Have you figured out the working status of UC284×?
Understanding how the UC284× works involves knowledge of analog and digital electronics. Today, let's break it down together; hopefully, you'll find it helpful.
Figure 1 shows a representative block diagram of the UC284×. Since each external condition can affect the output, we try to work backward from the output, deriving the corresponding input relationships based on the conditions for the switching transistors to turn on and off.
Figure 1
First, let's analyze the output circuit of the driver. Referring to Figure 2, the driver output consists of an OR gate and a push-pull circuit. When the switch is off, the push-pull output is low, the inverted OR gate output is low, and the non-inverted OR gate output is high. At this time, the OR gate input is high. This means that as long as the OR gate input is high, the switch will definitely be off; conversely, if it is low, the switch will be on. The condition is quite simple.
Figure 2
Let's analyze the inputs of the OR gate. The inputs of the OR gate include: 1. Undervoltage lockout; 2. Oscillator output; 3. PWM latch output. Under what conditions do their high-level signals (switch off) occur?
Figure 3
Figure 3 shows the output of the reference undervoltage lockout section. When the power supply voltage of the UC284× is lower than the power supply undervoltage lockout value (the upper and lower thresholds of the VCC comparator are: UC2844 16V/10V; UC2845 8.4V/ 7.6V , respectively), that is, the output of the reference and regulator is lower than 3.6V , the reference undervoltage lockout output is low, which becomes high after inversion. What does this mean? It means that output is not allowed when the power supply voltage is insufficient. When the circuit is working normally, the reference undervoltage lockout has no effect on the output of the UC284×.
Figure 4
Figure 4 shows the oscillator output. When the timing capacitor is charging, the oscillator outputs a low potential; when the timing capacitor is discharging, the oscillator outputs a high potential (the above principle is fully explained in the UC284× datasheet). This means that the switching transistor is allowed to conduct when the timing capacitor is charging, but not when it is discharging.
Figure 5
Figure 5 shows the PWM latch. When the oscillator outputs a high level, the PWM latch is set, and its inverting output is low. When the current sensing comparator outputs a high level, the PWM latch is cleared, and its inverting output is high. When the voltage sent from the error amplifier to the input (-) of the current sensing comparator is greater than the current feedback voltage, the current sensing comparator outputs a low level, and the output of the UC284× is high under normal conditions, allowing the switching transistor to conduct. When the voltage sent from the error amplifier to the input (-) of the current sensing comparator is less than the current feedback voltage, the current sensing comparator outputs a high level, the PWM latch is cleared, its inverting output is low, and the output of the UC284× is low under normal conditions, turning off the switching transistor.
Currently, it has been found that the product specification sheet contains an error in the labeling of the inverting and non-inverting inputs of the current sensing comparator, as shown in Figure 6. The correct labeling should be: the error amplifier input to the inverting input of the current sensing comparator should be the error amplifier input, and the current feedback voltage input to the non-inverting input should be the current feedback voltage input.
Figure 6
In summary:
When the power supply voltage is low, the switching transistor is not allowed to conduct. When the voltage is normal, the switching transistor's on/off state is unaffected by the power supply voltage.
The switching transistor is allowed to conduct when the timer capacitor is charging, but not when the timer capacitor is discharging.
When the voltage supplied by the error amplifier to the input terminal (-) of the current detection comparator is greater than the current feedback voltage, the switch is allowed to turn on; when the voltage supplied by the error amplifier to the input terminal (-) of the current detection comparator is less than the current feedback voltage, the switch is turned off.
Disclaimer: This article is a reprint. If it involves copyright issues, please contact us promptly for deletion (QQ: 2737591964). We apologize for any inconvenience.
Disclaimer: This article is provided by the company. If it involves copyright or confidentiality issues, please contact us promptly for deletion (QQ: 2737591964). We apologize for any inconvenience.
