The LM3645 features four selectable inductor current limits that are programmable through the I²C-compatible interface. When the inductor current limit is reached, the LM3645 terminates the charging phase of the switching cycle. Switching resumes at the start of the next switching period. If the overcurrent condition persists, the device operates continuously in current limit.

Since the current limit is sensed in the NMOS switch, there is no mechanism to limit the current when the device operates in Pass Mode (current does not flow through the NMOS in pass mode). In Boost mode or Pass mode if V_OUT falls below 2.3 V, the device stops switching, and the P-channel MOSFET operates as a current source limiting the current to 200 mA. This prevents damage to the LM3645 and excessive current draw from the battery during output short-circuit conditions. The output enable bits are not cleared upon a Current Limit event, but a flag is set (bit [4] in reg 0x14).

The over current protection (OCP) circuit compares the inductor current to the input current limit (I_CL) setting after a fixed blanking time of t_DLY. The blanking time ensures that the OCP signal does not false fire as the low-side NMOS of the boost converter turns on. Use Equation 1 to estimate the maximum recommended total LED current to ensure proper current limit level operation accounting for the delay time.

Equation 1.

where

• D is the duty cycle
• η is the boost efficiency
• I_CL is the current limit setting
• L is the inductance value
• f_SW is the boost switching frequency

(See Equation 2)

(See Equation 1)

Figure 6-2 Current Limit Timing

Equation 2.