LTC3454 PWM

[ripper121]

Hello ,
i want to use the LTC3454 for my Flashlight as a 1A Led Driver.
For this i want to controle the LTC3454 via a PWM signal from a attiny85

http://cds.linear.com/docs/en/datasheet/3454fa.pdf

Page 11 shows how to connect this PWM signal. But i dont know which Resistor and Which Capacitor Values i should use for this.

Also there are 2 ISET's. Is the first for the 0-500mA range and the second for the 500-1000mA?
If yes, is the PWM signal for 500mA ISET1 PWMsignal 0-100% and for 1000mA ISET1 PWMsignal 100% + ISET2 PWMsignal 0-100%?

[NottheDan]

Also there are 2 ISET's. Is the first for the 0-500mA range and the second for the 500-1000mA?

Err, no. Look at page 8 on the data sheet the table at the bottom right.

EN1	EN2	ILOAD (A)
GND	GND	0 (SHUTDOWN)
VIN	GND	3850 • 0.8V/RISET1
GND	VIN	3850 • 0.8V/RISET1
VIN	VIN	3850 • (0.8V/RISET1 + 0.8V/RISET2)

[ripper121]

Yeah but this is no PWM controle!
And i dont think i can put a PWM signal to EN1 and EN2, because then they hadn't wrote a extra info about PWM.

[kxenos]

The resistor which is shown as Riset/100 and the capacitor form a low pass filter that converts the PWM signal into an analog voltage. Say you have duty cycle 20% and a 5V PWM signal then you will have about 0.2*5=1V on average. For this to work ok the f-3dB of this filter has to be at least 2 orders of magnitude lower than the freq of the PWM signal. For PWM freq of 10kHz that would be 1/(2piRC)=100.

[danadak]

PWM Filter design, attached.



Regards, Dana.

[ripper121]

So i found a tool to calc this RC Filter.

http://sim.okawa-denshi.jp/en/PWMtool.php