STMicroelectronics Evaluation board for A5970AD EVALA5970AD EVALA5970AD Scheda Tecnica
Codici prodotto
EVALA5970AD
Functional description
A5970AD
12/42
DocID14661 Rev 5
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 500 kHz (see
sense the current in order to avoid a false overcurrent signal is too short to obtain
a sufficiently low duty cycle at 500 kHz (see
), the output current in
strong overcurrent or short-circuit conditions could be not properly limited. For this reason
the switching frequency is also reduced, thus keeping the inductor current under its
maximum threshold. The frequency shifter (
the switching frequency is also reduced, thus keeping the inductor current under its
maximum threshold. The frequency shifter (
) functions based on the feedback
voltage. As the feedback voltage decreases (due to the reduced duty cycle), the switching
frequency decreases also.
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:
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.
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