ADM1276-3ACPZ-RL(Rev0) View Datasheet(PDF) - Analog Devices

Part Name

Description

Manufacturer

ADM1276-3ACPZ-RL
(Rev.:Rev0)

Hot Swap Controller and Digital Power and Energy Monitoring with PMBus Interface

Analog Devices 

ADM1276

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 ADM1276 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 ADM1276 can reside on the backplane or on the

removable board.

POWERING THE ADM1276

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

ADM1276 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 ADM1276, 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 con-

nected via separate traces to prevent accuracy loss in the sense

voltage measurement (see Figure 40).

2.95V TO 20V

RSENSE

Q1

SENSE+

SENSE–

VCC

ADM1276

GATE

GND

Figure 40. Powering the ADM1276

To protect the ADM1276 from unnecessary resets due to transient

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

as shown in Figure 41. Choose the values of these components so

as 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. Unless a resistor

is used to limit the inrush current, do not place a supply decoupling

capacitor on the rail before the FET.

2.95V TO 20V

RSENSE

Q1

22Ω

SENSE+

SENSE–

VCC

330nF

ADM1276

GATE

GND

Figure 41. 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 42). 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

Q1

SENSE+

SENSE–

+–

×50

VCC

+

REFERENCE –

OVER-

CURRENT

GATE

ADM1276

GND

Figure 42. 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 ADM1276. The current flowing through the sense resistors

creates an offset, resulting in reduced accuracy.

To achieve better accuracy, the averaging resistors sum the current

from the nodes of each sense resistor, as shown in Figure 43. The

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

resistors are chosen to balance the input current to both sense

pins to within 5 μA. This ensures that the same offset is seen by

both sense inputs.

Rev. 0 | Page 18 of 48

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