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AN8014S Datasheet(PDF) 10 Page - Panasonic Semiconductor |
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AN8014S Datasheet(HTML) 10 Page - Panasonic Semiconductor |
10 / 18 page AN8014S Voltage regulators 10 s Application Notes (continued) [1] Function descriptions (continued) 6. Remote circuit It is possible to switch on or off the IC control by using an external control signal. When the OFF pin (pin 16) voltage is lowered to below approximately 0.8 V, the internal reference voltage goes down thereby stopping the IC control and reducing the circuit current to 5 µA or less. When the OFF pin voltage is increased to approximately 2.0 V or more, the internal reference voltage rises thereby starting the control operation. 7. PWM comparator block The PWM comparator controls the on-period of output pulse in accordance with the input voltage. While the triangular wave voltage on the CT pin (pin 3) is lower than both the error amplifier's output voltage on pin 8 and the voltage on the DTC pin (pin 4), the output on the Out pin (pin 13) will be set to high level. Then the switching element (n-channel MOSFET) will be turned on. The dead-time is set by adjusting the voltage VDTC on the DTC pin (pin 5) as shown in figure 4. The DTC pin has constant current output determined by the resistor RT . Therefore VDTC is adjusted by connecting the DTC and GND pins through the external resistor RDTC . When the oscillator frequency fOSC is 200 kHz, the output duty cycle will be 0% at VDTC of 0.44 V typical and 100% at VDTC of 1.32 V typical. The levels of overshooting and undershooting of the peak value VCTH and the trough value VCTL of the triangular wave vary with the oscillator frequency. DTC CT waveform DTC waveform Out waveform Figure 4. Setting the dead-time Off Off On tOFF tON VCTH VDTC VCTL IDTC PWM VREF RDTC CDTC CT FB Output duty ratio Du and DTC pin voltage VDTC are expressed by the following formulas; Du = tON × 100 [%] = VDTC − VCTL × 1.1 × 100 [%] tON − tOFF (VCTH − VCTL) × 1.1 IDTC = VRT × 1 [A] RT 2 VDTC = IDTC × RDTC = VRT × RDTC × 1 [V] RT 2 Example) When fOSC = 215 [kHz] (RT = 15 kΩ, CT = 120 pF) and RDTC = 75 [kΩ] VCTH is approximately 1.32 V, VCTH is approximately 0.44 V, and VRT is approximately 0.4 V. Therefore, the following are obtained. IDTC ≈ 13.3 [µA] VDTC ≈ 0.99 [V] Du ≈ 52.3 [%] There may be an operational delay of the PWM comparator and a difference in peak and trough values of the triangular wave oscillation. Discrepancies between the values obtained from the above formulas and the actual values may occur, in which case adjust the values on the mounting substrate. In starting, if the capacitor CDTC is added in parallel to the external resistor RDTC , and the output pulse width are gradually widened, the AN8014S will be in soft-start operation. Thus the overshoot at the output of DC-DC converters can be prevented. |
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