BD6590MUV
Technical Note
●How to set over voltage limit
This section is especially mentioned here because the spec shown electrical characteristic is necessary to explain this section.
Over voltage limit
LED control voltage
min 0.96V typ 1.00V max 1.04V
min 0.56V typ 0.70V max 0.84V
LED terminal over voltage protect min 10.0V typ 11.5 V max 13.0V
1. Calculate the conditions that the total value of LED VF is MAX.
Example) In the case of serial 8 LEDs with VF=2.9V(min), 3.2V(typ), 3.5V(max) => 3.5V x 8=28V
2.Then calculate the biggest value of output with the following formula.
The biggest value of output = the biggest value calculated for 1 + the biggest value of LED terminal voltage. (0.84V)
Example) The biggest value of output = 28V + 0.84V =28.84V
3.Set the smallest value of over voltage larger than the biggest value of output.
If over voltage is closer to the total value of VF, it could be occurred to detect over voltage by ripple, noise, and so on.
It is recommended that some margins should be left on the difference between over voltage and the total value of VF.
This time around 6% margin is placed.
Example) Against the biggest value of output = 28.84V, the smallest value of over voltage = 28.84V x 1.06 = 30.57V
Ic over voltage limit min=0.96V,typ=1.00V, max=1.04V
typ = 30.57V×(1.00V/0.96V) = 31.8V
max = 31.8V×(1.04V/1.00V) = 33.1V
4. The below shows how to control resistor setting over voltage
Please fix resistor high between VDET and output and then set over voltage after changing resistor between VDET and
GND. While PWM is off, output voltage decreases by minimizing this resistor. Due to the decrease of output voltage, ripple
of output voltage increases, and singing of output condenser also becomes bigger.
Example) Selecting OVP resistor.
(Example 1) VF=3.6V max, serial = 7 LED
OVP = 1.0V, R1 = 2.2MΩ, R2 = 78.7kΩ
VOUT = 1.0 × (2.2MΩ + 78.7kΩ)/ 78.7kΩ = 28.95V
VOUT
(Example 2) VF=3.6V max, serial = 8 LED
OVP = 1.0V, R1 = 2.2MΩ, R2 = 69.8kΩ
VOUT = 1.0 × (2.2MΩ + 69.8kΩ)/ 69.8kΩ = 32.52
R1
(Example 3) VF=3.6V max, serial = 9 LED
OVP = 1.0V, R1 = 2.2MΩ, R2 = 62kΩ
VOUT = 1.0 × (2.2MΩ + 62kΩ)/ 62kΩ = 36.48V
VDET
R2
(Example 4) VF=3.6V max, serial = 10 LED
OVP = 1.0V, R1 = 1.0MΩ, R2 = 26.7kΩ
VOUT = 1.0 × (1.0MΩ + 26.7kΩ)/ 26.7kΩ = 38.45V
Fig. 7 Control resistor setting
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