# How can I calculate the Cimon in a DC-DC converter?

I would like to use LT3649 IC, but I don't know how can I calculate the following in the datasheet: Output current monitoring and regulation section in page 10. The datasheet: LT3649 If I use 10k resistor for Rimon and 10nF capacitor for Cimon how can I calculate the value?

The answer is in the datasheet, at page 10.

$$C_{IMON}R_{IMON} \geq \frac{1.59}{f_{SW}}$$

$$(10 \text{ nF})(10 \text{ k}\Omega) \geq \frac{1.59}{f_{SW}} \Rightarrow f_{SW} \geq 15.9 \text{ kHz}$$
The switching frequency is set by $R_T$ as described at page 11.
$$f_{SW} = \frac{10^5}{R_T \text{ (k}\Omega)} \Rightarrow R_T < 6.289\text{ k}\Omega$$
• Then, $C_{IMON}R_{IMON} \geq 0.00000159$. For example, choosing $C_{IMON} = 10 nF$, you need $R_{IMON} \geq 0.00000159/10n = 159 \Omega$, which is pretty low. I suggest you use $C_{IMON} = 1 nF, R_{IMON} = 1.8k\Omega$ – altai Dec 27 '17 at 15:17