ETS2228 Maintenance Manual
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Page 16, Total 43
Figure16 External power over voltage protection module
2. DC/DC power module
The DC/DC power module generates 4.4V main power necessary for the operation of the whole
machine. It also fulfills the constant current charging function. The maximum output current is 830mA.
We mainly use the PWM control chip (U302) and the switch transistor (Q301) to complete the
second-time switch power circuit. It is the switch power supply that converts the external power into
4.4V.
Figure 5 DC/DC Power transfer circuit
The PWM control chip adopts TL494, including 2 differential amplifiers, 1 external adjustable
oscillator, 1 pulse comparer, 1 time overflow controller, and 1 output control circuit.