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Figure 5.

Current limitation circuitry

4.5

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:

Table 5.

Uncompensated error amplifier

Note:

The error amplifier output is compared with the oscillator sawtooth to perform PWM control.

4.6

PWM comparator and power stage

This block compares the oscillator sawtooth and the error amplifier output signals

generating the PWM signal for the driving stage. The power stage is a very critical block

cause it has to guarantee a correct turn on and turn OFF of the PDMOS. The turn ON of the

power element, or better, the rise time of the current at turn on, is a very critical parameter to

compromise.

At a first approach, it looks like the faster it is the rise time, the lower are the turn on losses.

But there is a limit introduced by the recovery time of the recirculation diode. In fact when the

current of the power element equals the inductor current, the diode turns off and the drain of

the power is free to go high. But during its recovery time, the diode can be considered as an

Tranconductance

2300

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