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M51996AP Datasheet(PDF) 14 Page - Mitsubishi Electric Semiconductor |
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M51996AP Datasheet(HTML) 14 Page - Mitsubishi Electric Semiconductor |
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14 / 22 page  SWITCHING REGULATOR CONTROL M51996AP/FP MITSUBISHI (Dig./Ana. INTERFACE) ( / 22 ) 14 Just after the start-up,the Icc current is supplied from Cvcc,however,under the steady state condition ,IC will be supplied from the third winding or bias winding of transformer,the winding ratio of the third winding must be designed so that the induced voltage may be higher than the operation-stop voltage Vcc(STOP). The Vcc voltage is recommended to be 12V to 17V as the normal and optimum gate voltage is 10 to 15V and the output voltage(VOH) of type M51996AP/FP is about(Vcc-2V). It is not necessary that the induced voltage is settled higher than the operation start-up voltage Vcc(START),and the high gate drive voltage causes high gate dissipation,on the other hand,too low gate drive voltage does not make the MOS-FET fully on- state or the saturation state. SUPPLY VOLTAGE Vcc(V) 0 10 20 30 40 0 Ta=85°C Ta=25°C Ta=-30°C OVP RESET POINT 8.87V(-30°C) 8.94V(25°C) 9.23V(85°C) 1 2 3 4 5 6 7 8 Fig.16 CIRCUIT CURRENT VS. SUPPLY VOLTAGE (OVP OPERATION) M51996A GND Vcc R1 VF CVcc THIRD WINDING OR BIAS WINDING MAIN TRANSFORMER RECTIFIED DC VOLTAGE FROM SMOOTHING CAPACITOR Fig.24 Start-up circuit diagram when it is not necessary to set the start and stop input voltage It is required that the output circuit have the high sink and source abilities for MOS-FET drive.It is well known that the "totempole circuit has high sink and source ability.However,it has the demerit of high through current. For example,the through current may reach such the high current level of 1A,if type M51996A has the "conventional" totempole circuit.For the high frequency application such as higher than 100kHz,this through current is very important factor and will cause not only the large Icc current and the inevitable heat-up of IC but also the noise voltage. This IC uses the improved totempole circuit,so without deteriorating the characteristic of operating speed,its through current is approximately 100mA. Output section Fig.17 shows one of the example circuit diagram of the start-up circuit which is used when it is not necessary to set the start and stop voltage. It is recommended that the current more than 300µA flows through R1 in order to overcome the operation start-up current Icc(START) and Cvcc is in the range of 10 to 47µF.The product of R1 by Cvcc causes the time delay of operation,so the response time will be long if the product is too much large. APPLICATION NOTE OF TYPE M51996AP/FP Design of start-up circuit and the power supply of IC (1)The start-up circuit when it is not necessary to set the start and stop input voltage M51996A GND Vcc R1 VF CVcc THIRD WINDING OF TRANSFORMER PRIMARY WINDING OF TRANSFORMER RECTIFIED DC VOLTAGE FROM SMOOTHING CAPACITOR Fig.18 Start-up circuit diagram when it is not necessary to set the start and stop input voltage NP NB VIN R2 It is recommend to use the third winding of "forward winding" or "positive polarity" as shown in Fig.18,when the DC source voltages at both the IC operation start and stop must be settled at the specified values. The input voltage(VIN(START)),at which the IC operation starts,is decided by R1 and R2 utilizing the low start-up (2)The start-up circuit when it is not necessary to set the start and stop input voltage |
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