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# LM3429-Q1: output capacitor capacitance in boost circuit

Part Number: LM3429-Q1
Other Parts Discussed in Thread: LM3429, TPS92692

Hello,

I am having trouble designing LED Boost driver using LM3429.

Since it is being used for automotive applications, the input voltage is 9 - 16V and the output current set to be 0.65A.

However when calculating the capacity of the output capacitor using the formula specified in the datasheet which is Co = Iled x D / (rd x delta(Iled-pp), the capacitance is over 1 Farad.

Values are used 0.65A for Iled, 0.75 for D, rd = 7.2 ( leds - 1.2ohm each, total 6 leds) and the delta(Iled-pp) - 50mA - 0.05A and the result is 1.35 Farad.
Using such capacitor or capacitor bank is illogical.

• Hi Rokas,

I assume you are looking at equation 42 on page 28.  This equation has a typo error in it.  It should have fsw on the denominator also.  So you would need to divide the 1.3 by the switching frequency (fsw).

If you are designing for automotive application you might want to look at the TPS92692 which is a newer part and has spread spectrum modulation which will help you with EMI along with more comprehensive fault protection and more features.

Thanks Tuan

• Hello Tuan,

thank you for responding so quickly. I have figured it out, since in the examples the correct formula was used.

However, due to super limited space I am opting for minimal pin count on the chip, and so TPS92692 is larger is size than a already a bit too large LM3429. It is a bit of topic I believe, but maybe you have a better solution for non dimmable LED array driver which can produce atleast 40V at 1A and has a smaller footprint than aforementioned chips.

Best regards,
Rokas

• Hi Rokas,

I understand...Thanks Tuan