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LM3477MM Datasheet(PDF) 11 Page - Texas Instruments |
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LM3477MM Datasheet(HTML) 11 Page - Texas Instruments |
11 / 35 page VOVP (HIGH) VOVP (LOW) VFB Hysteretic Mode: PWM Mode: - Light load current - D < DMIN - Transient response overshoot too high - Normal operation t Feedback Voltage VOVP(HYS) LM3477 www.ti.com SNVS141K – OCTOBER 2000 – REVISED MARCH 2013 OVP will cause the drive pin to go low, forcing the power MOSFET off. With the MOSFET off, the output voltage will drop. The LM3477/A will begin switching again when the feedback voltage reaches VFB + (VOVP - VOVP(HYS)). See Electrical Characteristics for limits on VOVP(HYS). OVP can be triggered by any event that causes the output voltage to rise out of regulation. There are several common circumstances in which this can happen, and it is beneficial for a designer to be aware of these for debugging purposes, since the mode of operation changes from the normal Pulse Width Modulation (PWM) mode to the hysteretic mode. In the hysteretic mode the output voltage is regulated between a high and low value that results in a higher ripple magnitude and lower ripple frequency than in the PWM mode, see Figure 20. See different Ripple Components in PWM and Hysteretic Modes. Figure 20. The Feedback Voltage is related to the Output Voltage If the load current becomes too low, the LM3477/A will increase the duty cycle, causing the voltage to rise and trigger the OVP. The reasons for this involve the way the LM3477/A regulates the output voltage, using a control waveform at the pulse width modulator. This control waveform has upper and lower bounds. Another way OVP can be tripped is if the input voltage rises higher than the LM3477/A is able to regulate in pulse width modulation (PWM) mode. The output voltage is related to the input voltage by the duty cycle as: VOUT = VIN*D. The LM3477/A has a minimum duty cycle of 16.5% (typical), due to the blank-out timing, TMIN. If the input voltage increases such that the duty cycle wants to be less than DMIN, the duty cycle will hold at DMIN and the output voltage will increase with the input voltage until it trips OVP. It is useful to plot the operational boundaries in order to illustrate the point at which the device switches into hysteretic mode. In Figure 19, the limits shown are with respect to the peak voltage across the sense resistor RSN, (VSNpk); they can be referred to the peak inductor current by dividing through by RSN. VSNpk is bound to the shaded regions. In normal circumstances VSNpk is required to be in the shaded region, and the LM3477/A will operate in the PWM mode. If operating conditions are chosen such that VSNpk would not normally fall in the shaded regions, then the mode of operation is changed so that VSNpk will be in the shaded region, and the part will operate in the hysteretic mode. What actually happens is that the LM3477/A will not allow VSNpk to be outside of the shaded regions, so the duty cycle is adjusted. The output voltage transient response overshoot can also trigger OVP. As discussed in Output Capacitor Selection, if the capacitance is too low or ESR too high, the output voltage overshoot will rise high enough to trigger OVP. However, as long as there is room for the duty cycle to adjust (the converter is not near DMIN or DMAX), the LM3477/A will return to PWM mode after a few cycles of hysteretic mode operation. There is one last way that OVP can be triggered. If the unregulated input voltage crosses 7.2V, the output voltage will react as shown in Figure 21. The internal bias of the LM3477/A switches supplies at 7.2V. When this happens, a sudden small change in bias voltage is seen by all the internal blocks of the LM3477/A. The control voltage, VC, shifts because of the bias change, the PWM comparator tries to keep regulation. To the PWM comparator, the scenario is identical to step change in the load current, so the response at the output voltage is the same as would be observed in a step load change. Hence, the output voltage overshoot here can also trigger OVP. The LM3477/A will regulate in hysteretic mode for several cycles, or may not recover and simply stay in hysteretic mode until the load current drops. Note that the output voltage is still regulated in hysteric mode. Predicting whether or not the LM3477/A will come out of hysteretic mode in this scenario is a difficult task, however it is largely a function of the output current and the output capacitance. Triggering hysteretic mode in this way is only possible at higher load currents. The method to avoid this is to increase the output capacitance. Copyright © 2000–2013, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM3477 |
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