STMicroelectronics Evaluation board for A5970AD EVALA5970AD EVALA5970AD Fiche De Données

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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 

), 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 (

) functions based on the feedback 

voltage. As the feedback voltage decreases (due to the reduced duty cycle), the switching 
frequency decreases also.

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.

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