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MAX8733EEI Fiches technique(PDF) 26 Page - Maxim Integrated Products |
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MAX8733EEI Fiches technique(HTML) 26 Page - Maxim Integrated Products |
26 / 32 page High-Efficiency, Quad-Output, Main Power- Supply Controllers for Notebook Computers 26 ______________________________________________________________________________________ When using low-capacity filter capacitors such as polymer types, capacitor size is usually determined by the capacity required to prevent VSAG and VSOAR from tripping the undervoltage and overvoltage fault latches during load transients in ultrasonic mode. For low input-to-output voltage differentials (VIN / VOUT < 2), additional output capacitance is required to main- tain stability and good efficiency in ultrasonic mode. The amount of overshoot due to stored inductor energy can be calculated as: where IPEAK is the peak inductor current. Stability Considerations Stability is determined by the value of the ESR zero (fESR) relative to the switching frequency (f). The point of instability is given by the following equation: where: For a typical 300kHz application, the ESR zero frequen- cy must be well below 95kHz, preferably below 50kHz. Low-ESR capacitors (especially polymer or tantalum), in widespread use at the time of publication, typically have ESR zero frequencies lower than 30kHz. In the design example used for inductor selection, the ESR needed to support a specified ripple voltage is found by the equation: where LIR is the inductor ripple current ratio and ILOAD is the average DC load. Using LIR = 0.35 and an aver- age load current of 5A, the ESR needed to support 50mVP-P ripple is 28mΩ. Do not place high-value ceramic capacitors directly across the fast-feedback inputs (OUT_ to GND for inter- nal feedback, FB_ divider point for external feedback) without taking precautions to ensure stability. Large ceramic capacitors can have a high-ESR zero frequency and cause erratic, unstable operation. Adding a discrete resistor or placing the capacitors a couple of inches downstream from the junction of the inductor and OUT_ may improve stability. Unstable operation manifests itself in two related but distinctly different ways: double pulsing and fast-feed- back loop instability. Noise on the output or insufficient ESR may cause double pulsing. Insufficient ESR does not allow the amplitude of the voltage ramp in the output signal to be large enough. The error comparator mistak- enly triggers a new cycle immediately after the 350ns minimum off-time period has expired. Double pulsing results in increased output ripple, and can indicate the presence of loop instability caused by insufficient ESR. Loop instability results in oscillations or ringing at the output after line or load perturbations, causing the out- put voltage to fall below the tolerance limit. The easiest method for checking stability is to apply a very fast zero-to-max load transient (refer to the MAX8734 EV kit data sheet) and observe the output voltage-ripple envelope for overshoot and ringing. Monitoring the inductor current with an AC current probe may also provide some insight. Do not allow more than one cycle of ringing of under- or overshoot after the initial step response. Input Capacitor Selection The input capacitors must meet the input-ripple-current (IRMS) requirement imposed by the switching current. The MAX8732/MAX8733/MAX8734 dual switching regu- lators operate at different frequencies. This interleaves the current pulses drawn by the two switches and reduces the overlap time where they add together. The input RMS current is much smaller in comparison than with both SMPSs operating in phase. The input RMS cur- rent varies with load and the input voltage. The maximum input capacitor RMS current for a single SMPS is given by: When V+ = 2 x VOUT_(D = 50%), IRMS has maximum current of ILOAD / 2. The ESR of the input-capacitor is important for deter- mining capacitor power dissipation. All the power (IRMS2 x ESR) heats up the capacitor and reduces effi- ciency. Nontantalum chemistries (ceramic or OS-CON) are preferred due to their low ESR and resilience to power-up surge currents. Choose input capacitors that exhibit less than +10°C temperature rise at the RMS input current for optimal circuit longevity. Place the drains of the high-side switches close to each other to share common input bypass capacitors. II VV V V RMS LOAD OUT OUT ≈ + − () + __ ESR V LIR I RIPPLE P P LOAD () = × − f RC ESR ESR OUT = 1 2π f f ESR ≤ π V IL CV SOAR PEAK OUT OUT = 2 2 _ |
Numéro de pièce similaire - MAX8733EEI |
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Description similaire - MAX8733EEI |
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