LED7707 datasheet(15/48 Pages) STMICROELECTRONICS

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LED7707

Operation description

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During the soft-start phase the floating rows detection is also performed. In presence of one

or more floating rows, the voltage across the involved current generator drops to zero. This

voltage becomes the inverting input of the error amplifier through the minimum voltage drop

selector (see Figure 5). As a consequence the error amplifier is unbalanced and the loop

reacts by increasing the output voltage. When it reaches the floating row detection (FRD)

threshold (which coincides with the OVP threshold, see Section 5.1.4), the floating rows are

managed according to Table 7 (see Section 5.3 on page 22). After the SS voltage reaches a

2.4 V threshold, the start-up finishes and all the protections turn active. The soft-start

capacitor CSS can be calculated according to equation 2.

Equation 2

Where ISS = 5 µA and tSS is the desired soft-start duration.

5.1.4

Over-voltage protection

An adjustable over-voltage protection is available. It can be set feeding the OVSEL pin with a

partition of the output voltage. The voltage of the central tap of the divider is thus compared

to a fixed 1.145 V threshold. When the voltage of the OVSEL pin exceeds the OV threshold,

the switching activity is suspended. It is resumed as OVSEL returns below the OV threshold.

A 10 mV hysteresis is provided. No device turn-off is performed. Normally, the value of the

high-side resistors of the divider is in the order of 100 k

Ω to reduce the output capacitor

discharge when the boost converter is off (during the off phase of the dimming cycle),

whereas the low-side resistor can be calculated as:

Equation 3

An additional filtering capacitor CF (typically in the 100 pF-330 pF range) may be required to

improve noise rejection at the OVSEL pin (see Figure 9).

Figure 9.

OVP threshold setting

⋅

≅

145

MAX

OUT

−

⋅

OVSEL

SGND

LED7707

VOUT

VIN

C10

COUT

C13

AM00586v1

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