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MC34280FTB Folha de dados(PDF) 10 Page - ON Semiconductor |
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MC34280FTB Folha de dados(HTML) 10 Page - ON Semiconductor |
10 / 20 page MC34280 http://onsemi.com 10 DETAILED OPERATING DESCRIPTION General The MC34280 is a power supply integrated circuit which provides two boost regulated outputs and some power management supervisory functions. Both regulators apply Pulse–Frequency–Modulation (PFM). The main boost regulator output can be externally adjusted from 2.7V to 5V. An internal synchronous rectifier is used to ensure high efficiency (achieve 87%). The auxiliary regulator with a built–in power transistor can be configured to produce a wide range of positive voltage (can be used to supply a LCD contrast voltage). This voltage can be adjusted from +5V to +25V by an external potentiometer; or by a microprocessor, digitally through a 6–bit internal DAC. The MC34280 has been designed for battery powered hand–held products. With the low start–up voltage from 1V and the low quiescent current (typical 35 µA); the MC34280 is best suited to operate from 1 to 2 AA/ AAA cell. Moreover, supervisory functions such as low battery detection, CPU power–on reset, and back–up battery control, are also included in the chip. It makes the MC34280 the best one–chip power management solution for applications such as electronic organizers and PDAs. Pulse Frequency Modulation (PFM) Both regulators apply PFM. With this switching scheme, every cycle is started as the feedback voltage is lower than the internal reference. This is normally performed by internal comparator. As cycle starts, Low–Side switch (i.e. M1 in Figure 1) is turned ON for a fixed ON time duration (namely, Ton) unless current limit comparator senses coil current reaches its preset limit. In the latter case, M1 is OFF instantly. So Ton is defined as the maximum ON time of M1. When M1 is ON, coil current ramps up so energy is being stored inside the coil. At the moment just after M1 is OFF, the Synchronous Rectifier (i.e. M2 in Figure 1) or any rectification device (such as Schottky Diode of Auxiliary Regulator) is turned ON to direct coil current to charge up the output bulk capacitor. Provided that coil current is not reached, every switching cycle delivers fixed amount of energy to the bulk capacitor. So for higher loading, larger amount of energy (Charge) is withdrawn from the bulk capacitor, and as output voltage is needed to regulated, larger amount of Charge is needed to be supplied to the bulk capacitor, that means switching frequency is needed to be increased; and vice–versa. Main Regulator Figure 20 shows the simplified block diagram of Main Regulator. Notice that precise bias current Iref is generated by a VI converter and external resistor RIref, where Iref + 0.5 RIref (A) This bias current is used for all internal current bias as well as setting VMAIN value. For the latter application, Iref is doubled and fed as current sink at Pin 1. With external resistor RMAINb tied from Pin1 to Pin32, a constant level shift is generated in between the two pins. In close–loop operation, voltage at Pin 1 (i.e. Output feedback voltage) is needed to be regulated at the internal reference voltage level, 1.22V. Therefore, the delta voltage across Pin 1 and Pin 32 which can be adjusted by RMAINb determines the Main Output voltage. If the feedback voltage drops below 1.22V, internal comparator sets switching cycle to start. So, VMAIN can be calculated as follows. VMAIN + 1.22 ) RMAINb RIref (V) From the above equation, although VMAIN can be adjusted by RMAINb and RIref ratio, for setting VMAIN, it is suggested, by changing RMAINb value with RIref kept at 480K. Since changing RIref will alter internal bias current which will affect timing functions of Max ON time (TON1) and Min OFF time (TOFF1). Their relationships are as follows; T ON 1 + 1.7 10–11 RIref (S) T OFF 1 + 6.4 10–12 RIref (S) Continuous Conduction Mode and Discontinuous Conduction Mode In Figure 21, regulator is operating at Continuous Conduction Mode. A switching cycle is started as the output feedback voltage drops below internal voltage reference VREF. At that instant, the coil current does not drop to zero yet, and it starts to ramp up for the next cycle. As the coil current ramps up, loading makes the output voltage to decrease as the energy supply path to the output bulk capacitor is disconnected. And after Ton elapsed, M1 is OFF, M2 becomes ON, energy is dumped to the bulk capacitor. Output voltage is increased as excessive charge is pumped in, then it is decreased after the coil current drops below the loading. Notice the abrupt spike of output voltage is due to ESR of the bulk capacitor. Feedback voltage can be resistor–divided down or level–shift down from the output voltage. As this feedback voltage drops below VREF, next switching cycle starts. |
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