EVAL-ADM1275EBZ Datasheet, PDF (21/48 Pages) Analog Devices – Hot-Swap Controller and Digital Power

Data Sheet
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
R_SENSE
Q₁
SENSE+
SENSE–
VCC
ADM1275
GATE
GND
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.
ADM1275
2.95V TO 20V
R_SENSE
Q₁
22Ω
SENSE+
SENSE–
VCC
330nF
ADM1275
GATE
GND
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.
R_SENSE
Q₁
SENSE+
SENSE–
+–
×50
VCC
+
REFERENCE ^–
OVER-
CURRENT
GATE
ADM1275
GND
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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