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LM2586 Datasheet(PDF) 23 Page - National Semiconductor (TI) |
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LM2586 Datasheet(HTML) 23 Page - National Semiconductor (TI) |
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23 / 28 page  Application Hints (Continued) stable or constant voltage supply to the LM2586, a storage capacitor ( ≥100 µF) is required. If the input source is a recti- fied DC supply and/or the application has a wide tempera- ture range, the required rms current rating of the capacitor might be very large. This means a larger value of capaci- tance or a higher voltage rating will be needed for the input capacitor. The storage capacitor will also attenuate noise which may interfere with other circuits connected to the same input supply voltage. In addition, a small bypass capacitor is required due to the noise generated by the input current pulses. To eliminate the noise, insert a 1.0 µF ceramic capacitor between V IN and ground as close as possible to the device. SWITCH VOLTAGE LIMITS In a flyback regulator, the maximum steady-state voltage ap- pearing at the switch, when it is off, is set by the transformer turns ratio, N, the output voltage, V OUT, and the maximum in- put voltage, V IN (Max): V SW(OFF) = VIN (Max) + (VOUT +VF)/N where V F is the forward biased voltage of the output diode, and is typically 0.5V for Schottky diodes and 0.8V for ultra-fast recovery diodes. In certain circuits, there exists a voltage spike, V LL, superimposed on top of the steady-state voltage (see Figure 5, waveform A). Usually, this voltage spike is caused by the transformer leakage inductance and/or the output rectifier recovery time. To “clamp” the volt- age at the switch from exceeding its maximum value, a tran- sient suppressor in series with a diode is inserted across the transformer primary (as shown in the circuit in Figure 4 and other flyback regulator circuits throughout the datasheet). The schematic in Figure 43 shows another method of clamp- ing the switch voltage. A single voltage transient suppressor (the SA51A) is inserted at the switch pin. This method clamps the total voltage across the switch, not just the volt- age across the primary. If poor circuit layout techniques are used (see the “Circuit Layout Guideline” section), negative voltage transients may appear on the Switch pin (pin 5). Applying a negative voltage (with respect to the IC’s ground) to any monolithic IC pin causes erratic and unpredictable operation of that IC. This holds true for the LM2586 IC as well. When used in a flyback regulator, the voltage at the Switch pin (pin 5) can go nega- tive when the switch turns on. The “ringing” voltage at the switch pin is caused by the output diode capacitance and the transformer leakage inductance forming a resonant circuit at the secondary(ies). The resonant circuit generates the “ring- ing” voltage, which gets reflected back through the trans- former to the switch pin. There are two common methods to avoid this problem. One is to add an RC snubber around the output rectifier(s), as in Figure 43. The values of the resistor and the capacitor must be chosen so that the voltage at the Switch pin does not drop below −0.4V. The resistor may range in value between 10 Ω and1kΩ, and the capacitor will vary from 0.001 µF to 0.1 µF. Adding a snubber will (slightly) reduce the efficiency of the overall circuit. The other method to reduce or eliminate the “ringing” is to in- sert a Schottky diode clamp between pins 5 and 4 (ground), also shown in Figure 43. This prevents the voltage at pin 5 from dropping below −0.4V. The reverse voltage rating of the diode must be greater than the switch off voltage. DS012516-62 FIGURE 43. Flyback Regulator www.national.com 23 |
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