LED7708 datasheet(23/53 Pages) STMICROELECTRONICS

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LED7708

Recommended operating conditions

Doc ID 022698 Rev 1

23/53

The two signals IOK and XVOK are connected (wired-or) to all the slave devices sharing the

same output voltage rail: if a channel has a low-voltage drop (below VRWL), the device that

owns that channel forces low the IOK signal. In a similar way, the XVOK is released if the

voltage across all the channels of a slave is higher than required (above VRWH). The wire-or

connection of this signal ensures that the output voltage reduction takes place only if all the

slaves (and the master itself) need this operation. Refer to the special functions section for a

detailed explanation of the daisy chain connection between the master and slave devices.

The boost converter is always active regardless of how many channels are active (i.e. even

if all channels are off) in order to improve the load transient dynamic response.

The output voltage regulation can be enabled/disabled (the OVRE bit of the DEVCFG0

In real applications, the spread of the forward voltage of the LEDs may cause the channels

to have different voltage drops. In any case, the leading channel (i.e. the channel requiring

the highest voltage to drive its LED string) is kept inside the regulation window.

The reference voltage, provided by the internal DAC, spans between the minimum and

maximum values, programmable through the VMIN pin (see Section 5: Electrical

characteristics).

As a consequence, the output voltage can be varied by the device in the following range:

Equation 2

Equation 3

where R1 and R2 are the resistors of the divider connected to the output rail and VVFB is the

reference voltage at the VFB pin.

The LED7708 regulates the output voltage in order to optimize the overall efficiency and, to

do this, it needs to read the voltage drop across the active current generators. Unfortunately,

in some cases (e.g. during the soft-start phase or when all the channels are set to 0%

brightness), the current generators are off and there is no useful information coming from

them. Therefore the output voltage is regulated to the minimum value (Equation 2: ). It must

be ensured that the minimum output voltage is below the minimum expected output voltage

Table 9.

Handshake signals summary

Function

IOK

Used by the slaves to increase the output voltage (algorithm is overdriven)

XVOK

Used by the slaves to ask for an output voltage reduction

XOVF

Used by the master to inform that the maximum output voltage has been reached (each

slave uses this information to tag as “faulty” its open channels)

XMSK

Used by the master to inform the slaves that the output voltage is out of regulation and

the LED fault detection must be masked to prevent false detection

BOOST min

--------------------

⎝⎠

⎛⎞

VFB min

⋅

BOOST MAX

--------------------

⎝⎠

⎛⎞

VFB MAX

⋅

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