NCL30161

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7

Inductor Selection

The inductor that is used directly affects the switching

frequency the driver operates at. The value of the inductor

sets the slope at which the output current rises and falls

during the switching operation. The slope of the current, in

turn, determines how long it takes the current to go from the

valley point of the current ripple to the peak when the FET

is on and the current and rising, and how long it takes the

current to go from the peak point of the current to the valley

when the FET is off and the current is falling. These times

can be approximated from the following equations:

+

L

DI

DS

L

DI

forward voltage of the catch diode.

The switching frequency can then be approximated from

the following:

1

Higher values of inductance lead to slower rates of rise

and fall of the output current. This allows for smaller

discrepancies between the expected and actual output

current ripple due to propagation delays between sensing at

the CS pin and the turning on and off of the power MOSFET.

However, the inductor value should be chosen such that the

peak output current value does not exceed the rated

saturation current of the inductor.

Catch Diode Selection

The catch diode needs to be selected such that the average

current through the diode does not exceed the rated average

forward current of the diode. The average current through

the diode can be calculated as:

It is also important to select a diode that is capable of

withstanding the peak reverse voltage it will see in the

application. It is recommended to select a diode with a rated

reverse voltage greater than VIN. It is also recommended to

use a low−capacitance Schottky diode for better efficiency

performance

.

Selecting The Off−Time Setting Resistor

NCL30161 with the initial time duration that the MOSFET

is turned off when the switching operation begins. During

subsequent switching cycles, the voltage at the CS pin is

sensed every time the MOSFET is turned on, and the

off−time will be adjusted depending on how much of a

discrepancy exists between the sensed value and the CS

value allows the system to quickly achieve the intended

the following equation:

operation, calculated in the Inductor Selection section

above.

Every time the DIM/EN pin is brought from a low state to

a high state, the initial off−time program is reset. The first

off−time of the MOSFET after the DIM/EN is brought high

will be set by the ROT value. The off−time will then be

adjusted in subsequent switching cycles.

Input Capacitor

A decoupling capacitor from VIN to ground should be

used to provide the current needed when the power

MOSFET turns on. A 10

mF ceramic capacitor is

recommended.