Adjusting Frequency Response of

the LM3464 Circuit

The frequency response of the LM3464 evaluation board can

be adjusted by changing the value of the capacitor, C

Higher capacitance of C

sponse of the LM3464 driver stage. In order to ensure stable

system operation, it is recommended to set the dominant pole

of the LM3464 one decade lower than the dominant pole of

the AC/DC converter. The default value of the C

evaluation board is 0.22uF. For applications with slow re-

sponse AC/DC power supply (e.g. converters with active

PFC), the CDHC value should be increased to make the fre-

quency response of the board slower than that of the AC/DC

power supply. The cut-off frequency of the LM3464 driver

stage is governed by the following equation:

(9)

Thermal Foldback Control

The LM3464 evaluation board features an interface that en-

ables thermal foldback control by connecting a NTC thermal

sensor to the THM+ and THM- terminals. With the NTC sen-

sor attached to the chassis of the LED arrays, the thermal

foldback control feature reduces the average LED current and

effectively reduces the LED temperature to prevent thermal

breakdown of the LEDs. The thermal foldback control reduces

the LED currents by means of PWM dimming which the dim-

ming frequency is set by the capacitor CTHM following the

equation shows below:

(10)

The default value of the CTHM on the LM3464 evaluation

board is 68nF, which set the thermal foldback dimming fre-

quency at 258Hz.

Thermal foldback control is activated when the voltage at the

Thermal pin, V

figure 5. Thermal foldback control begins when V

low 3.25V. The LED current will be reduced to zero as

V

cording to the Thermal pin voltage following the equation:

(11)

30127113

FIGURE 5. Changes of Average LED Current with

Thermal Foldback Control

Design Example

A NTC thermistor is connected to the THM+ and THM- ter-

minals of the LM3464 evaluation board as shown in figure 6

to activate the thermal foldback control function.

7

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