NCP4545
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8
APPLICATIONS INFORMATION
On−Resistance
The MOSFET gate voltage in the NCP4545 is driven by
a charge pump in the controller circuit. The output voltage
of the charge pump is dependent on the voltage on V
CC
. The
R
ON
of the MOSFET is in turn dependent on its V
GS
. Care
must be taken to ensure a sufficient V
CC
voltage is used to
create the desired R
ON
given the anticipated input voltage.
Enable Control
The NCP4545 has two separate part numbers,
NCP4545IMNTWG and NCP4545IMNTWG-L, that only
differ in the polarity of the enable control.
The NCP4545IMNTWG allows for enabling the
MOSFET in an Active-High configuration. When the EN
pin is at a logic high level and the V
CC
supply pin has an
adequate voltage applied, 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
GND on the EN pin ensures that the MOSFET will be
disabled when not being driven.
The NCP4545IMNTWG-L allows for enabling the
MOSFET in an Active-Low configuration. When the EN
pin is at a logic low level and the V
CC
supply pin has an
adequate voltage applied, 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.
Parametric Adjustments
The NCP4545 can be used in several configurations
depending on the need to control turn-on delay, slew rate, and
bleed resistance. Default values of each parameter are built
into the part without the requirement of external components.
This configuration is shown above in Figure 1.
Figures 22 and 23 show alternate configurations where
external components are used to modify the turn-on delay,
slew rate, and total bleed resistance.
Figure 22 shows an external capacitor, C
COMBO
,
connected from the V
OUT
pin to both the DELAY and SR
pins. This allows for one external capacitor to be used to
modify the turn-on delay and slew rate. Note that the value
of C
COMBO
is used in Equations 1 and 2 (below) in place of
C
DEL
and C
SR
respectively.
Figure 23 shows the use of two external capacitors for
independent control of the turn-on delay and slew rate.
Turn−On Delay
The NCP4545 provides a time delay between the input
transition on EN to the MOSFET turning on. The turn-on
delay can be increased with an external capacitor added
between the DELAY pin and either ground or the V
OUT
pin
as shown in Figures 22 and 23.
The total delay is calculated by adding the default turn-on
delay to a delta term which is calculated as follows:
Dt + K
DEL
*C
DEL
(eq. 1)
where K
DEL
is a constant and C
DEL
is the off-chip
capacitance added between the DELAY pin to either ground
or the V
OUT
pin (see table below). When no external
capacitor is present, the delay will be the specified default
turn-on delay.
Slew Rate Control
The NCP4545 is equipped with controlled output slew
rate which provides soft start functionality. This limits the
inrush current caused by capacitor charging and enables this
device to be used in hot swapping applications. The slew rate
can be decreased with an external capacitor added between
the SR pin and the V
OUT
pin as shown in Figures 22 and 23.
The slew rate can be calculated as follows:
SR +
K
SR
C
SR
) C
CHIP
(eq. 2)
where K
SR
is a constant, C
SR
is the off-chip capacitance
added between the SR pin and the V
OUT
pin, and C
CHIP
is
the on-chip capacitance (see table below). Note that this
equation is only valid for C
SR
≥ 470 pF. When no external
capacitor is present, the slew rate will be the specified
default slew rate.
Load Bleed
The NCP4545 has an on−chip bleed resistor that can be
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 which is enabled whenever the MOSFET
is disabled. Delays are added to the enable of this switch to
ensure that both the MOSFET and the bleed switch are not
concurrently active. The total bleed resistance can be
increased by adding a resistor between the BLEED pin and
the V
OUT
pin as shown in Figures 22 and 23. If the load
bleed function is not desired, the BLEED pin should be tied
to ground or left floating.
SLEW RATE AND TURN−ON DELAY
Parameter Conditions Symbol Min Typ Max Unit
Turn−on Delay Constant K
DEL
1.77
MW
External Delay Capacitance (Note 13) Between DEL pin and GND C
DEL
10 nF
Slew Rate Constant K
SR
1.42
mA
External Slew Rate Capacitance (Note 13) Between SR pin and V
OUT
pin C
SR
2 nF
Internal Slew Rate Capacitance C
CHIP
150 pF
External Combo Capacitance (Note 13) Between SR & DEL pins and V
OUT
pin C
COMBO
2 nF
13.Recommended range, larger values may be used but may degrade the performance of the part
