PEB2023 Datasheet, PDF (16/25 Pages) Siemens Semiconductor Group – ISDN DC Converter Circuit IDCC

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Functional Description
PEB 2023
PEF 2023
Functional Description
The PEB/F 2023 contains the following functional blocks:
• Supply and Biasing
• Undervoltage Detection
• Temperature Compensated Voltage Reference
• Sawtooth Oscillator
• Error Amplifier
• Pulse Width Modulator
• Digital Current Limiting
• Soft Start
• Control Logic (double pulse inhibit)
• Output Driver
The reference voltage provides 4V for the regulation loop. A high gain error amplifier
compares the reference voltage to the output voltage. The output of the error amplifier is
then compared to a periodic ramp, which is generated by the sawtooth-oscillator circuit.
The comparator output is a fixed-frequency, variable pulse width logic signal, which
passes through logic circuits and the output driver and out to the external high voltage
power-switching-FET.
A digital current limiting device suppresses the PWM logic signal when the voltage
difference between current limit sense input I_P and GND reaches 100 mV to protect the
external power-switching-FET.
Non-isolated and isolated SMPS-configurations are possible. Logic and analog circuits
are implemented in BICMOS in order to achieve low power dissipation.
Start-Up Procedure
Before the switched-mode DC/DC converter starts, a sequence of several conditions has
to be passed in order to avoid any system malfunction.
An integrated 6V linear voltage regulator supplies the internal low-voltage BICMOS-
circuits from the input voltage V_S. The generated supply voltage is connected to pin V_EXT
and has to be buffered by an external capacitor (C_min = 1µF). Power dissipation of the
linear voltage regulator can be reduced, if an external supply is used for that purpose by
connecting it to pin V_EXT. If the input voltage at V_EXT is greater than 6.2V, the internal
linear voltage regulator turns off and the internal BICMOS-circuits are then fed from the
external voltage source (power housekeeping input V_EXT). In this case, the input current
at V_EXT is approximately 0.6mA.
Semiconductor Group
16
08.97