SC667 datasheet(20/47 Pages) SEMTECH

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SC667

ongoing fade, the bank will turn off immediately. When

the backlight bits are re-enabled and BxFEN = 1, the back-

light currents will begin at 0mA and fade to the value

determined by the backlight current register bits IBx[4:0].

If the backlight enable bits are re-enabled and BxFEN = 0,

the main backlights will proceed immediately to the value

of IBx[4:0]. Note that the words“target value”are not used

to describe the final value after a fade operation. “Target

value” is reserved for describing the backlight settings at

the end of the blink or breathe effect cycles.

Non-Programmable Backlight Steps

In addition to the 32 programmable backlight steps, there

are 75 non-programmable steps which are used only

during a fade or breathe operation. Table 1 provides the

total number of steps between the starting and ending

value of any fade or breathe operation. The value from

Table 1 is multiplied by the fade rate to determine the

total fade time. The maximum possible fade-in duration,

from 0mA to 25mA, or fade-out duration, from 25mA to

0mA, is equal to 106 x 16ms = 1696ms.

Figures 10 through 14 provide additional information

about the non-programmable steps. Each figure repre-

sents one linear segment of the overall fade range shown

in Figure 15. The overall fade range is a piece-wise linear

approximation of a logarithmic function which provides

for a very smooth visual fading or breathing effect.

The fade rate may be changed dynamically when a fade

operation is active by writing new values to the fade reg-

ister. When a new backlight level is written during an

ongoing fade operation, the fade will be redirected to the

new value from the present state. An ongoing fade opera-

tion may be cancelled by disabling fade, which will result

in the backlight current changing immediately to the final

value. If fade is disabled, the current level will change

immediately to the final value without the fade delay.

PWM Operation on Bank #1

A PWM signal on the PWM pin can be used to adjust the

DC current through the LEDs in bank #1. When the duty

cycle is 100%, the backlight current through each LED (I

BL)

equals the full scale current value set for bank #1. The

PWM input samples voltage at the PWM pin and converts

Applications Information (continued)

the duty cycle to a DC current level. A DC current is passed

through the LEDs, providing lower noise compared to the

more conventional pulsed current PWM method.

PWM Sampling

The sampling system that translates the PWM signal to a

DC current requires the PWM pin to have a minimum high

time t

HIGH_MIN to set the DC level. High time less than tHIGH_MIN

impacts the accuracy of the target I

BL. The minimum duty

cycle needed to support the minimum high time specifi-

cation varies with the applied PWM frequency (see Figure

9). Note that use of a lower PWM frequency, from 200Hz

to 10kHz, will support a lower minimum duty cycle and an

extended backlight dimming range.

0.2

PWM Frequency (kHz)

tHIGH_MIN = 1µs

Figure 9 — Minimum Duty Cycle

Ambient Light Sense

The SC667 includes a general purpose sigma-delta ADC

that is designed to interface with an ambient light sensor.

The ADC input accepts the output of an external ambient

light sensor circuit. When the ADC is enabled via the I2C

bus, the analog signal produced by the ambient light

sensor is compared with two user programmable thresh-

old levels. The result of the comparison is then used to

automatically change the brightness of the LEDs in bank

#1 to a user defined value. This function is used to com-

pensate for ambient lighting conditions — increasing

brightness where brighter ambient conditions exist and

decreasing brightness in lower lighting conditions.

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