Datasheet

Data Sheet ADM1276
Rev. C | Page 19 of 48
Q
1
SENSE–
GND
GATEVCC
SENSE+
BIAS
CURRENT
2.95
V
TO 20V
09718-043
Figure 43. 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 ADM1276 limits the current during an over-
current 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 pro-
grammed 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
Q
1
SENSE–
GND
GATE
VCC
ISET
SS
FLB
SENSE+
ADM1276
OVER-
CURRENT
×50
09718-044
Figure 44. 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 45 provides an
example of how the soft start, foldback, and ISET voltages interact
during startup as the ADM1276 is enhancing the FET and charging
the load capacitances. Depending on how the soft start and fold-
back features are configured, the hand-off point can vary to
ensure that the FET is being operated within the correct limits.
SS
FLB
ISET1V
0.2V
0.1V
V
t
CURRENT-LIMIT
REFERENCE
09718-045
Figure 45. 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 ADM1276 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 46).
ADM1276
VCAP
ISET
C1
GND
09718-046
Figure 46. 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 47).