NCP5355

http://onsemi.com

7

APPLICATIONS INFORMATION

Enable Pin

The Enable pin allows logic level On/Off control of the

NCP5355. A Logic Level Low (< 0.8 V) disables the part by

forcing both TG and BG low. Bringing both gates low

prevents the output voltage from ringing below ground at

turn−off. A Logic Level High (> 2.0 V) enables the part by

allowing CO to control TG and BG.

If the Enable function is not being used, the Enable pin

should be left unconnected. This will Enable the part by

default, as the Enable pin will be internally pulled high by

a 2.0

mA current. The maximum high voltage level is 5.0 V.

Voltages greater than this may damage the part.

Undervoltage Lockout

exceeding the UVLO threshold allows CO to take control of

the Start Threshold.

Supply Capacitor Selection

A 1.0

and to reduce noise. A small 1.0

additional filtering in noisy environments.

Bootstrap Capacitor Selection

in Figure 4 ) is determined from the following equation:

CBST w

QTtopFETs

DVBST

where

charge,

is the maximum change in voltage across

the bootstrap capacitor and is typically

designed for a drop of less than a 1.0 V.

For example, a circuit using one Top MOSFET with a

Internal or External Bootstrap Diode

For convenience, a bootstrap diode is internally provided

by the NCP5355. This internal diode reduces system cost

and parts count.

However, this diode will have higher losses than a

standard small signal switching or Schottky diode. By using

improvement in efficiency can be achieved as illustrated by

the graph in Figure 5.

While the difference in efficiency is relatively small, this

difference represents heat loss in the driver and on average

driver temperature may be reduced by about 10

°C if using

an external diode. If an external diode is used, it should be

a

Schottky

or

switching

diode.

(For

example:

ON Semiconductor

Part

Number

BAT54HT1

or

BAS16HT1.)

Adaptive Nonoverlap

The NCP5355 includes adaptive nonoverlap protection to

prevent top and bottom MOSFET cross conduction.

When CO goes low signaling TG to turn off the top

MOSFET, BG does not go high until the switch node

(DRN pin) has fallen below 5.0 V and a fixed amount of

MOSFET is off before the bottom MOSFET is turned on.