LED7708 datasheet(28/53 Pages) STMICROELECTRONICS

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Recommended operating conditions

LED7708

28/53

Doc ID 022698 Rev 1

The two current levels are set by connecting a resistor on both the ISETH and ISETL pins:

Equation 12

where KH= 1200 V.

Equation 13

where KL= 4.0 V.

The lower level is generally used to keep the voltage across the LED string under control

during the off-phase of the dimming period; if a zero-current low level is required, the ISETL

pin must be connected to the device supply rail (LDO3).

The current generators of the LED7708 have been designed to be grouped (i.e. connected

in parallel) if required by the application. For stability reasons, the parallelization of the

current generators should be done when the programmed current is above 40 mA per

channel.

The current accuracy among different devices is guaranteed to be lower than ±2% through

the full output current range (20 mA-85 mA). Two current ranges are selectable in order to

optimize the overall current accuracy.

Table 10 provides some typical output currents for standard E96 values.

Unused channels should be disabled through the CHSEL register and can be physically left

floating or shorted to ground.

Table 10.

LED current vs. external resistor setting

Nominal LED current

ICHX,ON [mA]

RISETH [kΩ]

E96 series (1%)

Actual LED current

ICHX,ON [mA]

14.3

83.9

16.2

74.1

20.0

60.0

24.3

49.4

30.1

39.9

40.2

29.9

60.4

19.9

80.6

14.9

CHX ON

ISETH

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

CHX OFF

ISETL

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

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