Datasheet
Operation description LED7707
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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
Ta b l e 6 (see Section 5.3 on page 21). After the SS voltage reaches a
2.4 V threshold, the start-up finishes and all the protections turn active. The soft-start
capacitor C
SS
can be calculated according to equation 2.
Equation 2
Where I
SS
= 5 µA and t
SS
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 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
An additional filtering capacitor C
F
(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
5.1.5 Switching frequency selection and synchronization
The switching frequency of the boost converter can be set in the 250 kHz-1 MHz range by
connecting the FSW pin to ground through a resistor. Calculation of the setting resistor is
made using equation 4 and should not exceed the 100 kΩ-400 kΩ range.
4.2
tI
C
SSSS
SS
⋅
≅
V145.1V4V
V145.1
RR
MAX,OUT
12
−+
⋅=
AM00586v1AM00586v1
LX
OVSEL
SGND
LED7707
V
OUT
V
IN
R
1
R
2
C
F
C
OUT