4 Increasing Current Limit

The R_SL resistor offers flexibility in choosing the ramp of the slope compensation. Slope compensation affects the minimum inductance for stability (see the Slope Compensation section in the LM3477 High Efficiency High-Side N-Channel Controller for Switching Regulator Data Sheet), but also helps determine the current limit and hysteretic threshold. As an example, R_SL can be disconnected and replaced by a 0-Ω resistor so that no extra slope compensation is added to the current sense waveform to increase the current limit. A more conventional way to adjust the current limit is to change R_SN. R_SL is used here to change current limit for the sake of simplicity and to demonstrate the dependence of current limit to R_SL. By changing R_SL to 0 Ω, the following conditions can be met:

4.5 V ≤ V_IN ≤ 15 V

V_OUT = 3.3 V

0 A ≤ I_OUT ≤ 3 A

The current limit is a weak function of slope compensation and a strong function of the sense resistor. By decreasing R_SL, slope compensation is decreased, and as a result the current limit increases. The hysteretic mode threshold will also increase to about 1 A (see Figure 3-1).

Figure 4-1 shows a bode plot of LM3477 open loop frequency response using the modified (R_SL = 0 Ω) components to achieve higher output current capability.

Magnitude = 20 dB/Decade, Bandwidth = 55.3 kHz, Phase = 45°/Decade, Phase Margin = 42°

Figure 4-1 Open Loop Frequency Response V_IN = 5 V, V_OUT = 3.3 V, I_OUT = 3 A