Hi All,
I am trying to learn how to construct a circuit to produce a dual rail supply with a DC-DC converter + a Charge Pump circuit, as shown in the attached pic. From the calculation equations for the component values for the charge pump circuit, which states that:
1. ΔV = Ichg1 x 1/Cchg x Ton = Iout2/Cchg x Tsw
In my learning example, I have set ΔV to be 87 mV and the switching frequency = 1.2MHz, giving a Tsw of 0.833 uS and Iout of 400mA
From this equation, the value of Cchg, i.e. the charging capacitor is calculated as 3.9 uF, so I chose the closest value which is 4.7uF.
2. Rchg x Cchg = Tsw
From what I read, it is suggested to choose R according to the above equation, which means the time constant of the RCHG and CCHG is equal to the switching cycle, and in this case Rchg is calculated to be 0.2R.
Now 0.2R seems to be very low. Schematics that I can find on the internet use values of 1R, and from what I read high RCHG will result in low COUT2 voltage. So my question is, should I adopt the commonly used value, 1R or stick to the calculated value and use 0.22R (which does exist)?
Best regards
Roger
I am trying to learn how to construct a circuit to produce a dual rail supply with a DC-DC converter + a Charge Pump circuit, as shown in the attached pic. From the calculation equations for the component values for the charge pump circuit, which states that:
1. ΔV = Ichg1 x 1/Cchg x Ton = Iout2/Cchg x Tsw
In my learning example, I have set ΔV to be 87 mV and the switching frequency = 1.2MHz, giving a Tsw of 0.833 uS and Iout of 400mA
From this equation, the value of Cchg, i.e. the charging capacitor is calculated as 3.9 uF, so I chose the closest value which is 4.7uF.
2. Rchg x Cchg = Tsw
From what I read, it is suggested to choose R according to the above equation, which means the time constant of the RCHG and CCHG is equal to the switching cycle, and in this case Rchg is calculated to be 0.2R.
Now 0.2R seems to be very low. Schematics that I can find on the internet use values of 1R, and from what I read high RCHG will result in low COUT2 voltage. So my question is, should I adopt the commonly used value, 1R or stick to the calculated value and use 0.22R (which does exist)?
Best regards
Roger
