LED Flexi Strip
30/09/2013 09:21
The role of the error amplifier is sampled feedback voltage , outputs a control signal is then input LED Bulb to the PWM comparator to control the size of the peak current . LED work , due to process variations , each one on the LED forward voltage drop will not be the same , so that each channel LED voltage sampling point voltage will not be the same size . In order to ensure that each channel LED can work , the circuit should be the minimum sampling voltage signal input to the error amplifier, and the reference voltage . This error amplifier has a self-selection function , the circuit shown in Figure 4 .
Figure 4 error amplifier. LED Module
Since the PMOS input pair of the bias current will flow all the way to the lowest of the gate voltage , the gate voltage is relatively high will be the other three-way shut -off , to ensure the circuit to work at the inverting input of the error amplifier is only one transistor at work .
Can be seen from Figure 4 , the circuit is a single stage folded cascode structure, which has a high output resistance , to ensure a high voltage gain of the circuit . Combined with a basic knowledge of analog integrated circuits , circuit static gain can be obtained :
Where , gm is the input differential pair transconductance , / / representative resistors in parallel , gm14 and gm35 transistors MN14 and MP35 are the transconductance , ro1, ro14, ro35, ro30 were differential input pair and MN14, MP35, MP30 the output resistance.
This error amplifier has only one dominant pole , the output of the amplifier , the output of which the size of the resistors and capacitors , its size by p :
