02-08-2015, 08:15 PM
Hi Lawrence,
Yes. I left off that detail because it's all in the datasheet, but basically make the resistance to ground variable. Pots, fixed resistors and switch, fixed resistors and a pot, or a bit of all
The resistor between output and adjust should be around 6k according to the datasheet (they say 6.04k - go figure!). Just like the LM317, the regulator maintains a constant voltage across that resistor of 1.2V, and at the same time, it tries its best to not draw current via the adjust pin*. That means that whatever current exists across the first resistor (R1 on the datasheet), the same current exists in the resistance between "adjust" and ground (R2 on the datasheet).
With 6.04k as the resistance, the current is 0.2mA.
So to get 250V, we need 1.2M. That's just (250V-1.2V)/0.2mA
*Actually, I've double-checked the datasheet, and apparently "adjust" draws a constant 10uA, so that increases the output voltage for a given value of R2. In the case of 1.2M, it's 12V greater than the prediction above.
Like the LM317, there is a minimum current requirement - in this case it's 0.5mA. For such a wide output range, it's probably worth using a transistor current sink rather than a fixed resistance. Or, you might be able to reduce the values of R1 and R2 to achieve that.
Yes. I left off that detail because it's all in the datasheet, but basically make the resistance to ground variable. Pots, fixed resistors and switch, fixed resistors and a pot, or a bit of all
The resistor between output and adjust should be around 6k according to the datasheet (they say 6.04k - go figure!). Just like the LM317, the regulator maintains a constant voltage across that resistor of 1.2V, and at the same time, it tries its best to not draw current via the adjust pin*. That means that whatever current exists across the first resistor (R1 on the datasheet), the same current exists in the resistance between "adjust" and ground (R2 on the datasheet).
With 6.04k as the resistance, the current is 0.2mA.
So to get 250V, we need 1.2M. That's just (250V-1.2V)/0.2mA
*Actually, I've double-checked the datasheet, and apparently "adjust" draws a constant 10uA, so that increases the output voltage for a given value of R2. In the case of 1.2M, it's 12V greater than the prediction above.
Like the LM317, there is a minimum current requirement - in this case it's 0.5mA. For such a wide output range, it's probably worth using a transistor current sink rather than a fixed resistance. Or, you might be able to reduce the values of R1 and R2 to achieve that.
