R5973D datasheet(12/39 Pages) STMICROELECTRONICS

12 / 39 page

Functional description

R5973D

12/39

DocID026876 Rev 1

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 250 kHz (see Section 8.4 on page 26), 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 (Figure 4) functions based on the feedback

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

frequency decreases also.

Figure 6. 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.

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

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