Datasheet

ADM1276 Data Sheet
Rev. C | Page 20 of 48
ADM1276
GND
VCAP
ISET
C1 R1
R2
09718-047
Figure 47. Adjustable 5 mV to 25 mV Current Sense Limit
The VCAP pin has a 2.7 V internal generated voltage that can
be used to set a voltage at the ISET pin. Assuming that V
ISET
equals the voltage on the ISET pin, size the resistor divider to
set the ISET voltage as follows:
V
ISET
= V
SENSE
× 50
where V
SENSE
is the current sense voltage limit.
The VCAP rail can also be used as the pull-up supply for setting
the I
2
C address. Do not use the VCAP pin for any other purpose.
To guarantee accuracy specifications, do not load the VCAP pin
by more than 100 μA.
SOFT START
A capacitor connected to the SS pin determines the inrush current
profile. Before the FET is enabled, the output voltage of the
current-limit reference selector block is clamped at 100 mV.
This, in turn, holds the hot swap sense voltage current limit,
V
SENSECL
, at approximately 2 mV. When the FET receives a
request to turn on, the SS pin is held at ground until the voltage
between the SENSE+ and SENSE− pins (V
SENSE
) reaches the
circuit breaker voltage, V
CB
.
V
CB
= V
SENSECL
− V
CBOS
where V
CBOS
is typically 0.88 mV, making V
CB
= 1.12 mV.
When the load current generates a sense voltage equal to V
CB
,
a 10 μA current source is enabled, which charges the SS capa-
citor and results in a linear ramping voltage on the SS pin. The
current-limit reference also ramps up accordingly, allowing the
regulated load current to ramp up while avoiding sudden transients
during power-up. The SS capacitor value is given by
ISET
SS
SS
V
tI
C


where:
I
SS
= 10 μA.
t = SS ramp time.
For example, a 10 nF capacitor gives a soft start time of 1 ms.
Note that the SS voltage may intersect with the FLB (foldback)
voltage, and the current-limit reference may change to follow
FLB (see Figure 45). This change has minimal impact on startup
because the output voltage rises at a similar rate to the SS voltage.
GATE
SENSE+
ADM1276
GND
SENSE–
×50
SS
CURRENT
LIMIT
FLB
ISET
TIMEOUT
CURRENT
LIMIT
CONTROL
REF
SELECT
1.0V
CURRENT
LIMIT
VCAP
10µA
GATE
DRIVE/
LOGIC
+
+
–
–
V
CP
09718-048
Figure 48. Soft Start
FOLDBACK
Foldback is a method to actively reduce the current limit as the
voltage drop across the FET increases. It keeps the power across
the FET to a minimum during power-up, overcurrent, or short-
circuit events. It also avoids the need to oversize the FET to
accommodate worst-case conditions, resulting in board size
and cost savings.
The ADM1276 detects the voltage drop across the FET by
looking at a resistor divided version of the output voltage. It is
assumed that the supply voltage remains constant and within
tolerance. The device, therefore, relies on the principle that the
drain of the FET is at the maximum expected supply voltage,
and that the magnitude of the output voltage is relative to that
of the V
DS
of the FET. Using a resistor divider from the output
voltage to the FLB pin, a relationship from V
OUT
, and thus V
DS
,
to V
FLB
can be derived.
Design the resistor divider to output a voltage equal to ISET
when V
OUT
falls below the desired level. This should be well
below the working tolerance of the supply rail. As V
OUT
continues
to drop, the current-limit reference follows V
FLB
because it is
now the lowest voltage input to the current-limit reference
selector block. This results in a reduction of the current limit
and, therefore, the regulated load current. To prevent complete
current flow restriction, a clamp becomes active when the
current-limit reference reaches 200 mV. The current limit
cannot drop below this level.
To suit the SOA characteristics of a particular FET, the required
minimum current for this clamp varies from design to design.
However, the current-limit reference fixes this clamp at 200 mV,
which equates to 4 mV at the sense resistor. Therefore, the main
ISET voltage can be adjusted to align this clamp to the required
percentage current reduction. For example, if ISET equals 0.8 V,
the clamp can be set at 25% of the maximum current.