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

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

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

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

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 21 and 22 show alternate configurations where

external components are used to modify the turn-on delay,

slew rate, and total bleed resistance.

pins. This allows for one external capacitor to be used to

modify the turn-on delay and slew rate. Note that the value

Figure 22 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

as shown in Figures 21 and 22.

The total delay is calculated by adding the default turn-on

delay to a delta term which is calculated as follows:

(eq. 1)

capacitance added between the DELAY pin to either ground

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 slew rate can be calculated as follows:

SR +

(eq. 2)

the on-chip capacitance (see table below). Note that this

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

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

1.77

MW

External Delay Capacitance (Note 13)

Between DEL pin and GND

10

nF

Slew Rate Constant

1.42

mA

External Slew Rate Capacitance (Note 13)

2

nF

Internal Slew Rate Capacitance

150

pF

External Combo Capacitance (Note 13)

2

nF

13.Recommended range, larger values may be used but may degrade the performance of the part