Other Parts Discussed in Thread: TPS92200
Hi TI,
I have a few question regarding TPS54200 as below:
1) From the datasheet, analog dimming equation stated that the ILED is the maximum LED current, but may I know it is based on Isense or PWM output? This is because maximum LED current will change based on PWM duty cycle value.
2) For analog dimming mode:
If let say ILED is based on PWM output, I found that if PWM = 100% duty cycle, ILED maximum = 800mA (Isense = 800mA), IL (ripple) = 240mA with current ripple ratio = 30% (K=0.3) thus inductance = 22uH selected.
But what if the PWM duty change to 10%, the calculated min inductance value will be a lot lower.
3) In other word, inductor current ripple ratio is relative to the ILED max value, so CR ratio = 30% when 100% duty cycle, but CR ratio = ~1% when 1% duty cycle. Is there any lower limit and upper limit for CR ratio and duty cycle to assure loop stability?
4) In TPS92200, datasheet stated that inductor current ripple be larger than 300mA to assure loop stability. Any reason that this IC is based on >300mA instead of current ripple ratio? This is because if Isense = 600mA and inductor current ripple = 300mA, the inductor current ripple ratio is >50%, which is not following the 0.2 to 0.4 requirement.
5) Any reason TPS54200 do not have the above statement in datasheet?
6) TPS54200's COUT is connected to Rsense but TPS92200's COUT is connected to GND, may i know any reason cause to this different?
7) If i am using TPS54200 in my application, how should I place the extra filtering components (ex: mid and high frequency filter) after the COUT? It should be connected to Rsense as well? What if i connect to GND since i need to filter those high frequency noise.
8) When I simulate TPS54200 using TINA simulator, found that it is still able to operate and the noise level slightly reduced if COUT connected to GND. Why?
9) When I simulate TPS54200 using TINA simulator, found that even the inductor current ripple ratio >90%, it is still functioning, even the inductor fall on discontinuous conduction mode. Why?
10) Any document i can refer to more understand on duty cycle vs ILED vs inductor ripple current vs loop stability?
