The LM3477 is a current mode, high-side N channel FET controller. It is most commonly used in buck configurations, as shown in Figure 1-1. All the power conducting components of the circuit are external to the LM3477, so a large variety of inputs, outputs, and loads can be accommodated by the LM3477.

The LM3477 evaluation board comes ready to operate at the following conditions:

• 4.5 V ≤ V_IN ≤ 15 V
• V_OUT = 3.3 V
• 0 A ≤ I_OUT ≤ 1.6 A

The circuit and BOM for this application are given in Figure 1-1 and Table 1-1.

Figure 1-1 LM3477 Buck Converter

Figure 2-1 to Figure 2-2 show some benchmark data taken from the circuit above on the LM3477 evaluation board. This evaluation board can also be used to evaluate a buck regulator circuit optimized for a different operating point or to evaluate a trade-off between cost and some performance parameter. For example, the conversion efficiency can be increased by using a lower R_DS(ON) MOSFET, ripple voltage can be lowered with lower ESR output capacitors, and the hysteretic threshold can be changed as a function of the R_SN and R_SL resistors.

The conversion efficiency can be increased by using a lower R_DS(ON) MOSFET, however, it drops as input voltage increases. The efficiency reduces because of increased diode conduction time and increased switching losses. Switching losses are due to the Vds × Id transition losses and to the gate charge losses, both of which can be lowered by using a FET with low gate capacitance. At low duty cycles, where most of the power loss in the FET is from the switching losses, trading off higher R_DS(ON) for lower gate capacitance will increase efficiency.

Figure 2-1 Efficiency vs Load V_OUT = 3.3 V

Figure 2-2 Efficiency vs V_IN V_OUT = 3.3 V, I_OUT = 2 A

Figure 3-1 shows a bode plot of LM3477 open loop frequency response using the external components listed in Table 1-1.

Magnitude = 20 dB/Decade, Bandwidth = 39.8 kHz, Phase = 45°/Decade, Phase Margin = 41°

Figure 2-3 Open Loop Frequency Response V_IN = 5 V, V_OUT = 3.3 V, I_OUT = 1.5 A

As the load current is decreased, the LM3477 will eventually enter a 'hysteretic' mode of operation. When the load current drops below the hysteretic mode threshold, the output voltage rises slightly. The overvoltage protection (OVP) comparator senses this rise and causes the power MOSFET to shut off. As the load pulls current out of the output capacitor, the output voltage drops until it hits the low threshold of the OVP comparator and the part begins switching again. This behavior results in a lower frequency, higher peak-to-peak output voltage ripple than with the normal pulse width modulation scheme. The magnitude of the output voltage ripple is determined by the OVP threshold levels, which are referred to the feedback voltage and are typically 1.25 V to 1.31 V. For more information, see the Electrical Characteristics table in the LM3477 High Efficiency High-Side N-Channel Controller for Switching Regulator Data Sheet. In the case of a 3.3-V output, this translates to a regulated output voltage between 3.27 V and 3.43 V. The hysteretic mode threshold point is a function of R_SN and R_SL. Figure 3-1 shows the hysteretic threshold versus V_IN for the LM3477 evaluation board with and without R_SL.

Figure 3-1 I_TH vs V_IN

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