An error amplifier is most commonly encountered in feedback unidirectional voltage control circuits, where the sampled output voltage of the circuit under control, is fed back and compared to a stable reference voltage. Wikipedia
An error amplifier is essentially what its name says, that is, it amplifies an error signal. This error is based on the difference between a reference signal and the input signal. It can also be treated as the difference between the two inputs. These are usually used in unison with feedback loops, owing to their self-correcting mechanism. They have an inverting and a non-inverting input pin set, which is what is responsible for the output to be the difference of the inputs.
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Schematic diagram of voltage error amplifier in PWM IC unit
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COMP — Output of error amplifier. An external resistor and capacitor network is typically connected from this pin to ground to provide loop compensation.
Fb — Inverting input to the error amplifier. This pin is connected directly to the output of the regulator via resistor divider to set the output voltage and provide feedback to the error amplifier.
Vp — Non-inverting input of error amplifier. This pin can be used for tracking application.
Soft-Start
The IR3638 has programmable soft-start to control the output voltage rise and limit the inrush current during start-up.
To ensure correct start-up, the soft-start sequence initiates when Vcc and Enable rise above their threshold and generate the Power On Ready (POR) signal. The soft-start function operates by sourcing current to charge an external capacitor to about 3V.
Initially, the soft-start function clamps the output of error amplifier by injecting a current (64uA) into the Fb pin and generates a voltage about 1.6V (64ux25K) across the negative input of error amplifier (see figure 6). The magnitude of the injected current is inversely proportional to the voltage at the soft-start pin. As the soft-start voltage ramps up, the injected current decreases linearly and so does the voltage at negative input of error amplifier. When the soft-start capacitor is around 1V, the voltage at the negative input of the error amplifier is approximately 0.8V (32uAx25K).
As soon as the Fb reaches the Vp voltage, the output of error amplifier will start increasing and generating the first PWM signal. As the soft-start capacitor voltage continues to ramp up, the current flowing into the Fb pin will keep decreasing.
The feedback voltage increases linearly as the soft start voltage ramps up. When soft-start voltage is around 2V the output voltage is reached the steady state and the injected current is zero.
Figure 7 shows the theoretical operational waveforms during soft-start.
The output voltage start-up time is the time period when soft-start capacitor voltage increases from 1V to 2V.
The start-up time will be dependent on the size of the external soft-start capacitor and can be estimate by.
Soft-Start Programming
The soft-start timing can be programmed by selecting the soft-start capacitance value. The start-up time of the converter can be calculated by using:
Where Tstart is the desired start-up time (ms) For a start-up time of 5ms, the soft-start capacitor will be 0.1uF. Choose a ceramic capacitor at 0.1uF.
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The error amplifier is the heart of the power management IC’s which indicates the accuracy of the entire circuitry. The specifications are considered which is suitable for power management applications.
INTRODUCTION TO ERROR AMPLIFIERS
In all power management ICs, there will be a negative feedback system that will perform the desirable control actions using operational amplifiers. The control ICs for power factor corrections make use of error amplifiers for the generation of the necessary template for shaping the input current. The functional block diagram is shown in Figure 3.1 where in, an error amplifier compares the output voltage with a reference generating the required magnitude of the error signal. The error signal, being the function of variation in output voltage, is multiplied with the sinusoidal current template thereby the magnitude of the current template is set. The current template so generated is compared with a ramp signal in an op-amp to generate the necessary gate signals for the converter. From the above discussion it is very clear that operation amplifiers are essential components in any power management ICs. The ability of the controller to shape the input current in the form of a sine function is governed by the specifications of op-amps such as DC gain, UGB, slew rate, PSRR, CMRR. The error amplifiers used in commercial chips are not optimized and the design guidelines are often proprietary in nature. The present scope of the research is to design a power factor controller with all the necessary control features. In this paper, guidelines to design an error amplifier with the desirable specifications is presented.
BASIC STRUCTURE OF TWO STAGE OPAMP
Figure 3.2 shows the block diagram for a two stage opamp. The first stage consists of differential amplifier [6] [7], followed by the second stage which is a common source amplifier. A differential amplifier being a single stage opamp with single pole role off, has insufficient gain for any practical applications in power management IC’s. Hence two stage operational amplifiers are needed to boost the gain so as to meet the requirement. The basics of two stage opamps are discussed in further paragraphs, highlighting the design aspect.
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https://www.youtube.com/watch?v=gIZW6jJe8Tc
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http://www.rom.by/files/sc6105.pdf
