NCP45560
www.onsemi.com
10
APPLICATIONS INFORMATION
Enable Control
The NCP45560 has two part numbers, NCP45560−H and
NCP45560−L, that only differ in the polarity of the enable
control.
The NCP45560−H device allows for enabling the
MOSFET in an active−high configuration. When the V
CC
supply pin has an adequate voltage applied and the EN pin
is at a logic high level, the MOSFET will be enabled.
Similarly, when the EN pin is at a logic low level, the
MOSFET will be disabled. An internal pull down resistor to
ground on the EN pin ensures that the MOSFET will be
disabled when not being driven.
The NCP45560−L device allows for enabling the
MOSFET in an active−low configuration. When the V
CC
supply pin has an adequate voltage applied and the EN pin
is at a logic low level, the MOSFET will be enabled.
Similarly, when the EN pin is at a logic high level, the
MOSFET will be disabled. An internal pull up resistor to
V
CC
on the EN pin ensures that the MOSFET will be
disabled when not being driven.
Power Sequencing
The NCP45560 devices will function with any power
sequence, but the output turn−on delay performance may
vary from what is specified. To achieve the specified
performance, there are two recommended power sequences:
1. V
CC
→ V
IN
→ V
EN
2. V
IN
→ V
CC
→ V
EN
Load Bleed (Quick Discharge)
The NCP45560 devices have an internal bleed resistor,
R
BLEED
, which is used to bleed the charge off of the load to
ground after the MOSFET has been disabled. In series with
the bleed resistor is a bleed switch that is enabled whenever
the MOSFET is disabled. The MOSFET and the bleed
switch are never concurrently active.
It is required that the BLEED pin be connected to V
OUT
either directly (as shown in Figure 31) or through an external
resistor, R
EXT
(as shown in Figure 30). R
EXT
should not
exceed 1 kW and can be used to increase the total bleed
resistance.
Care must be taken to ensure that the power dissipated
across R
BLEED
is kept at a safe level. The maximum
continuous power that can be dissipated across R
BLEED
is
0.4 W. R
EXT
can be used to decrease the amount of power
dissipated across R
BLEED
.
Power Good
The NCP45560 devices have a power good output (PG)
that can be used to indicate when the gate of the MOSFET
is fully charged. The PG pin is an active−high, open−drain
output that requires an external pull up resistor, R
PG
, greater
than or equal to 1 kW to an external voltage source, V
TERM
,
compatible with input levels of other devices connected to
this pin (as shown in Figures 30 and 31).
The power good output can be used as the enable signal for
other active−high devices in the system (as shown in
Figure 32). This allows for guaranteed by design power
sequencing and reduces the number of enable signals needed
from the system controller. If the power good feature is not
used in the application, the PG pin should be tied to GND.
Slew Rate Control
The NCP45560 devices are equipped with controlled
output slew rate which provides soft start functionality. This
limits the inrush current caused by capacitor charging and
enables these devices to be used in hot swap applications.
The slew rate can be decreased with an external capacitor
added between the SR pin and ground (as shown in
Figures 30 and 31). With an external capacitor present, the
slew rate can be determined by the following equation:
Slew Rate +
K
SR
C
SR
[Vńs]
(eq. 1)
where K
SR
is the specified slew rate control constant, found
in Table 4, and C
SR
is the slew rate control capacitor added
between the SR pin and ground. The slew rate of the device
will always be the lower of the default slew rate and the
adjusted slew rate. Therefore, if the C
SR
is not large enough
to decrease the slew rate more than the specified default
value, the slew rate of the device will be the default value.
The SR pin can be left floating if the slew rate does not need
to be decreased.
Short−Circuit Protection
The NCP45560 devices are equipped with short−circuit
protection that is used to help protect the part and the system
from a sudden high−current event, such as the output, V
OUT
,
being shorted to ground. This circuitry is only active when
the gate of the MOSFET is fully charged.
Once active, the circuitry monitors the difference in the
voltage on the V
IN
pin and the voltage on the BLEED pin.
In order for the V
OUT
voltage to be monitored through the
BLEED pin, it is required that the BLEED pin be connected
to V
OUT
either directly (as shown in Figure 31) or through
a resistor, R
EXT
(as shown in Figure 30), which should not
exceed 1 kW. With the BLEED pin connected to V
OUT
, the
short−circuit protection is able to monitor the voltage drop
across the MOSFET.
If the voltage drop across the MOSFET is greater than or
equal to the short−circuit protection threshold voltage, the
MOSFET is immediately turned off and the load bleed is
activated. The part remains latched in this off state until EN
is toggled or V
CC
supply voltage is cycled, at which point the
MOSFET will be turned on in a controlled fashion with the
normal output turn−on delay and slew rate. The current
through the MOSFET that will cause a short−circuit event
can be calculated by dividing the short−circuit protection
threshold by the expected on−resistance of the MOSFET.
