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MAX5090AATE Datasheet(PDF) 11 Page - Maxim Integrated Products |
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MAX5090AATE Datasheet(HTML) 11 Page - Maxim Integrated Products |
11 / 17 page 2A, 76V, High-Efficiency MAXPower Step-Down DC-DC Converters ______________________________________________________________________________________ 11 Thermal-overload protection is intended to protect the MAX5090 in the event of a fault condition. For normal circuit operation, do not exceed the absolute maximum junction temperature rating of TJ = +150°C. Setting the Output Voltage The MAX5090A/MAX5090B have preset output volt- ages of 3.3V and 5.0V, respectively. Connect FB to VOUT for the preset output voltage (Figure 1). The MAX5090C offers an adjustable output voltage. Set the output voltage with a resistive divider connected from the circuit’s output to ground (Figure 2). Connect the center node of the divider to FB. Choose R4 less than 15k Ω, then calculate R3 as follows: The MAX5090 features internal compensation for opti- mum closed-loop bandwidth and phase margin. Because of the internal compensation, the output must be sensed immediately after the primary LC. Inductor Selection The MAX5090 is a fixed-frequency converter with fixed internal frequency compensation. The internal fixed compensation assumes a 100µH inductor and 100µF output capacitor with 50m Ω ESR. It relies on the loca- tion of the double LC pole and the ESR zero frequency for proper closed-loop bandwidth and the phase mar- gin at the closed-loop unity-gain frequency. See Table 2 for proper component values. Usually, the choice of an inductor is guided by the voltage difference between VIN and VOUT, the required output current and the operating frequency of the circuit. However, use the recommended inductors in Table 2 to ensure stable operation with optimum bandwidth. Use an inductor with a maximum saturation current rat- ing greater than or equal to the maximum peak current limit (5A). Use inductors with low DC resistance for a higher efficiency converter. Selecting a Rectifier The MAX5090 requires an external Schottky rectifier as a freewheeling diode. Connect this rectifier close to the device using short leads and short PC board traces. The rectifier diode must fully conduct the inductor cur- rent when the power FET is off to have a full rectifier function. Choose a rectifier with a continuous current rating greater than the highest expected output current. Use a rectifier with a voltage rating greater than the maximum expected input voltage, VIN. Use a low for- ward-voltage Schottky rectifier for proper operation and high efficiency. Avoid higher than necessary reverse- voltage Schottky rectifiers that have higher forward-volt- age drops. Use a Schottky rectifier with forward-voltage drop (VF) less than 0.55V and 0.45V at +25°C and +125°C, respectively, and at maximum load current to avoid forward biasing of the internal parasitic body diode (LX to ground). See Figure 3 for forward-voltage drop vs. temperature of the internal body diode of the MAX5090. Internal parasitic body-diode conduction may cause improper operation, excessive junction tem- perature rise, and thermal shutdown. Use Table 1 to choose the proper rectifier at different input voltages and output current. Input Bypass Capacitor The discontinuous input current waveform of the buck converter causes large ripple currents in the input capacitor. The switching frequency, peak inductor cur- rent, and the allowable peak-to-peak voltage ripple reflecting back to the source dictate the capacitance requirement. The MAX5090 high switching frequency allows the use of smaller value input capacitors. The input ripple is comprised of ∆VQ (caused by the capacitor discharge) and ∆VESR (caused by the ESR of the capacitor). Use low-ESR aluminum electrolytic capacitors with high-ripple current capability at the input. Assuming that the contribution from the ESR and capaci- tor discharge is equal to 90% and 10%, respectively, cal- culate the input capacitance and the ESR required for a specified ripple using the following equations: R3 = − (. ) . V xR OUT 1 228 1 228 4 Table 1. Diode Selection VIN (V) DIODE PART NUMBER MANUFACTURER B340LB Diodes Inc. RB051L-40 Central Semiconductor 6.5 to 36 MBRS340T3 ON Semiconductor MBRM560 Diodes Inc. RB095B-60 Central Semiconductor 6.5 to 56 MBRD360T4 ON Semiconductor 50SQ80 IR 6.5 to 76 PDS5100H Diodes Inc. |
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