PEB2023 Datasheet, PDF (19/25 Pages) Siemens Semiconductor Group – ISDN DC Converter Circuit IDCC

PEB 2023
PEF 2023
Functional Description
Error Amplifier
Conventional operational amplifier for closed-loop gain and phase compensation.
Low output impedance: unity-gain stable.
Pulse Width Modulator
The pulse width modulator compares the sawtooth-voltage of the oscillator output with
the output of the error amplifier and with the voltage of the external soft start capacitor at
pin C_SS.
Current Limiting
When the sense voltage reaches a threshold voltage of 100mV a shutdown signal is sent
to the control logic. Sense voltage is the voltage between pin I_P and pin GND. Because
of the small value of the current-sensing-resistor (R₅ in figure 4, 5 or 6) the board layout
has to be done carefully.
Soft Start
The soft start circuit protects the power transistors and rectifier diodes from high current
surges during power supply turn-on. When the supply voltage is connected to the PEB/
F 2023 the undervoltage lockout circuit holds the soft start capacitor voltage at zero.
When the supply voltage reaches the normal operating range, an internal 1.5µA current
source will charge the external soft start capacitor. As the soft start voltage ramps up to
+5 volts, the duty cycle of the PWM linearly increases to whatever value the regulation
loop requires.
Control Logic
The control logic inhibits double pulses during one duty cycle and limits the maximum
duty cycle to 50%.
Disable Input Realization
One way to disable the function of the PEB/F 2023 is to connect an external n-channel
MOS-transistor between the pins UV(drain) and GND(source). By switching on this
external transistor the PEB/F 2023 detects an ’undervoltage’ and turns off. After
switching off this external transistor the PEB/F 2023 turns on and starts the DC/DC
conversion with a soft start.
A second posibility is to connect an external n-channel MOS-transistor between the pins
C_SS(drain) and GND(source). By switching on this external transistor the PEB/F 2023
disables the gate output driver because of the soft start circutry. After switching off this
external transistor the PEB/F 2023 turns on and starts the DC/DC conversion with a soft
start.
Semiconductor Group
19
08.97