EVAL-ADM1276EBZ Analog Devices, EVAL-ADM1276EBZ Datasheet - Page 19

• Manufacturer Part Number: EVAL-ADM1276EBZ
• Description: Power Management IC Development Tools EVALUATION BOARD
• Manufacturer: Analog Devices
• Type: Hot Swap & Power Distributionr
• Series: ADM1276r

Specifications of EVAL-ADM1276EBZ

• Rohs: yes
• Tool Is For Evaluation Of: ADM1276
• Input Voltage: 20 V
• Factory Pack Quantity: 1

Data Sheet
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.
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.
Figure 43. Connection of Multiple Sense Resistors to the SENSE± Pins
ISET
VCC
FLB
SS
TO 20V
VCC
2.95V
Figure 44. Current-Limit Reference Selection
SENSE+
SENSE+
R
CURRENT
SENSE
×50
BIAS
GND
GND
SENSE–
ADM1276
OVER-
CURRENT
SENSE–
GATE
Q
Q
1
GATE
1
Rev. B | Page 19 of 48
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).
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).
Figure 45. Interaction of Soft Start, Foldback, and ISET Current Limits
0.2V
0.1V
1V
V
ISET
Figure 46. Fixed 20 mV Current Sense Limit
C1
VCAP
ISET
CURRENT-LIMIT
REFERENCE
ADM1276
FLB
SS
GND
ADM1276
t