LP5521 datasheet(25/48 Pages) TI1

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PWM Control

Value

Current value

Step time = 31.2 ms

End of 1st Ramp command,

start next command

Steps

End of 2nd Ramp command,

start next command

Rising ramp,

Sign = 0

Downward

ramp, Sign = 1

Increment = 4

=> 5 cycles

LP5521

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SNVS441H – JANUARY 2007 – REVISED MAY 2016

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X means do not care whether 1 or 0.

7.5.3.1 RAMP/WAIT

Ramp command generates a PWM ramp starting from current value. At each ramp step the output is

incremented by one. Time for one step is defined with Prescale and Step time bits. Minimum time for one step is

0.49 ms and maximum time is 63 × 15.6 ms = 1 second/step, so it is possible to program very fast and also very

slow ramps. Increment value defines how many steps are taken in one command. Number of actual steps is

Increment + 1. Maximum value is 127d, which corresponds to half of full scale (128 steps). If during ramp

command PWM reaches minimum/maximum (0/255) ramp command is executed to the end, and PWM stays at

minimum/maximum. This enables ramp command to be used as combined ramp and wait command in a single

instruction.

Ramp command can be used as wait instruction when increment is zero.

Setting register 00H bit LOG_EN sets the scale from linear to logarithmic. When LOG_EN = 0 linear scale is

used, and when LOG_EN = 1 logarithmic scale is used. By using logarithmic scale the visual effect of the ramp

command seems linear to the eye.

Table 15. Ramp/Wait Command

Pre-

scale

Step time

Sign

Increment

NAME

VALUE(d)

DESCRIPTION

Prescale

Divides master clock (32 768Hz) by 16 = 2048 Hz, 0.49 ms cycle time

Divides master clock (32 768Hz) by 512 = 64 Hz, 15.6 ms cycle time

Step time

1-63

One ramp increment done in (step time) x (clock after prescale) Note: 0 means Set

PMW command.

Sign

Increase PWM output

Decrease PWM output

Increment

0-127

The number of steps is Increment + 1. Note: 0 is a wait instruction.

Application example:

For example if following parameters are used for ramp:

•

Prescale = 1

→ cycle time = 15.6 ms

•

Step time = 2

→ time = 15.6 ms x 2 = 31.2 ms

•

Sign = 0

→ rising ramp

•

Increment = 4

→ 5 cycles

Ramp command will be: 0100 0010 0000 0100b = 4204H

If current PWM value is 3, and the first command is as described above and next command is a ramp with

otherwise same parameters, but with Sign = 1 (Command = 4284H), the result will be like in Figure 25:

Figure 25. Example of 2 Sequential Ramp Commands.

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