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MAX1951ESA+ Datasheet(PDF) 9 Page - Maxim Integrated Products |
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MAX1951ESA+ Datasheet(HTML) 9 Page - Maxim Integrated Products |
9 / 15 page 1MHz, All-Ceramic, 2.6V to 5.5V Input, 2A PWM Step-Down DC-to-DC Regulators _______________________________________________________________________________________ 9 Design Procedure Output Voltage Selection: Adjustable (MAX1951) or Preset (MAX1952) The MAX1951 provides an adjustable output voltage between 0.8V and VIN. Connect FB to output for 0.8V output. To set the output voltage of the MAX1951 to a voltage greater than VFB (0.8V typ), connect the output to FB and GND using a resistive divider, as shown in Figure 2a. Choose R2 between 2k Ω and 20kΩ, and set R3 according to the following equation: R3 = R2 x [(VOUT/VFB) – 1] The MAX1951 PWM circuitry is capable of a stable min- imum duty cycle of 18%. This limits the minimum output voltage that can be generated to 0.18 ✕ VIN. Instability may result for VIN/VOUT ratios below 0.18. The MAX1952 provides a preset output voltage. Connect the output to FB, as shown in Figure 2b. Output Inductor Design Use a 2µH inductor with a minimum 2A-rated DC cur- rent for most applications. For best efficiency, use an inductor with a DC resistance of less than 20m Ω and a saturation current greater than 3A (min). See Table 2 for recommended inductors and manufacturers. For most designs, derive a reasonable inductor value (LINIT) from the following equation: LINIT = VOUT x (VIN - VOUT)/(VIN x LIR x IOUT(MAX) x fSW) where fSW is the switching frequency (1MHz typ) of the oscillator. Keep the inductor current ripple percentage LIR between 20% and 40% of the maximum load cur- rent for the best compromise of cost, size, and perfor- mance. Calculate the maximum inductor current as: IL(MAX) = (1 + LIR/2) x IOUT(MAX) Check the final values of the inductor with the output ripple voltage requirement. The output ripple voltage is given by: VRIPPLE = VOUT x (VIN - VOUT) x ESR / (VIN x LFINAL x fSW) where ESR is the equivalent series resistance of the output capacitors. Input Capacitor Design The input filter capacitor reduces peak currents drawn from the power source and reduces noise and voltage ripple on the input caused by the circuit’s switching. The input capacitor must meet the ripple current requirement (IRMS) imposed by the switching currents defined by the following equation: For duty ratios less than 0.5, the input capacitor RMS current is higher than the calculated current. Therefore, use a +20% margin when calculating the RMS current at lower duty cycles. Use ceramic capacitors for their low ESR, equivalent series inductance (ESL), and lower cost. Choose a capacitor that exhibits less than 10°C temperature rise at the maximum operating RMS cur- rent for optimum long-term reliability. After determining the input capacitor, check the input ripple voltage due to capacitor discharge when the high-side MOSFET turns on. Calculate the input ripple voltage as follows: VIN_RIPPLE = (IOUT x VOUT)/(fSW x VIN x CIN) Keep the input ripple voltage less than 3% of the input voltage. Output Capacitor Design The key selection parameters for the output capacitor are capacitance, ESR, ESL, and the voltage rating requirements. These affect the overall stability, output ripple voltage, and transient response of the DC-to-DC converter. The output ripple occurs due to variations in the charge stored in the output capacitor, the voltage drop due to the capacitor’s ESR, and the voltage drop due to the capacitor’s ESL. Calculate the output voltage ripple due to the output capacitance, ESR, and ESL as: VRIPPLE = VRIPPLE(C) + VRIPPLE(ESR) + VRIPPLE(ESL) where the output ripple due to output capacitance, ESR, and ESL is: VRIPPLE(C) = IP-P/(8 x COUT x fSW) VRIPPLE(ESR) = IP-P x ESR VRIPPLE(ESL) = (IP-P/tON) x ESL or (IP-P/tOFF) x ESL, whichever is greater and IP-P the peak-to-peak inductor current is: IP-P = [ (VIN - VOUT )/fSW x L) ] x VOUT/VIN Use these equations for initial capacitor selection, but determine final values by testing a prototype or evalua- tion circuit. As a rule, a smaller ripple current results in less output voltage ripple. Since the inductor ripple current is a factor of the inductor value, the output voltage ripple decreases with larger inductance. Use ceramic capacitors for their low ESR and ESL at the switching frequency of the converter. The low ESL of ceramic capacitors makes ripple voltages negligible. Load transient response depends on the selected output capacitor. During a load transient, the output instantly changes by ESR x ILOAD. Before the controller can respond, the output deviates further, depending on the inductor and output capacitor values. After a short time (see the Load Transient Response graph in the IV I V V V RMS IN OUT OUT IN OUT =× − ×× (/ ) ( ( )) 1 2 |
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