R-2R ladder and pwm

[cosminnci]

Hi,

I seen a bench power supply and for driving the output it uses 5 bit pwm and 7 bit r-2r ladder operated by 7 digital pins of microcontroller, the explication for using this is faster control of the output.
I understand that the pwm outputs pulses which filtered can be converted to dc,
and that the r-2r ladder offers a output voltage in steps and in powers of 2
but how can the pwm and r-2r be interconnected to create a 12 bit dac ?

regards,
Cosmin

[daveatol]

The R-2R ladder would likely be the MS bits and the PWM fed through the LSb. You obviously understand how the R-2R works. If we have the LSb high, then it contributes 1/64 Vref to the output. If the LSb is low, then it doesn't. If we have the LSb high for 50% of the time and low the remainder, then it contributes 1/64*0.50, which is the same as having an additional bit below the LSb high. If the LSb is high for 25% of the time, then it contributes 1/64*0.25, which is the same  another extra bit - i.e. two additional bits below the LSb, with the lowest one high.

Following this for 75% high time, the two additional bits would be '11', or 62.5% high time gives '101' (all below the LSb), etc.

[cosminnci]

the part I don`t understand how do we feed the pwm as LSB.
would it be connected in series with the r2r ladder ?

[daveatol]

The PWM is fed into the resistor corresponding to the least significant bit of the ladder.

[daveatol]

This schematic shows where the 6 most significant bits go, and where the PWM is goes. (there will also be a capacitor connected to Vout to smooth the signal)

(ref: image derived from image on google images)

[cosminnci]

Thank you, this is clear now.