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LED7707 Datasheet, PDF (15/48 Pages) STMicroelectronics – 6-rows 85 mA LEDs driver with boost regulator for LCD panels backlight
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LED7707
Operation description
5.1.4
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
CSS
≅
ISS ⋅ tSS
2.4
Where ISS = 5 µA and tSS is the desired soft-start duration.
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 kΩ 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
R2
=
R1 ⋅
1.145V
VOUT,MAX + 4V − 1.145V
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
VIN
VOUT
LX
OVSEL
LED7707
SGND
C13
COUT
R1
R2
C10
AM00586v1
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