MCP1664 datasheet(16/28 Pages) MICROCHIP

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MCP1664 LED Driver Evaluation Board

DS50002363A-page 16

 2015 Microchip Technology Inc.

2.3.1.4

PWM DIMMING BLOCK

The MCP1664 allows PWM dimming by turning on and off the LED with a PWM signal

applied to the EN pin. The maximum frequency for dimming is limited by the start-up

and the load. By varying the duty cycle of the PWM signal applied on the EN pin, the

LED current changes linearly.

In order to test this feature, a small variable PWM generator, created using three

operational amplifiers from a single quad-op-amp chip, was included on the board.

FIGURE 2-4:

PWM Dimming Block.

The circuit consists of a triangular wave generator which is made up of U2B

(functioning as an integrator), U2A (acting as a comparator with hysteresis) and a

comparator (U2C). U2B non-inverting input is biased at VDD/2.

A virtual connection between the inverting and non-inverting inputs allows a constant

current through R1 equal to I = VDD/(2R1) which charges the capacitor C2. This way,

the U2B integrator output increases linearly with time. When it reaches 95% VDD, the

comparator output changes to its maximum output voltage VDD.

At that point, the output voltage of the integrator decreases linearly. When it reaches

5% VDD, the comparator output voltage switches to zero and the cycle repeats. Thus,

the integrator output is a triangular wave that swings between the levels of 5% and

95% VDD.

U2C compares the triangular wave against the DC level given by the R7 variable

resistor. Its output is a square wave, with a duty cycle that varies from 0% to 100%.

The frequency of the PWM signal is determined by R1, C2, R2 and R4:

EQUATION 2-2:

PWM DIMMING FREQUENCY

4 U2A

MCP6004

11 U2C

MCP6004

Triangular wave output

PWM output

4.7 k



1.8 k



10 k



10 k



0.1 µF

0.47 µF

10 k



0.1 µF

VDD

MCP6004

U2B

VDD

4R1R4C2

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