into a live backplane,

the supply bypass capacitors on the boards draw high

peak currents from the backplane power bus as they

charge up. The transient currents can permanently

damage the connector pins and glitch the system sup-

ply, causing other boards in the system to reset.

Power-Up Sequence

The power supply on a board is controlled by placing

an external n-channel MOSFET (Q1) in the power path

and capacitor C1 provides control of the GATE slew

rate. Resistor R6 provides current control-loop compen-

sation, while R5 prevents high-frequency oscillations in

Q1. Resistors R1 and R2 provide undervoltage sensing.

After the power pins first make contact, transistor Q1 is

turned off. When the voltage at ON exceeds the turn-on

undervoltage lockout threshold, and when the voltage

on TIMER is less than 1.233V, transistor Q1 turns on

(Figure 6).

The voltage at GATE rises with a slope equal to

10µA/C1 and the supply inrush current is set at:

When the voltage across the current-sense resistor

ed by the internal current-limit circuitry that adjusts the

voltage on GATE to maintain a constant voltage across

the sense resistor.

Once the voltage at the output has reached its final value,

as sensed by resistors R3 and R4, PWRGD goes high or

PWRGD goes low.

Short-Circuit Protection

The MAX5933_/MAX5947_ feature a programmable fold-

back current limit with an electronic circuit breaker that

protects against short circuits or excessive supply cur-

rents. The current limit is set by placing a sense resistor

To prevent excessive power dissipation in the pass

transistor and to prevent voltage spikes on the input

supply during short-circuit conditions at the output, the

current folds back as a function of the output voltage

that is sensed at FB (Figure 7).

When the voltage at FB is 0V, the current-limit circuit

drives GATE to force a constant 12mV drop across the

sense resistor. As the output voltage at FB increases,

the voltage across the sense resistor increases until FB

reaches 0.5V. At this point, the voltage across the

sense resistor is held constant at 47mV.

The maximum current limit is calculated as:

For a 0.025

Ω sense resistor, the current limit is set at

1.88A and folds back to 480mA when the output is

shorted to ground.

The MAX5933_/MAX5947_ also feature a variable over-

current response time. The time required to regulate

Q1’s drain current depends on:

1) Q1’s input capacitance

2) GATE capacitor C1 and compensation resistor R6

3) The internal delay from SENSE to GATE

Figure 8 shows the delay from a voltage step at SENSE

until GATE voltage starts falling, as a function of

overdrive.

POWER-UP WAVEFORMS

2A/div

PWRGD

50V/div

OUTPUT

50V/div

GATE

50V/div

20ms/div

Figure 6. Power-Up Waveforms

12mV

47mV

0.5V

0V

Figure 7. Current-Limit Sense Voltage vs. Feedback Voltage