MAX5933CESA+ Datasheet MAX5947BESA+, MAX5933CESA+, MAX5933EESA+ | P10-P12

MAX5933A–MAX5933F/MAX5947A/B/C

Positive High-Voltage, Hot-Swap Controllers

10 ______________________________________________________________________________________

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

(Figure 5). Resistor R

SENSE

provides current detection

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

threshold voltage, the voltage on V

exceeds the

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:

INRUSH

= C

x 10µA/C1

When the voltage across the current-sense resistor

SENSE

reaches V

SENSETRIP

, the inrush current is limit-

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

between V

(pin 8) and SENSE (pin 7).

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:

LIMIT

= 47mV / R

SENSE

For a 0.025Ω 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

SENSE

2A/div

PWRGD

50V/div

OUTPUT

50V/div

GATE

50V/div

20ms/div

Figure 6. Power-Up Waveforms

- V

SENSE

12mV

47mV

0.5V0V

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

MAX5933A–MAX5933F/MAX5947A/B/C

Positive High-Voltage, Hot-Swap Controllers

______________________________________________________________________________________ 11

TIMER

TIMER provides a method for programming the maxi-

mum time the device is allowed to operate in current

limit. When the current-limit circuitry is not active,

TIMER is pulled to GND by a 3µA current source. After

the current-limit circuit becomes active, an 80µA pullup

current source is connected to TIMER, and the voltage

rises with a slope equal to 77µA/C

TIMER

, as long as the

current-limit circuit remains active. Once the desired

maximum current-limit time is chosen, the capacitor

value is:

C(nF) = 65 x t(ms)

LIMIT

= (C

TIMER

/80µA) x 1.233V

When the current-limit circuit turns off, TIMER is dis-

charged to GND by the 3µA current source.

Whenever TIMER reaches 1.233V, the internal fault

latch is set. GATE is immediately pulled to GND and

TIMER is pulled back to GND by the 3µA current

source. When TIMER falls below 0.5V, ON is pulsed low

to reset the internal fault latch.

The waveform in Figure 9 shows how the output latches

off following a short circuit. The drop across the sense

resistor is held at 12mV as the timer ramps up. Since

the output did not rise, FB remains below 0.5V and the

circuit latches off. For Figure 9, C

= 100nF.

Undervoltage and Overvoltage Detection

ON can be used to detect an undervoltage condition at

the power-supply input. ON is internally connected to

an analog comparator with 80mV of hysteresis. If ON

falls below its threshold voltage (1.233V), GATE is

pulled low and is held low until ON is high again.

Figure 10 shows an overvoltage detection circuit. When

the input voltage exceeds the Zener diode’s breakdown

voltage, D1 turns on and starts to pull TIMER high. After

TIMER is pulled higher than 1.233V, the fault latch is set

and GATE is pulled to GND immediately, turning off

transistor Q1 (see Figure 11). Operation is restored

either by interrupting power or by pulsing ON low.

Power-Good Detection

The MAX5933_/MAX5947_ include a comparator for mon-

itoring the output voltage. The noninverting input (FB) is

compared against an internal 1.233V precision reference

and exhibits 80mV hysteresis. The comparator’s output

(PWRGD) is open drain and capable of operating from a

pullup as high as 80V. The PWRGD is similar to PWRGD

with an opposite polarity (active low) output.

The PWRGD (PWRGD) can be used to directly

enable/disable a power module with an active-high

enable input. Figure 12 shows how to use PWRGD to

control an active-low enable-input power module. Signal

inversion is accomplished by transistor Q2 and R7.

Supply Transient Protection

The MAX5933_/MAX5947_ are 100% tested and guar-

anteed to be safe from damage with supply voltages

up to 80V. However, spikes above 85V may damage

the device. During a short-circuit condition, the large

change in currents flowing through the power-supply

traces can cause inductive voltage spikes which could

exceed 85V. To minimize the spikes, the power-trace

parasitic inductance should be minimized by using

wider traces or heavier trace plating and a 0.1µF

bypass capacitor placed between V

and GND. A

transient voltage suppressor (TVS) at the input can also

prevent damage from voltage surges.

RESPONSE TIME TO OVERCURRENT

- V

SENSE

(mV)

PROPAGATION DELAY (µs)

200100

0300

Figure 8. Response Time to Overcurrent

SHORT-CIRCUIT WAVEFORMS

SENSE

1A/div

TIMER

1V/div

OUTPUT

50V/div

GATE

50V/div

10ms/div

Figure 9. Short-Circuit Waveforms

GATE Voltage

A curve of Gate Drive vs. V

is shown in Figure 13.

GATE is clamped to a maximum voltage of 18V above the

input voltage. At a minimum input-supply voltage of 33V,

the minimum gate drive voltage is 10V. When the input

supply voltage is higher than 20V, the gate-drive voltage

is at least 10V and a standard n-channel MOSFET can be

used. Using the MAX5947 in applications over a 9V to

20V range, a logic-level N-FET must be used with a prop-

er protection Zener diode between its gate and source

(see D1 in Figure 5).

Thermal Shutdown

If the MAX5933_/MAX5947_ die temperature reaches

+150°C, an overtemperature fault is generated. As a

result, GATE goes low and turns the external MOSFET off.

The MAX5933_/MAX5947_ die temperature must cool

down below +130°C before the overtemperature fault

condition is removed.

1kΩ

10nF

CMPZ5248B

IRF530

10Ω

SENSE

0.025Ω

59kΩ

3.57kΩ

24kΩ

3.4kΩ

49.9kΩ

0.68µF

MAX5933B

MAX5947B

PWRGD

GND

TIMER

PWRGD

GND

SENSE GATE

876

30V

1N5256B

SHORT

0.1µF

MAX5933A–MAX5933F/MAX5947A/B/C

Positive High-Voltage, Hot-Swap Controllers

12 ______________________________________________________________________________________

OVERVOLTAGE WAVEFORMS

SENSE

5A/div

TIMER

10V/div

50V/div

GATE

50V/div

10µs/div

OUTPUT

50V/div

Figure 11. Overvoltage Waveforms

Figure 10. Overvoltage Detection

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15

MAX5933CESA+

Mfr. #:

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Manufacturer:

Maxim Integrated

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Hot Swap Voltage Controllers Positive Hot-Swap Controller

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MAX5933CESA+

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