Datasheet

0.245V

DS(ON)

OUT

0.24V

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Board Features

MOSFET switch current can also be sensed directly across the R

DS(ON)

of MOSFET Q5, eliminating the

need for a sense resistor (see Table 5).

Figure 5. MOSFET R

DS(ON)

Sensing Configuration

Table 5. MOSFET R

DS(ON)

Sensing Configuration

Jumper Operation

J2 Close Jumper

J3 Open Jumper

J4A, J4B Close Jumper

R6 No Load

The trade-off will be less accuracy and performance flexibility for reduced component count, and

increased efficiency. The current limit (I

) using this sense method is calculated by the equation:

(13)

Substituting in the resistor values as listed in the board schematic and an R

DS(ON)

of 0.025Ω for Q5

(SUD40N10-25) gives a current limit I

of approximately 10A.

5.5 PWM Dimming

The average LED forward current is often controlled or reduced with a pulse-width modulated (PWM)

signal. By reducing the average LED current, light from the LEDs is reduced.

This dimming method allows the converter to operate the LEDs at a specific peak output current level (i

which is usually a set point determined by the LED manufacturer. This allows the LED to illuminate with a

consistent light color while still having the ability to reduce its lumens output.

The dimming frequency should be fast enough so that the ON and OFF blinking of the LEDs is not

perceived by the human eye. Usually the dimming frequency should be greater than 120 Hz, but less than

5 kHz for best results.

The LM3423 evaluation board implements PWM dimming by placing a series connected MOSFET in

series with the LED stack. The PWM signal is applied to this MOSFET, and the LED current is interrupted

when the MOSFET turns OFF.

SNVA376A–December 2008–Revised May 2013 AN-1907 LM3423 Buck-Boost Configuration Evaluation Board

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