LED7708TR datasheet(22/53 Pages) STMICROELECTRONICS

22 / 53 page

Recommended operating conditions

LED7708

22/53

Doc ID 022698 Rev 1

Figure 5.

Simplified output regulation circuitry

During normal operation, the regulation algorithm is the following:

●

if at least one active channel has a voltage drop below the lower threshold, the internal

DAC counts up and the output voltage is increased

●

if all the active channels have a voltage drop higher than the upper threshold, the

internal DAC counts down and the output voltage is reduced.

Therefore, the possible operating configurations are shown in Table 8.

The algorithm can also manage faulty conditions that may occur on the LED array (open

channels and LED short-circuit) in order to prevent excessive power dissipation.

The LED7708 has been designed to interface with other slave devices and can provide them

with the correct LED supply voltage by extending the above mentioned output voltage

optimization. For this purpose, some handshake signals have been reserved:

Table 8.

Output voltage states during regulation

Output voltage

Condition

Excessive

All the channels have their voltage above the upper threshold of the

regulation window.

Optimal

All the channels have their voltage inside the regulation window.

Regulated

Most of the channels have their voltage inside the regulation window; the

remaining channels have a voltage drop above the upper threshold.

Insufficient

At least one channel has its voltage drop below the lower threshold of the

regulation window.

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