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LM3477AMM データシート(PDF) 11 Page - National Semiconductor (TI) |
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LM3477AMM データシート(HTML) 11 Page - National Semiconductor (TI) |
11 / 23 page Functional Description (Continued) The important differences between the LM3477 and the LM3477A are summarized in Figure 1. The voltages in Fig- ure 1 can be referred to the switch current by dividing through by R SN. The LM3477A has a lower hysteretic thresh- old voltage V HYS, and thus will operate in PWM mode for a larger load range than the LM3477. Typically, V HYS = 32mV for the LM3477, while V HYS = 11mV for the LM3477A. The difference in area between the shaded regions give a graphi- cal representation of this. The lightly shaded region is the extra PWM operating area gained by using the LM3477A. Thus the benefits of operating in PWM mode such as a well regulated output voltage with low noise ripple are extended to a larger load range when the LM3477A is used. While less significant, the other noteworthy difference between the two parts is in the short circuit current limit V SC. V SC is a ceiling limit for the peak sense voltage VSNpk (see the SHORT CIRCUIT CURRENT LIMIT section). V SC is lower in the LM3477A than in the LM3477 (see the ELEC- TRICAL CHARACTERISTICS section for limits). Over Voltage Protection The LM3477/A has over voltage protection (OVP) for the output voltage. OVP is sensed at and is in respect to the feedback pin (pin 3). If at anytime the voltage at the feedback pin rises to V FB +VOVP, OVP is triggered. See ELECTRICAL CHARACTERISTICS section for limits on V FB and VOVP. 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 V FB +(VOVP -VOVP(HYS)). See ELECTRICAL CHARACTERISTICS for limits on V OVP(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 2. 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 wave- form 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: V OUT =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 D MIN, the duty cycle will hold at D MIN 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 hyster- etic mode. In Figure 1, the limits shown are with respect to the peak voltage across the sense resistor R SN,(VSNpk); they can be referred to the peak inductor current by dividing through by R SN.VSNpk is bound to the shaded regions. In normal circumstances V SNpk 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 V SNpk would not normally fall in the shaded regions, then the mode of opera- tion is changed so that V SNpk will be in the shaded region, and the part will operate in the hysteretic mode. What actu- ally happens is that the LM3477/A will not allow V SNpk 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 the OUTPUT CAPACITOR section, 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 D MIN or D MAX), 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 3. 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 sce- nario 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. 200033C1 FIGURE 2. The Feedback Voltage is related to the Output Voltage. See different Ripple Components in PWM and Hysteretic Modes www.national.com 11 |
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同様の説明 - LM3477AMM |
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