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MC33025DWR2G Fiches technique(PDF) 9 Page - ON Semiconductor |
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MC33025DWR2G Fiches technique(HTML) 9 Page - ON Semiconductor |
9 / 20 page MC34025, MC33025 http://onsemi.com 9 If the voltage at this pin exceeds 1.4 V, the second comparator is activated. This comparator sets a latch which, in turn, causes the Soft−Start capacitor to be discharged. In this way a “hiccup” mode of recovery is possible in the case of output short circuits. If a current limit resistor is used in series with the output devices, the peak current at which the controller will enter a “hiccup” mode is given by: Ishutdown + 1.4 V RSense Undervoltage Lockout There are two undervoltage lockout circuits within the IC. The first senses VCC and the second Vref. During power−up, VCC must exceed 9.2 V and Vref must exceed 4.2 V before the outputs can be enabled and the Soft−Start latch released. If VCC falls below 8.4 V or Vref falls below 3.6 V, the outputs are disabled and the Soft−Start latch is activated. When the UVLO is active, the part is in a low current standby mode allowing the IC to have an off−line bootstrap startup circuit. Typical startup current is 500 mA. Output The MC34025 has two high current totem pole outputs specifically designed for direct drive of power MOSFETs. They are capable of up to ±2.0 A peak drive current with a typical rise and fall time of 30 ns driving a 1.0 nF load. Separate pins for VC and Power Ground are provided. With proper implementation, a significant reduction of switching transient noise imposed on the control circuitry is possible. The separate VC supply input also allows the designer added flexibility in tailoring the drive voltage independent of VCC. Reference A 5.1 V bandgap reference is pinned out and is trimmed to an initial accuracy of ±1.0% at 25°C. This reference has short circuit protection and can source in excess of 10 mA for powering additional control system circuitry. Design Considerations Do not attempt to construct the converter on wire−wrap or plug−in prototype boards. With high frequency, high power, switching power supplies it is imperative to have separate current loops for the signal paths and for the power paths. The printed circuit layout should contain a ground plane with low current signal and high current switch and output grounds returning on separate paths back to the input filter capacitor. All bypass capacitors and snubbers should be connected as close as possible to the specific part in question. The PC board lead lengths must be less than 0.5 inches for effective bypassing or snubbing. Instabilities In current mode control, an instability can be encountered at any given duty cycle. The instability is caused by the current feedback loop. It has been shown that the instability is caused by a double pole at half the switching frequency. If an external ramp (Se) is added to the on−time ramp (Sn) of the current−sense waveform, stability can be achieved (see Figure 21). One must be careful not to add too much ramp compensation. If too much is added, the system will start to perform like a voltage mode regulator. All benefits of current mode control will be lost. Figures 29A and 29B show examples of two different ways in which external ramp compensation can be implemented. 1.25 V + + Ramp Input Current Signal Sn Ramp Compensation Se Figure 21. Ramp Compensation A simple equation can be used to calculate the amount of external ramp necessary to add that will achieve stability in the current loop. For the following equations, the calculated values for the application circuit in Figure 37 are also shown. Se + VO L NS NP (RS)Ai where: = DC output voltage = number of power transformer primary = or secondary turns = gain of the current sense network = (see Figures 26, 27 and 28) = output inductor = current sense resistance VO NP, NS Ai L RS + 0.115 V μs For the application circuit: Se + 5 1.8 μ 4 16 (0.3)(0.55) |
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Description similaire - MC33025DWR2G |
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