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MAX1522-MAX1524 Datasheet(PDF) 10 Page - Maxim Integrated Products |
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MAX1522-MAX1524 Datasheet(HTML) 10 Page - Maxim Integrated Products |
10 / 14 page Simple SOT23 Boost Controllers 10 ______________________________________________________________________________________ Optional Feed-Forward Capacitor Selection For proper control of peak inductor current during soft- start and for stable switching, the ripple at FB should be greater than 25mV. Without a feed-forward capaci- tor connected between the output and FB, the output’s ripple must be at least 2% of VOUT in order to meet this requirement. Alternatively, if a low-ESR output capacitor is chosen to obtain small output ripple, then a feed-for- ward capacitor should be used, and the output ripple may be as low as 25mV. The approximate value of the feed-forward capacitor is given by: Do not use a feed-forward capacitor that is much larger than this because line-transient performance will degrade. Do not use a feed-forward capacitor at all if the output ripple is large enough without it to provide stable switching because load regulation will degrade. Optional Feedback Capacitor Selection When using a feed-forward capacitor, it is possible to achieve too much ripple at FB. The symptoms of this include excessive line and load regulation and possibly high output ripple at light loads in the form of pulse groupings or “bursts.” Fortunately, this is easy to cor- rect by either choosing a lower-ESR output capacitor or by adding a feedback capacitor between FB and ground. This feedback capacitor (CFB), along with the feed-forward capacitor, form an AC-coupled ripple volt- age-divider from the output to FB: It is relatively simple to determine a good value for CFB experimentally. Start with CFB = CFF to cut the FB ripple in half; then increase or decrease CFB as needed. The ideal ripple at FB is from 25mV to 40mV, which will pro- vide stable switching, low output ripple at light and medium loads, and reasonable line and load regula- tion. Never use a feedback capacitor without also using a feed-forward capacitor. Input Capacitor Selection The input capacitor (CIN) in boost designs reduces the current peaks drawn from the input supply, increases efficiency, and reduces noise injection. The source impedance of the input supply largely determines the value of CIN. High source impedance requires high input capacitance, particularly as the input voltage falls. Since step-up DC-DC converters act as “constant- power” loads to their input supply, input current rises as input voltage falls. Consequently, in low-input-volt- age designs, increasing CIN and/or lowering its ESR can add as many as five percentage points to conver- sion efficiency. A good starting point is to use the same capacitance value for CIN as for COUT. The input capacitor must also meet the ripple current requirement imposed by the switching currents, which is about 30% of IPEAK in CCM designs and 100% of IPEAK in DCM designs. In addition to the bulk input capacitor, a ceramic 0.1µF bypass capacitor at VCC is recommended. This capaci- tor should be located as close to VCC and GND as pos- sible. In bootstrapped configuration, it is recommended to isolate the bypass capacitor from the output capaci- tor with a series 10 Ω resistor between the output and VCC. Power MOSFET Selection The MAX1522/MAX1523/MAX1524 drive a wide variety of N-channel power MOSFETs (NFETs). Since the out- put gate drive is limited to VCC, a logic-level NFET is required. Best performance, especially when VCC is less than 4.5V, is achieved with low-threshold NFETs that specify on-resistance with a gate-source voltage (VGS) of 2.7V or less. When selecting an NFET, key parameters include: 1) Total gate charge (Qg) 2) Reverse transfer capacitance or charge (CRSS) 3) On-resistance (RDS(ON)) 4) Maximum drain-to-source voltage (VDS(MAX)) 5) Minimum threshold voltage (VTH(MIN)) At high switching rates, dynamic characteristics (para- meters 1 and 2 above) that predict switching losses may have more impact on efficiency than RDS(ON), which predicts I 2R losses. Qg includes all capacitances associated with charging the gate. In addition, this parameter helps predict the current needed to drive the gate when switching at high frequency. The continuous VCC current due to gate drive is: Use the FET manufacturer’s typical value for Qg (see manufacturer’s graph of Qg vs. Vgs) in the above equation since a maximum value (if supplied) is usually too conservative to be of any use in estimating IGATE. IQg GATE SWITCHING =× ƒ Ripple Ripple C CC FB OUTPUT FF FB FF = + × C RR FF ≅× + − 310 1 1 1 2 6 |
Similar Part No. - MAX1522-MAX1524 |
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Similar Description - MAX1522-MAX1524 |
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