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A5973AD Datasheet(PDF) 11 Page - STMicroelectronics |
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A5973AD Datasheet(HTML) 11 Page - STMicroelectronics |
11 / 19 page A5973AD Functional description 11/19 4.5 Error amplifier The voltage error amplifier is the core of the loop regulation. It is a transconductance operational amplifier whose non inverting input is connected to the internal voltage reference (1.235 V), while the inverting input (FB) is connected to the external divider or directly to the output voltage. The output (COMP) is connected to the external compensation network. The uncompensated error amplifier has the following characteristics: The error amplifier output is compared with the oscillator sawtooth to perform PWM control. 4.6 PWM comparator and power stage This block compares the oscillator sawtooth and the error amplifier output signals generating the PWM signal for the driving stage. The power stage is a very critical block cause it has to guarantee a correct turn on and turn OFF of the PDMOS. The turn ON of the power element, or better, the rise time of the current at turn on, is a very critical parameter to compromise. At a first approach, it looks like the faster it is the rise time, the lower are the turn on losses. But there is a limit introduced by the recovery time of the recirculation diode. In fact when the current of the power element equals the inductor current, the diode turns off and the drain of the power is free to go high. But during its recovery time, the diode can be considered as an high value capacitor and this produces a very high peak current, responsible of many problems: ● Spikes on the device supply voltage that cause oscillations (and thus noise) due to the board parasitics. ● Turn ON overcurrent causing a decrease of the efficiency and system reliability. ● Big EMI problems. ● Shorter freewheeling diode life. The fall time of the current during the turn off is also critical. In fact it produces voltage spikes (due to the parasitics elements of the board) that increase the voltage drop across the PDMOS. In order to minimize all these problems, a new topology of driving circuit has been used and its block diagram is shown in Figure 7 on page 12. The basic idea is to change the current levels used to turn on and off the power switch, according with the PDMOS status and with the gate clamp status. This circuitry allow to turn off and on quickly the power switch and to manage the above question related to the freewheeling diode recovery time problem. Table 5. Uncompensated error amplifier Tranconductance 2300 μS Low frequency gain 65 dB Minimum sink/source voltage 1500 μA/300 μA Output voltage swing 0.4 V/3.65 V Input bias current 2.5 μA |
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