Datasheet

LED7707 Application information
35/47
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 V
OUT, MAX
is the maximum output voltage considering the worst case (all LEDs with
the maximum V
F
= V
F, m a x
= 3.7 V on the same row):
Equation 39
R
1
can be chosen is in the order of hundreds of kilo-ohms to reduce the leakage current in
the resistor divider. For example, setting R
1
= 510 kΩ leads to R
2
= 21.89 kΩ. The closest
standard commercial value is R
2
= 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 kΩ
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):
I
BOOST, PEAK
> 3.83 A
Therefore, using equation (7) and choosing I
BOOST, PEAK
= 4 A, R
BILIM
will be:
Equation 40
V145.1V4V
V145.1
RR
MAX,OUT
12
−+
⋅=
V6.26mV700VnLEDV
max,FOVP,OUT
=
+
⋅
=
Ω== k300
I
K
R
PEAK,BOOST
B
BILIM