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ADM1276
THEORY OF OPERATION
Data Sheet
When circuit boards are inserted into a live backplane, discharged
2.95V TO 20V
R SENSE
Q 1
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
22 ?
SENSE+
SENSE–
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
VCC
330nF
ADM1276
GATE
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 V GS 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
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, R SENSE (see Figure 42). An
internal current sense amplifier provides a gain of 50 to the
voltage drop detected across R SENSE . The result is compared to
an internal reference and used by the hot swap control logic to
detect when an overcurrent condition occurs.
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
SENSE+
R SENSE
SENSE–
Q 1
voltage measurement (see Figure 40).
+
×50
–
2.95V TO 20V
R SENSE
Q 1
VCC
+
OVER-
CURRENT
GATE
REFERENCE
–
SENSE+
SENSE–
ADM1276
GND
VCC
ADM1276
GATE
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
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
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.
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.
Rev. C | Page 18 of 48
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