ACT5880 datasheet(71/87 Pages) ACTIVE-SEMI

71 / 87 page

ACT5880

Rev 2, 03-Sep-13

- 71 -

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The ODO1, the ODO2 and the ODO3 are open-drain

drivers, which could operate as simple open-drain

outputs, or as constant current sinkers, set by

individual configuration. Each of them could also

output in PWM pattern for extended output power

adjustment range. One of the driver output, the

ODO1, has high operation voltage range of up to

40V and the other 2 have the ranges of up to 5V,

make them flexible and powerful.

Each driver is configured and manipulated through

the access to respective register bit. Refer to THE

for more details.

The BUFFER bit sets how a current setting or PWM

duty cycle affects the output. When the BUFFER is

set to 0, the data written into the respective current

or duty register affects the output instantly; When it

is set to 1, the data written into the register does not

affect the output until the bit is set to 0. This

BUFFER bit has power over all the 3 drivers, which

makes

the

outputs

drivers

change

simultaneously, independent to when the data is

set.

Operation Modes

Constant Current Driving

The current is set by configuring the weight for LSB

and a 6 bits current code. Combining the weight

and the current code, the adjustment to current for

each driver covers 0 to 100.8mA nominal.

PWM Switching

Each driver could be turned on and off with an

internal generated PWM pattern at nominal 244Hz.

The duty cycle of PWM pattern is either linear

coded or non-linear coded, optioned against

ordering. If linear coded, the duty cycle is the ratio

of the integer volume put into the duty register over

255. See respective tables for non-linear coded

duty cycle setting and the register setting. Figure 7

shows the duty cycle to code plot for both the linear

and non-linear code, with an exponential curve for

reference.

The non-linear coded duty cycle setting provides a

exponential

code

brightness

approximation,

which

gives

similar

feeling

difference in changing the code linearly, against the

Webber's law.

Phase Shift and Delay

In case of driving backlight assembly with long wire

and close to the EMI sensitive LCD display, both

PWM

phase

shift

and

sequential

are

implemented in the driver circuitry additional to the

current-on slew rate limit of 40mA/μs nominal,

which assure no flicker, no fake operation in various

size for high quality image display.

Figure 7:

The PWM Adjustment Curves.

Combined Modulations

The PWM pattern could modulate the constant

current output on and off, as well as the simple

open drain output on and off. The combined

modulation extends the dynamic range for backlight

brightness adjustment. For multi-strings backlight

application, the current setting could be used for

matching different strings while the PWM is used for

overall brightness control.

Inductive Load

For inductive load driving, like driving a vibrator

motor, an external dump clamp diode is desired to

avoid voltage overstress by current breaking

voltage surge caused by the serial inductance. As

specified in the characteristics table, the drivers

have a controlled current on slew rate of 40mA/μs

nominal. This slew rate control only applies on the

turning on of the current, not on the turning off.

Irregular Conditions

The driver block has her own overheat monitoring,

with a threshold temperature lower than the over

temperature shutdown threshold. If overheat is

detected in this block, the drivers are turned off

before the system is possibly forced into thermal-

shutdown. Each driver also has a LED open

detection circuit, which monitors the driving level of

the output stage. When it is driven saturated, then

OPEN-DRAIN DRIVERS

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

100

150

200

250

Non‐linear code

Expotential Curve

Linear code

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