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A5974D Datasheet(PDF) 11 Page - STMicroelectronics

Part # A5974D
Description  Zero load current operation
PDF  46 Pages
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Manufacturer  STMICROELECTRONICS [STMicroelectronics]
Direct Link  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

A5974D Datasheet(HTML) 11 Page - STMicroelectronics

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A5974D
Functional description
46
Figure 5. Oscillator circuit block diagram
Figure 6. Synchronization example
5.4
Current protection
The A5974D device features two types of current limit protection: pulse-by-pulse and
frequency foldback.
The schematic of the current limitation circuitry for the pulse-by-pulse protection is shown in
Figure 7. The output power PDMOS transistor is split into two parallel PDMOS transistors.
The smallest one includes a resistor in series, RSENSE. The current is sensed through
RSENSE and if it reaches the threshold, the mirror becomes unbalanced and the PDMOS is
switched off until the next falling edge of the internal clock pulse. Due to this reduction of the
ON time, the output voltage decreases. Since the minimum switch ON time necessary to
sense the current in order to avoid a false overcurrent signal is too short to obtain
a sufficiently low duty cycle at 250 kHz (see Section 8.5 on page 31), the output current in
strong overcurrent or short-circuit conditions could be not properly limited. For this reason
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