LED7706 Datasheet, PDF (21/31 Pages) STMicroelectronics – 6-rows 30 mA LEDs driver with boost regulator for LCD panels backlight

LED7706
Operation description - boost section
Figure 13. Effect of slope compensation on small inductor current perturbation (D > 0.5)
Inductor current (CCM)
0.35·T_SW
Programmed inductor peak current with
slope compensation (SE)
I_TRIP
Inductor current
∆I_L perturbation
T_SW
t
6.7
Boost current limit
The design of the external components, especially the inductor and the flywheel diode, must
be optimized in terms of size relying on the programmable peak current limit. The LED7706
improves the reliability of the final application giving the way to limit the maximum current
flowing into the critical components. A simple resistor connected between the BILIM pin and
ground sets the desired value. The voltage at the BILIM pin is internally fixed to 1.23 V and
the current limit is proportional to the current flowing through the setting resistor, according
to the following equation:
Equation 14
I_BOOST,PEAK
= ^KB
R_BILIM
where
K_B = 6 ⋅10⁵ V
The maximum allowed current limit is 5 A, resulting in a minimum setting resistor
R_BILIM > 120 kΩ. The maximum guaranteed RMS current in the power switch is 2 Arms. The
current limitation works by clamping the COMP pin voltage proportionally to R_BILIM. Peak
inductor current is limited to the above threshold decreased by the slope compensation
contribution.
In a boost converter the r.m.s. current through the internal MOSFET depends on both the
input and output voltages, according to equations 15 a (DCM) and 15 b (CCM).
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