LED7707 datasheet(36/48 Pages) STMICROELECTRONICS

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Application information

LED7707

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6.4.6

Over-voltage protection divider setting

The over-voltage protection (OVP) divider provides a partition of the output voltage to the

OVSEL pin. The OVP divider setting not only fixes the OVP threshold, but also the open-

channel detection threshold.

The proper OVP divider setting can be calculated by the equation (3):

Equation 38

where VOUT, MAX is the maximum output voltage considering the worst case (all LEDs with

the maximum VF = VF,max = 3.7 V on the same row):

Equation 39

R1 can be chosen is in the order of hundreds of kilo-ohms to reduce the leakage current in

the resistor divider. For example, setting R1 = 510 kΩ leads to R2 = 21.89 kΩ. The closest

standard commercial value is R2 = 22 kΩ.

6.4.7

Compensation network

For the compensation network, the suggestions provided in Section 6.1 are always valid.

In this condition, tentatively the following value of R3 and C8 (see Figure 24) are usually a

good choice for the loop stability:

R3 = 2.4 k

C8 = 4.7 nF

6.4.8

Boost current limit

The boost current limit is set to protect the internal power switch against excessive current.

The slope compensation may reduce the programmed current limit. Hence, to take into

account this effect, as a rule of thumb, the current limit can be set as twice as much the

maximum inductor peak current (see Section 6.4.4):

IBOOST, PEAK > 3.83 A

Therefore, using equation (7) and choosing IBOOST, PEAK = 4 A, RBILIM will be:

Equation 40

145

MAX

OUT

−

⋅

700

nLED

max

OVP

OUT

⋅

300

PEAK

BOOST

BILIM

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