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MAX1927 Datasheet(PDF) 9 Page - Maxim Integrated Products |
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MAX1927 Datasheet(HTML) 9 Page - Maxim Integrated Products |
9 / 12 page Applications Information Output Voltage Selection The MAX1927/MAX1928 have preset output voltages. In addition, the MAX1927R has an adjustable output. To set the output voltage at the preset voltage, connect FB to the output. See Table 1 for a list of the preset volt- ages and their corresponding part numbers. The output voltage for the MAX1927R is adjustable from 0.75V to the input voltage by connecting FB to a resistor-divider between the output and GND (Figure 2). To determine the values of the resistor-divider, first select a value for feedback resistor R2 between 5k Ω to 50k Ω. R1 is then given by: where VFB is 0.75V. Input Capacitor Selection Capacitor equivalent series resistance (ESR) is a major contributor to input ripple in high-frequency DC-DC converters. Ordinary aluminum-electrolytic capacitors have high ESR and should be avoided. Low-ESR alu- minum electrolytic capacitors are acceptable and rela- tively inexpensive. Low-ESR tantalum capacitors or polymer capacitors are better and provide a compact solution for space-constrained surface-mount designs. Ceramic capacitors have the lowest ESR overall. The input filter capacitor reduces peak currents and noise at the input voltage source. Connect a low-ESR bulk capacitor ( ≥10µF typ) to the input. Select this bulk capacitor to meet the input ripple requirements and voltage rating rather than capacitance value. Use the following equation to calculate the maximum RMS input current: Compensation, Stability, and Output Capacitor The MAX1927/MAX1928 are externally compensated with a resistor and a capacitor (see Figure 3, RC and CC) in series from COMP to GND. An additional capaci- tor (Cf) may be required from COMP to GND if high- ESR output capacitors are used. The capacitor inte- grates the current from the transimpedance amplifier, averaging output capacitor ripple. This sets the device speed for transient response and allows the use of small ceramic output capacitors because the phase- shifted capacitor ripple does not disturb the current regulation loop. The resistor sets the proportional gain of the output error voltage by a factor gm ✕ RC. Increasing this resistor also increases the sensitivity of the control loop to output ripple. The resistor and capacitor set a compensation zero that defines the system’s transient response. The load creates a dynamic pole, shifting in frequency with changes in load. As the load decreases, the pole fre- quency decreases. System stability requires that the compensation zero must be placed to ensure adequate phase margin (at least 30° at unity gain). The following is a design procedure for the compensation network: 1) Select an appropriate converter bandwidth (fC) to stabilize the system while maximizing transient response. This bandwidth should not exceed 1/10 of the switching frequency. 2) Calculate the compensation capacitor, CC, based on this bandwidth: For the MAX1927: C V IR g R RR f C OUT OUT MAX CS m C = × ×× + × () 12 12 1 2 π I I V VV V RMS OUT IN OUT IN OUT =× × − () RR V V OUT FB 12 1 =× − Low-Output-Voltage, 800mA, PWM Step-Down DC-DC Converters _______________________________________________________________________________________ 9 LX FB R1 R2 50k Ω MAX1927R Figure 2. Setting the Adjustable Output Voltage PART PRESET OUTPUT VOLTAGE MAX1927R 0.75V, Adjustable MAX1928-15 1.5 V MAX1928-18 1.8 V MAX1928-25 2.5 V Table 1. FB Regulation Voltages |
Similar Part No. - MAX1927 |
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Similar Description - MAX1927 |
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