Keywords: linear regular, low noise, noise reduction, noise filter
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APPLICATION NOTE 3672

Abstract: The MAX1864 is a low-cost, multiple-output converter consisting of a switching buck circuit and two positive linear regulators. Although this IC's basic application circuit offers a low-cost, multiple-output supply, applications where lower noise is desired can benefit from this application note. This application note details the design and implementation of noise reduction for the MAX1864 auxiliary linear regulator.

Figure 1 shows a capacitor C3 across Q1 base, this reduces noise that is amplified by the error amp. The rolloff frequency is quite high because of the low base impedance. In this case, with 0.150A emitter current the emitter resistance is:

With Re at 0.167Ω with a minimum Beta of 15, the worst-case impedance reflected at the base is 2.5Ω (Beta * Re). With C3 at 2.2nF, this filter begins to roll off at 29MHz. This is not an effective place for a filter to be applied because, at this point, the impedance is too low. The circuit in

Clearly, a 6kHz rolloff provides better filtering of 200kHz switching noise than the circuit in Figure 1. R7 must be selected so that it does not limit Q1 base drive current. This circuit, with R7 at 1.2k, can deliver a maximum of 12.6mA of base drive current to Q1. Power dissipation can also be an issue in R7. In this case, the maximum power in R7 is 200mW (1.2k * 12.6mA²). This assumes that there can be a maximum of 15.1V (15.8V - 0.7V) across R7. With a minimum Beta for Q1 of 15, the circuit can source a minimum of 190mA output current. In applications where lower output voltages are needed and the input voltage is lower, R7 will have to be reduced to allow the minimum base drive required for the specified output current. This might require a larger C12 capacitor to achieve the same filtering as in Figure 2.

To determine the frequency where C12 and R7 should roll off, the worst-case equivalent series resistance (ESR) for the output capacitor, C1, needs to be known. Then set the rolloff for R7/C12 to cancel the zero created by the ESR of the output capacitor. For example, if the lowest ESR of C1 is 0.6Ω, then this introduces a zero at:

Then, by setting Frol equal to Fzer, and if R7 has been selected at 1.2k, C12 can be found to be 22nF. In other words:

It is important to note that the circuit in Figure 1 also has a pole to cancel the ESR zero in the output cap. This pole is formed by the parallel combination of R2, R3, and capacitor C2. Unfortunately, this does little to filter the switching noise generated inside the chip. C12 in Figure 2 provides a dual-purpose role to both cancel the ESR zero, and filter internally generated noise at the chip output.

The MAX1864 is a versatile integrated circuit that can provide cost-effective, multiple-output converters. A practical application circuit using the MAX1864 is shown in

V_{in} |
I_{in} |
V_{out} |
I_{out} |
V2 | I2 | Efficiency (%) |

12.012 | 0.047 | 3.3220 | 0 | 5.0846 | 0 | — |

9.989 | 2.670 | 3.3262 | 7.007 | 5.0722 | 0.1505 | 0.902 |

12.008 | 2.259 | 3.3272 | 7.007 | 5.0743 | 0.1505 | 0.887 |

14.004 | 1.969 | 3.3276 | 7.007 | 5.0777 | 0.1505 | 0.873 |

Designation | Quantity | Description |

C1 | 1 | 3.3nF ceramic capacitor (0805) |

C2 | 1 | 33pF ceramic capacitor (0805) |

C3 | 1 | 2.2nF ceramic capacitor (0805) |

C4 | 1 | 1µF 10V X7R ceramic capacitor (0805) Taiyo Yuden LMK212BJ105MG |

C5 | 1 | 0.1µF ceramic capacitor (0805) |

C6 | 2 | 47µF 25V organic semiconductor cap Sanyo 25SC47M |

C7 | 1 | 4.7µF 25V X5R ceramic capacitor (1210) Taiyo Yuden TMK325BJ475MN |

C8 | 2 | 820µF 6V aluminum electrolytic cap Sanyo 6MV820EXR |

C9 | 1 | 10µF 10V X5R ceramic capacitor (1210) Taiyo Yuden LMK325BJ106MN |

C10 | 1 | 47nF ceramic capacitor (0805) |

C11 | 1 | 47µF 10V aluminum electrolytic cap Sanyo 10MV47HC |

D1 | 1 | 100mA 30V Schottky diode (SOT-23) Central Semi CMPSH-3 |

D2 | 1 | 3A 30V Schottky diode Nihon EC31QS03L |

L1 | 1 | 3.3µH 15A Power Inductor Coilcraft DO5022P-332HC |

N1, N2 | 1 | 12mΩ N-channel MOSFET (SO-8) International Rectifier IRF7811 |

Q2 | 1 | 1A 40V PNP transistor (D-PAK) Fairchild KSH30 |

R1 | 1 | 120kΩ 5% resistor (0805) |

R2 | 1 | 100kΩ 5% resistor (0805) |

R3, R4 | 2 | 3.3Ω 5% resistor (0805) |

R5 | 1 | 220Ω 0.25W 5% resistor (1210) |

R6 | 1 | 220Ω 5% resistor (0805) |

R7 | 1 | 30.9kΩ 1% resistor (0805) |

R8 | 1 | 10.0kΩ 1% resistor (0805) |

U1 |
1 |
MAX1864TEEE (16-QSOP) |