LDLL16ENTR datasheet(15/102 Pages) STMICROELECTRONICS

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4.2.2

PWM operation

In the LDLL16EN, the brightness of LED strings can be modified by PWM. The PWM dimming could be

performed in different ways:

•

Through serial bus

•

Through direct input

•

Through power line

When PWM operation is managed through the serial bus, there are 3 bits (PWM_FREQ, see the Section 6: FTP

and RAM memory mapping) to configure the PWM frequency of the channels.

The PWM frequency range is reported in the table below.

Table 5. PWM frequency range

PWM_FREQ [2÷0]

PWM_FREQ [Hz]

bit 2

bit 1

bit 0

200

300

400

500

700

1000

1200

1400

For all the non-mapped output channels (see Table 3), the PWM frequency and duty cycle is the same as DIN

mapped channels (as reported in the dimming with direct input in fail-safe/stand-alone mode section).

Dimming with serial bus

In the LDLL16EN, each channel can be individually dimmed on 8-bit exponential duty cycle to adequate LED

brightness change to human eye light perception, giving in this way the impression of brightness linear variation.

The exponential law used to calculate the dimming steps is the following:

toni=PWMperiod⋅αΛ N−i−12

(1)

Where toni is the LED ON-time during step number "i", 1 ≤ i ≤ 255, α ≈ 0.9471, N = 254.

For the step number "i" = 0, DC is set to "0".

The function of duty cycle vs 8-bit PWM_DUTY_CHx setting is shown in the following graph. The minimum duty

cycle is clamped to 0.1% in any case.

Figure 9. Duty cycle vs PWM_DUTY_CHx

LDLL16EN

Functional description

DS14111 - Rev 5

page 15/102

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