EVAL-ADM1275EBZ Datasheet, PDF (22/48 Pages) Analog Devices – Hot-Swap Controller and Digital Power

ADM1275
2.95V
TO 20V
Q₁
SENSE+
SENSE–
VCC
BIAS
CURRENT
GATE
GND
Figure 49. Connection of Multiple Sense Resistors to the SENSE Pins
CURRENT LIMIT REFERENCE
The current limit reference voltage determines the load current
level to which the ADM1275 limits the current during an
overcurrent event. This reference voltage is compared to the
gained-up current sense voltage to determine whether the limit
is reached.
An internal current limit reference selector block continuously
compares the ISET, soft start, and foldback voltages to determine
which voltage is the lowest at any given time; the lowest voltage
is used as the current limit reference. This ensures that the
programmed current limit, ISET, is used in normal operation,
and that the soft start and foldback features reduce the current
limit when required during startup and/or fault conditions.
R_SENSE
Q₁
SENSE+
SENSE–
VCC
×50
ISET
SS
FLB
OVER-
GATE
CURRENT
ADM1275
GND
Figure 50. Current Limit Reference Selection
The foldback and soft start voltages vary during different modes
of operation and are, therefore, clamped to minimum levels of
200 mV and 100 mV, respectively, to prevent zero current flow
due to the current limit being too low. Figure 51 provides an
example of how the soft start, foldback, and ISET voltages
interact during startup as the ADM1275 is enhancing the FET
and charging the load capacitances. Depending on how the soft
start and foldback features are configured, the hand-off point
can vary to ensure that the FET is operated correctly.
V
1V
ISET
Data Sheet
SS
FLB
CURRENT LIMIT
REFERENCE
0.2V
0.1V
t
Figure 51. Interaction of Soft Start, Foldback, and ISET Current Limits
SETTING THE CURRENT LIMIT (ISET)
The maximum current limit is partially determined by selecting
a sense resistor to match the current sense voltage limit on the
controller for the desired load current. However, as currents
become larger, the sense resistor requirements become smaller,
and resolution can be difficult to achieve when selecting the
appropriate sense resistor. The ADM1275 provides an adjustable
current sense voltage limit to handle this issue. The device allows
the user to program the required current sense voltage limit
from 5 mV to 25 mV.
The default value of 20 mV is achieved by connecting the ISET
pin directly to the VCAP pin. This configures the device to use
an internal 1 V reference, which equates to 20 mV at the sense
inputs (see Figure 52).
VCAP
C1
ISET ^ADM1275
GND
Figure 52. Fixed 20 mV Current Sense Limit
To program the sense voltage from 5 mV to 25 mV, a resistor
divider is used to set a reference voltage on the ISET pin (see
Figure 53).
Rev. D | Page 22 of 48