NCP45541
www.onsemi.com
10
APPLICATIONS INFORMATION
Enable Control
The NCP45541 has two part numbers, NCP45541−H and
NCP45541−L, that only differ in the polarity of the enable
control.
The NCP45541−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 NCP45541−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 NCP45541 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 NCP45541 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 29) or through an external
resistor, R
EXT
(as shown in Figure 28). R
EXT
should not
exceed 100 MW and can be used to increase the total bleed
resistance and decrease the load bleed rate.
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 NCP45541 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 28 and 29).
The power good output can be used as the enable signal for
other active−high devices in the system (as shown in
Figure 30). 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 NCP45541 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 28 and 29). 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.
