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.

\begin{equation} C_{IMON}R_{IMON} \geq \frac{1.59}{f_{SW}} \end{equation}

Using your values,

\begin{equation} (10 \text{ nF})(10 \text{ k}\Omega) \geq \frac{1.59}{f_{SW}} \Rightarrow f_{SW} \geq 15.9 \text{ kHz} \end{equation}

The switching frequency is set by \$R_T\$ as described at page 11.

\begin{equation} f_{SW} = \frac{10^5}{R_T \text{ (k}\Omega)} \Rightarrow R_T < 6.289\text{ k}\Omega \end{equation}

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  • \$\begingroup\$ If i want to use 1MHz frequency how do the values change? \$\endgroup\$ – Lobi Dec 27 '17 at 15:13
  • \$\begingroup\$ 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 \$ \$\endgroup\$ – altai Dec 27 '17 at 15:17

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