Datasheet

A5973D Functional description
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threshold. The frequency shifter (Figure 5) functions based on the feedback voltage. As the
feedback voltage decreases (due to the reduced duty cycle), the switching frequency
decreases also.
Figure 7. Current limitation circuitry
5.5 Error amplifier
The voltage error amplifier is the core of the loop regulation. It is a transconductance
operational amplifier whose non inverting input is connected to the internal voltage
reference (1.235 V), while the inverting input (FB) is connected to the external divider or
directly to the output voltage. The output (COMP) is connected to the external compensation
network. The uncompensated error amplifier has the following characteristics:
The error amplifier output is compared to the oscillator sawtooth to perform PWM control.
5.6 PWM comparator and power stage
This block compares the oscillator sawtooth and the error amplifier output signals to
generate the PWM signal for the driving stage.
The power stage is a highly critical block, as it functions to guarantee a correct turn ON and
turn OFF of the PDMOS. The turn ON of the power element, or more accurately, the rise
time of the current at turn ON, is a very critical parameter. At a first approach, it appears that
the faster the rise time, the lower the turn ON losses.
However, there is a limit introduced by the recovery time of the recirculation diode.
Table 5. Uncompensated error amplifier characteristics
Description Values
Transconductance 2300 µS
Low frequency gain 65 dB
Minimum sink/source voltage 1500 µA/300 µA
Output voltage swing 0.4 V/3.65 V
Input bias current 2.5 µA