Datasheet

Data Sheet ADM1275
Rev. D | Page 23 of 48
ADM1275
GND
VCAP
ISET
C1 R1
R2
08931-035
Figure 53. Adjustable 5 mV to 25 mV Current Sense Limit
The VCAP pin has a 2.7 V (±1.5%) 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, the resistor divider
should be sized 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. The VCAP pin should not be used for any other
purpose. To guarantee accuracy specifications, care should be
taken not to 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 is requested 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 capacitor
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 51). This change has minimal impact on startup
because the output voltage rises at a similar rate to the SS voltage.
GATE
SENSE+
ADM1275
GND
SENSE–
×50
SS
CURRENT
LIMIT
FLB
ISET
TIMEOUT
CURRENT
LIMIT
CONTROL
REF
SELECT
1.0V
CURRENT
LIMIT
VCAP
10µA
GATE
DRIVE/
LOGIC
+
+
–
–
08931-036
V
CP
Figure 54. 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 ADM1275 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.
The resistor divider should be designed 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.