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M51995AP Datasheet(PDF) 22 Page - Mitsubishi Electric Semiconductor |
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M51995AP Datasheet(HTML) 22 Page - Mitsubishi Electric Semiconductor |
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22 / 27 page  SWITCHING REGULATOR CONTROL M51995AP/FP MITSUBISHI (Dig./Ana. INTERFACE) ( / 27 ) 22 Fig.31 and 32 show the example circuit diagrams around the CLM+ and CLM- terminal.It is required to connect the low pass filter,as the main current or drain current contains the spike current especially during the turn-on duration of MOS-FIT. 1,000pF to 22,000pF is recommended for CNF and the RNF1 and RNF2 have the functions both to adjust the "current- detecting-sensitivity" and to consist the low pass filter. (1)Peripheral circuit of CLM+,CLM- terminal Current limiting circuit M51995A GND Vcc Fig.31 Peripheral circuit diagram of CLM+ terminal R1 Cvcc INPUT SMOOTHING CAPACITOR CFIN COLLECTOR CLM+ VOUT EMITTER CNF RNF1 RNF2 RCLM M51995A GND Vcc Fig.32 Peripheral circuit diagram of CLM- terminal R1 Cvcc INPUT SMOOTHING CAPACITOR CFIN COLLECTOR CLM+ VOUT EMITTER CNF RNF1 RNF2 RCLM To design the RNF1 and RNF2,it is required to consider the influence of CLM terminal source current(IINCLM+ or INFCLM-), which value is in the range of 90 to 270µA. In order to be not influenced from these resistor paralleled value of RNF1 and RNF2,(RNF1/RNF2)is recommended to be less than 100 Ω. The RCLM should be the non-inductive resistor. Fig.33 shows the primary and secondary current wave-forms under the current limiting operation. At the typical application of pulse by pulse primary current detecting circuit,the secondary current depends on the primary current.As the peak value of secondary current is limited to specified value,the characteristics curve of output voltage versus output current become to the one as shown in Fig.34. (a)In case of feed forward system (2)Over current limiting curve I1 I2 RCLM CLM Fig.33 Primary and secondary current waveforms under the current limiting operation condition on feed forward system (a) Feed forward system IP1 IP2 I1 I2 (b) Primary and secondary current OUTPUT CURRENT Fig.34 Over current limiting curve on feed forward system The demerit of the pulse by pulse current limiting system is that the output pulse width can not reduce to less than some value because of the delay time of low pass filter connected to the CLM terminal and propagation delay time TPDCLM from CLM terminal to output terminal of type M51995A.The typical TPDCLM is 100ns. As the frequency becomes higher,the delay time must be shorter.And as the secondary output voltage becomes higher,the dynamic range of on-duty must be wider;it means that it is required to make the on-duration much more narrower. So this system has the demerit at the higher oscillating frequency and higher output voltage applications. To improve these points,the oscillating frequency is set low using the characteristics of VF terminal.When the current limiting circuit operates under the over current condition,the oscillating frequency decreases in accordance with the decrease of VF terminal voltage,if the VF is lower than 3.5V.And also the dead time becomes longer. |
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