Datasheet AL8861Q (Diodes) - 10

AL8861Q Application Information
(continued) 1600 R =0.067 S 1400 R =0.1 S R =0.15 1200 S R =0.3 S (mA) 1000
T
t n
C
rre u 800
U
C t
D
u p 600 t
O
u O
R
400
P W
200
E N
0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 VSET Pin Voltage (V)
Figure 1. Analog Dimming Curve Soft-Start
The default soft-start time for AL8861Q is 0.1ms, this provides very fast turn-on of the output, improving the PWM dimming accuracy. Nevertheless, adding an external capacitor from the VSET pin to ground will provide a longer soft-start delay. This is achieved by increasing the time of the VSET voltage rising to the turn-on threshold, and by slowing down the rising rate of the control voltage at the input of hysteresis comparator. The additional soft-start time is related to the capacitance between the VSET pin and GND pin, and the typical value will be 1.5ms/nF.
Capacitor Selection
A low ESR capacitor should be used for input decoupling, as the ESR of this capacitor appears in series with the supply source impedance and will reduce the overall efficiency. This capacitor can supply the relatively high peak current to the coil and smooth the ripple on the input current. The minimum capacitance needed is determined by input power, cable’s length and peak current. 4.7μF to 10μF is a commonly used value for most cases. A higher value will improve the performance at lower input voltages, especially when the source impedance is high. The input capacitor should be placed as close as possible to the IC. For maximum stability of over temperature and voltage, capacitors with X7R, X5R or better dielectric are recommended. Capacitors with Y5V dielectric are not suitable for decoupling in this application and should NOT be used.
Diode Selection
For maximum efficiency and performance, the freewheeling diode (D1) should be a fast low capacitance Schottky diode with low reverse leakage current. It also provides better efficiency than the silicon diodes, due to the lower forward voltage and reduced recovery time. It is important to select parts with a peak current rating above the peak coil current, and a continuous current rating higher than the maximum output load current. It is very important to control the reverse leakage current of the diode when operating above +85°C. Excess leakage current will increase power dissipation. The higher forward voltage and overshoot due to the reverse recovery time in silicon diodes will increase the peak output voltage on the LX pins. If a silicon diode is used, more care should be taken to ensure that the total voltage appearing on the LX pins including supply ripple won’t exceed the specified maximum value. AL8861Q 10 of 15 May 2019 Document number: DS41798 Rev. 2 - 2
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