Block diagram of MC44603
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8 | DETECT | DEMAGNETISATION | REFERENCE | SUPPLY |
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10 CT |
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15 RF STANDBY |
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| =1 | 1Reset |
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12 RP STANDBY |
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| (REDUCED FREQUENCY) |
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| 2.5V |
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| THERMAL |
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| SHUTDOWN |
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| VOLTAGE | ERROR |
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14 FEEDBACK | AMP |
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13 E/A OUT |
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| CONTROL | Vref |
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| FOLDBACK |
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| FOLDBACK | CURRENT SENSE |
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| INPUT | INPUT |
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5 | 7 | 11 | CL 96532076_032.eps |
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| 290799 |
Figure
Operating description of MC44603
The input voltage Vcc (pin 1) is monitored by a comparator with hysteresis, enabling the circuit at 14.5V and disabling the circuit below 7.5V. The error amplifier compares a voltage Vfb (pin 14) related to the output voltage of the power supply, with an internal 2.5V reference. The current sense comparator compares the output of the error amplifier with the switch current Isense (pin 7) of the power supply. The output of the current sense comparator resets a latch, which is set every cycle by the oscillator. The output stage is a totem pole, capable of driving a MOSFET directly.
Start up sequence of P816
t1: Charging the capacitors at Vcc
C829 will be charged via R823 and R832, C833 and
C811 v i a R829. The output is switched off during t1. t2: Charging of output capacitors
When the input voltage of the IC exceeds 14.5V, the circuit is enabled and starts to produce output pulses. The current consumption of the circuit increases to about 17mA, depending on the external loads of the IC. At first, the capacitors at the Vcc pin will discharge because the primary auxiliary voltage, coming from winding
t3: Regulation
The output voltage of the power supply is in regulation. t4: Overload
When the output is shorted, the supply voltage of the circuit will decrease and after some time drop below the lower threshold voltage. At that moment, the output will be disabled and the process of charging the Vcc capacitors starts again. If the output is still shorted at the next t2 phase, the complete start- and stop sequence will repeat. The power supply goes in a hiccup mode.
14.5V
Vcc
10V
7.5V
p.a.v.
0V
17mA
Icc 1mA
OUTPUT
short
Vo
0
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CL 96532076_035.eps 290799
Figure 8-14 ‘Start-up sequence’
Regulation of P816
Figure 8-14 shows the most relevant signals during the regulation phase of the power supply.
The oscillator voltage ramps up and down between V1 and V2. The voltage at the current sense terminal is compared every cycle with the output of the error amplifier Vcomp. The output
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