Datenblatt-Suchmaschine für elektronische Bauteile |
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CS51033YD8 Datenblatt(PDF) 6 Page - Cherry Semiconductor Corporation |
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CS51033YD8 Datenblatt(HTML) 6 Page - Cherry Semiconductor Corporation |
6 / 8 page 6 Applications Information: continued D = From this, the maximum duty cycle DMAX is 53%, this occurs when VIN is at its minimum while the minimum duty cycle DMIN is 0.35%. 2) Switching Frequency and on and off time calculations. FSW= 200KHz. The switching frequency is determined by COSC, whose value is determined by : COSC = 95 ≅ 470pF Fsw × (1-()-()2 ) T = = 5µs TON(MAX) = 5µs × 0.53 = 2.65µs TON(MIN) = 5µs × 0.35 = 1.75µs TOFF(MAX) = 5µs − 0.7µs = 4.3µs 3) Inductor selection Pick the inductor value to maintain continuous mode opera- tion down to 0.3 Amps. The ripple current ∆I = 2 × IOUT(MIN) = 2 × 0.3A = 0.6A. LMIN == ≅ 15µH The CS51033 will operate with almost any value of inductor. With larger inductors the ripple current is reduced and the regulator will remain in a continuous conduction mode for lower values of load current. A smaller inductor will result in larger ripple current. The core must not saturate with the maximum expected current, here given by: IMAX = = 3A + 0.6A/2 = 3.3A 4) Output Capacitor The output capacitor limits the output ripple voltage. The CS51033 needs a maximum of 15mV of output ripple for the feedback comparator to change state. If we assume that all the inductor ripple current flows through the output capaci- tor and that it is an ideal capacitor (i.e. zero ESR), the mini- mum capacitance needed to limit the output ripple to 50mV peak to peak is given by: CO = = ≅ 11.4µF The minimum ESR needed to limit the output voltage ripple to 50mV peak to peak is: ESR = = = 55mΩ The output capacitor should be chosen so that its ESR is at least half of the calculated value and the capacitance is at least ten times the calculated value. It is often advisable to use several capacitors in parallel to reduce the ESR. Low impedance aluminum electrolytic, tantalum or organic semiconductor capacitors are a good choice for an output capacitor. Low impedance aluminum are the cheapest but are not available in surface mount at present. Solid tantalum chip capacitors are available from a number of suppliers and offer the best choice for surface mount applications. The capacitor working voltage should be greater than the output voltage in all cases. 5) VFB Divider VOUT = 1.25V ()=1.25V( +1) The input bias current to the comparator is 4µA. The resistor divider current should be considerably higher than this to ensure that there is sufficient bias current. If we choose the divider current to be at least 250 times the bias current this gives a divider current of 1mA and simplifies the calcula- tions. = R1+R2 = 1.5KΩ Let R2 = 1K Rearranging the divider equation gives: R1 = R2 ( -1 )=1KΩ()=200Ω 6) Divider bypass capacitor Crr Since the feedback resistors divide the output voltage by a factor of 4, i.e. 5V/1.25V= 4 it follows that the output ripple is also divided by four. This would require that the output ripple be at least 60mV (4 × 15mV) to trip the feedback com- pactor. We use a capacitor Crr to act as an ac short so that the output ripple is not attenuated by the divider network. The ripple voltage frequency is equal to the switching fre- quency so we choose Crr so that: XC = is negligible at the switching frequency. In this case FSW is 200kHz if we allow XC = 3Ω then: C = ≅ 0.265µF 7) Soft start and Fault timing capacitor CS. CS performs several important functions. First it provides a dead time for load transients so that the IC does not enter a fault mode every time the load changes abruptly. Secondly it disables the fault circuitry during startup, it also provides soft start by clamping the reference voltage during startup 1 2πf3 1 2πfC 1.5V 1.25 VOUT 1.25 1.5V 1mA R1 R2 R1 + R2 R2 50 × 10-3 0.6A ∆V ∆I 0.6A 8 × (200 × 103 Hz) × (33 × 10-3V) ∆I 8 × FSW × ∆V IOUT + ∆I 2 2.1V × 4.3µs 0.6A VOUT + VD × ΤOFF(MAX) ∆I 1 FSW 30 × 10 3 FSW FSW 3 × 10 6 VOUT VIN |
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Ähnliche Beschreibung - CS51033YD8 |
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