Functional description

ST1S31

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DocID022998 Rev 4

4.3

Error amplifier and control loop stability

The error amplifier provides the error signal to be compared with the high-side switch

current through the current sense circuitry. The non inverting input is connected with the

internal 0.8 V reference, while the inverting input is the FB pin. The compensation network is

internal and connected between the E/A output and GND.

The error amplifier of the ST1S31 device is a transconductance operational amplifier, with

high bandwidth and high output impedance.

The ST1S31 device 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 on

the next pages of this section.

In Figure 5 the simple small signal model for the peak current mode control loop is shown.

Figure 5. Block diagram of the loop for the small signal analysis

Three main terms can be identified to obtain the loop transfer function:

1.

2.

3.

Table 5. Characteristics of the uncompensated error amplifier

Description

Value

DC gain

94 dB

gm

238 µA/V

Ro

96 M



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

AM11418v1