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MC33067P Fiches technique(PDF) 8 Page - ON Semiconductor |
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MC33067P Fiches technique(HTML) 8 Page - ON Semiconductor |
8 / 16 page ![]() MC34067 MC33067 8 MOTOROLA ANALOG IC DEVICE DATA The maximum oscillator frequency is set by the current through resistor RVFO. The current required to discharge COSC at the maximum oscillator frequency can be calculated by Equation 2: COSC 1.5C (2) OSC I 1 ƒ 5.1 – 3.6 = = (max) (max) ƒ(max) The discharge current through ROSC must also be known and can be calculated by Equation 3: ƒ (min) COSC ROSC 1.5 ε I 1 ƒ 5.1 – 3.6 = (min) R ROSC COSC ROSC = ROSC ε 1 (3) – – OSC Resistor RVFO can now be calculated by Equation 4: (4) R= VFO I(max) IR – 2.5 – VEAsat OSC One–Shot Timer The One–Shot is designed to disable both outputs simultaneously providing a deadtime before either output is enabled. The One–Shot capacitor (CT) is charged concurrently with the oscillator capacitor by transistor Q1, as shown in Figure 14. The one–shot period begins when the oscillator comparator turns off Q1, allowing CT to discharge. The period ends when resistor RT discharges CT to the threshold of the One–Shot comparator. The lower threshold of the One–Shot is 3.6 V. By choosing CT, RT can by solved by Equation 5: 5.1 3.6 (5) CT R t OS C 0.348 = = T T t OS n Errors in the threshold voltage and propagation delays through the output drivers will affect the One–Shot period. To guarantee accuracy, the output pulse of the control chip is trimmed to within 5% of 250 ns with nominal values of RT and CT. The outputs of the Oscillator and One–Shot comparators are OR’d together to produce the pulse tOS, which drives the Flip–Flop and output drivers. The output pulse (tOS) is initiated by the Oscillator and terminated by the One–Shot comparator. With zero–voltage resonant mode converters, the oscillator discharge time should never be set less than the one–shot period. Error Amplifier A fully accessible high performance Error Amplifier is provided for feedback control of the power supply system. The Error Amplifier is internally compensated and features dc open loop gain greater than 70 dB, input offset voltage of less than 10 mV and a guaranteed minimum gain–bandwidth product of 2.5 MHz. The input common mode range extends from 1.5 V to 5.1 V, which includes the reference voltage. Figure 15. Error Amplifier and Clamp RVFO Oscillator Control Current 3.1V Error Amp Output Noninverting Input Inverting Input Error Amp IOSC 7 8 6 3 Error Amp Charge When the Error Amplifier output is coupled to the IOSC pin by RVFO, as illustrated in Figure 15, it provides the Oscillator Control Current, IOSC. The output swing of the Error Amplifier is restricted by a clamp circuit to improve its transient recovery time. Output Section The pulse(tOS), generated by the Oscillator and One–Shot timer is gated to dual totem–pole output drives by the Steering Flip–Flop shown in Figure 16. Positive transitions of tOS toggle the Flip–Flop, which causes the pulses to alternate between Output A and Output B. The flip–flop is reset by the undervoltage lockout circuit during startup to guarantee that the first pulse appears at Output A. Figure 16. Steering Flip–Flop and Output Drivers Output A R 12 13 14 Output B Power Ground Q Q T Steering Flip–Flop ± ± Pwr Gnd VOE VOE Pwr Gnd The totem–pole output drivers are ideally suited for driving power MOSFETs and are capable of sourcing and sinking 1.5 A. Rise and fall times are typically 20 ns when driving a 1.0 nF load. High source/sink capability in a totem–pole driver normally increases the risk of high cross conduction current during output transitions. The MC34067 utilizes a unique design that virtually eliminates cross conduction, thus controlling the chip power dissipation at high frequencies. A separate power ground pin is provided to isolate the sensitive analog circuitry from large transient currents. |
Numéro de pièce similaire - MC33067P |
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Description similaire - MC33067P |
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