Functional description

ST1S40

8/30

Doc ID 17928 Rev 4

from 0 V to 0.8 V in around 1 ms.

5.2

Error amplifier and control loop stability

The error amplifier compares the FB pin voltage with the internal 0.8 V reference and it

provides the error signal to be compared with the output of the current sense circuitry, that is

the high side Power MOSFET current. Comparing the output of the error amplifier and the

peak inductor current implements the peak current mode control loop.

The error amplifier is a transconductance amplifier (OTA). The uncompensated

characteristics are listed inTable 5.

Table 5.

Error amplifier characteristics

The ST1S40 embeds the compensation network that assures the stability of the loop in the

whole operating range. All the tools needed to check the loop stability are shown below.

Figure 4. shows the simple small signal model for the peak current mode control loop.

Figure 4.

Block diagram of the loop for the small signal analysis

DC Gain

95 dB

Gm

251 µA/V

Ro

240 M

Ω

L

Cout

Current sense

Logic

And

Driver

Slope

Compensation

PWM comparator

Error Amp

Rc

Cc

R1

R2

0.8V

High side

Switch

Low side

Switch

VIN