ADM1275-1ACPZ-R7 datasheet(21/48 Pages) AD

21 / 48 page

Data Sheet

ADM1275

Rev. D | Page 21 of 48

THEORY OF OPERATION

When circuit boards are inserted into a live backplane, discharged

supply bypass capacitors draw large transient currents from the

backplane power bus as they charge. These transient currents can

cause permanent damage to connector pins, as well as dips on

the backplane supply that can reset other boards in the system.

The ADM1275 is designed to control the powering on and off

of a system in a controlled manner, allowing a board to be removed

from, or inserted into, a live backplane by protecting it from excess

currents. The ADM1275 can reside on the back-plane or on the

removable board.

POWERING THE ADM1275

A supply voltage from 2.95 V to 20 V is required to power the

ADM1275 via the VCC pin. The VCC pin provides the majority

of the bias current for the device; the remainder of the current

needed to control the gate drive and best regulate the VGS

voltage is supplied by the SENSE+ pin.

To ensure correct operation of the ADM1275, the voltage on

the VCC pin must be greater than or equal to the voltage on

the SENSE+ pin. No sequencing of the VCC and SENSE+ rails

is necessary. The SENSE+ pin can be as low as 2 V for normal

operation provided that a voltage of at least 2.95 V is connected

to the VCC pin. In most applications, both the VCC and SENSE+

pins are connected to the same voltage rail, but they are connected

via separate traces to prevent accuracy loss in the sense voltage

measurement (see Figure 46).

2.95V TO 20V

RSENSE

SENSE–

GND

GATE

VCC

SENSE+

ADM1275

Figure 46. Powering the ADM1275

To protect the ADM1275 from unnecessary resets due to transient

supply glitches, an external resistor and capacitor can be added,

as shown in Figure 47. The values of these components should be

chosen to provide a time constant that can filter any expected

glitches. The resistor should, however, be small enough to keep

voltage drops due to quiescent current to a minimum. A supply

decoupling capacitor should not be placed on the rail before the

FET unless a resistor is used to limit the inrush current.

2.95V TO 20V

RSENSE

SENSE–

GND

GATE

VCC

330nF

SENSE+

22Ω

ADM1275

Figure 47. Transient Glitch Protection Using an RC Network

CURRENT SENSE INPUTS

The load current is monitored by measuring the voltage drop

across an external sense resistor, RSENSE (see Figure 48). An

internal current sense amplifier provides a gain of 50 to the

voltage drop detected across RSENSE. The result is compared to

an internal reference and used by the hot-swap control logic to

detect when an overcurrent condition occurs.

RSENSE

SENSE–

GND

GATE

VCC

SENSE+

ADM1275

OVER-

CURRENT

REFERENCE

×50

–

Figure 48. Hot-Swap Current Sense Amplifier

The SENSE inputs may be connected to multiple parallel sense

resistors, which can affect the voltage drop detected by the

ADM1275. The current flowing through the sense resistors

creates an offset, resulting in reduced accuracy.

To achieve better accuracy, averaging resistors sum the current

from the nodes of each sense resistor, as shown in Figure 49. The

typical value for the averaging resistors is 10 Ω. The value of the

averaging resistors is chosen to be much greater than the trace

resistance between the sense resistors terminals and the inputs

to the ADM1275. This greatly reduces the effects of differences

in the trace resistances.

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