LP5569 datasheet(13/93 Pages) TI2

13 / 93 page

Cycle Time

1/fPWM

LED0

LED3

LED6

LED1

LED4

LED7

LED2

LED5

LED8

Phase 1

Phase 2

Phase 3

Phase 1

Phase 2

Phase 3

0 mA

1LSB

256 (9-bit)

2048 (12-bit)

256 4/8 (9-bit)

2052 (12-bit)

257 (9-bit)

2056 (12-bit)

LP5569

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SNVSAP8A – JULY 2017 – REVISED SEPTEMBER 2017

Product Folder Links: LP5569

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8.3 Feature Description

8.3.1 Current Sinks

8.3.1.1 Overview

The LP5569 LED drivers are constant-current sources. Maximum output-current scale can be programmed by

control registers up to 25.5 mA. The overall maximum current is set by 8-bit output current-control registers with

100-μA step size. Each of the 9 LED drivers has a separate output-current control register. The LED luminance

pattern (dimming) is controlled with a PWM technique, which has 12-bit resolution during ramping and 8-bit user

control. The LED current-sink PWM frequency is 20 kHz.

High 20-kHz PWM frequency and 12-bit control accuracy are achieved by using 3-bit dithering for PWM control.

There is a 9-bit pure PWM resolution generated in the PWM generators, and one least-significant bit (1 LSB) is

toggled in eight periods to output a smaller average step. For 3-bit dithering, every eighth pulse is made 1 LSB

longer to increase the average value by 1 / 8th. Figure 12 shows an example of 9-bit PWM value, step of 4 / 8

(0.5) and its 12-bit representation.

Figure 12. Dithering

A phase-shift PWM scheme allows delaying the time when each LED driver is active. When the LED drivers are

not activated simultaneously, the peak load current from the charge-pump output is greatly decreased. This also

reduces input-current ripple and ceramic-capacitor audible ringing. LED drivers are grouped into three different

phases. In phase 1, the rising edge of the PWM pulse is fixed in time. In phase 2, the middle point of the PWM

pulse is fixed, and the pulse spreads to both directions when PWM duty cycle is increased. Phase 3 has a fixed

falling edge, that is, the rising edge of the pulse is changed when the duty cycle changes.

Figure 13. LED Phase Shift

LED dimming is controlled according to an exponential or linear scale (see Figure 14) In exponential mode, the

PWM output percent can be approximated by the following two equations:

•

Less than or equal to code 64: y = 0.0125x – 0.0066.

•

Greater than code 64: y = 0.7835e0.0217x

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