MAX5949 datasheet(14/18 Pages) MAXIM

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When the DRAIN voltage of the MAX5949A is high with

respect to VEE or the GATE voltage is low, the internal

pulldown MOSFET Q2 is off and the PWRGD pin is in a

high-impedance state (Figure 14). The PWRGD pin is

pulled high by the module’s internal pullup current

source, turning the module off. When the DRAIN volt-

age drops below VDL and the GATE voltage is greater

than

∆VGATE - VGH, Q2 turns on and the PWRGD pin

pulls low, enabling the module.

The PWRGD signal can also be used to turn on an LED

or optoisolator to indicate that the power is good

(Figure 15) (see the Component Selection Procedure

section).

When the DRAIN voltage of the MAX5949B is high with

respect to VEE (Figure 13) or the GATE voltage is low, the

internal MOSFET Q3 is turned off so that I1 and the inter-

nal MOSFET Q2 clamp the PWRGD pin to the DRAIN pin.

MOSFET Q2 sinks the module’s pullup current, and the

module turns off.

When the DRAIN voltage drops below VDL and the

GATE voltage is greater than

∆VGATE - VGH, MOSFET

Q3 turns on, shorting I1 to VEE and turning Q2 off. The

pullup current in the module pulls the PWRGD pin high,

enabling the module.

GATE Pin Voltage Regulation

The GATE pin goes high when the following startup con-

ditions are met: the UV pin is high, the OV pin is low, the

supply voltage is above VUVLOH, and (VSENSE - VEE) is

less than 50mV. The gate is pulled up with a 45µA current

source and is regulated at 13.5V above VEE. The

MAX5949A/MAX5949B include an internal clamp that

ensures the GATE voltage of the external MOSFET never

exceeds 18V. During a fast-rising VDD, the clamp also

keeps the GATE and SENSE potentials as close as possi-

ble to prevent the FET from accidentally turning on. When

a fault condition is detected, the GATE pin is pulled low

with a 50mA current.

Applications Information

Sense Resistor

The circuit-breaker current-limit threshold is set to 50mV

(typically). Select a sense resistor that causes a drop

equal to or above the current-limit threshold at a current

level above the maximum normal operating current.

Typically, set the overload current to 1.5 to 2.0 times the

nominal load current plus the load-capacitance charging

current during startup. Choose the sense-resistor power

rating to be greater than (VCL)2 / RSENSE.

-48V Hot-Swap Controllers

with External RSENSE

______________________________________________________________________________________

VEE

SENSE

GATE

DRAIN

VDD

PWRGD

MAX5949A

-48V RTN

-48V

549k

Ω

6.49k

Ω

10k

Ω

0.02

Ω

C1**

470nF

25V

IRF530

15nF

100V

-48V RTN

(SHORT PIN)

100

µF

100V

*DIODES INC. SMAT70A.

**OPTIONAL.

PWRGD

MOC207

R7**

51k

Ω

Figure 15. Using PWRGD to Drive an Optoisolator

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