NCP45520, NCP45521

http://onsemi.com

5

APPLICATIONS INFORMATION

Enable Control

Both the NCP45520 and the NCP45521 have two part

numbers, NCP4552x-H and NCP4552x-L, that only differ

in the polarity of the enable control.

The NCP4552x-H devices allow for enabling the

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 NCP4552x-L devices allow for enabling the

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

disabled when not being driven.

Power Sequencing

The NCP4552x 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:

Load Bleed (Quick Discharge)

The NCP4552x devices have an internal bleed resistor,

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.

either directly (as shown in Figures 4 and 7) or through an

should not exceed 1 k

W and can be used to increase the total

bleed resistance.

Care must be taken to ensure that the power dissipated

Power Good

The NCP45520 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

than or equal to 1 k

that is compatible with input levels of all devices connected

to this pin (as shown in Figures 3 and 4).

The power good output can be used as the enable signal for

other active−high devices in the system (as shown in

Figure 5). 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 NCP4552x 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 of the NCP45521 can be decreased with an

external capacitor added between the SR pin and ground (as

shown in Figures 6 and 7). With an external capacitor

present, the slew rate can be determined by the following

equation:

Slew Rate +

[V s]

(eq. 1)

between the SR pin and ground. The slew rate of the device

will always be the lower of the default slew rate and the

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 NCP4552x devices are equipped with short−circuit

protection that is used to help protect the part and the system

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

BLEED pin, it is required that the BLEED pin be connected

which should not exceed 1 k

W. With the BLEED pin

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

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.

Thermal Shutdown

The thermal shutdown of the NCP4552x devices protects

the part from internally or externally generated excessive