1. Pin Functions of LM5021 The LM5021 uses SOP-8 and DIP-8 packages. The pin arrangement is shown in Figure 1. The functions of each pin are as follows: COMP: PWM control input. An internal 5kΩ resistor pulls the COMP pin up to the 5V power supply. It is controlled by the output feedback voltage after optocoupler isolation. VIN: Internal bias circuit input. The internal regulator is activated when the input voltage reaches the threshold. This pin is clamped to 36V by an internal Zener diode. VCC: Internal bias circuit output. A capacitor must be connected between this pin and GND. Its output voltage is typically 8.5V. OUT: PWM control output. This pin is connected to the MOSFET driver. GND: Common ground. CS: Current monitoring pin. This pin is used for sampling the current-mode control signal and monitoring overcurrent signals. RT/SYNC: Clock signal input. An external resistor is connected between this pin and GND to set the internal crystal oscillator frequency. An external clock pulse signal can also be directly input. SS: Soft-start or "hiccup" mode timing input. The external capacitor connected between this terminal and GND determines the soft-start time and the "hiccup" mode restart frequency. 2. Working Principle of LM5021 The internal structure block diagram of LM5021 is shown in Figure 2. It includes a startup circuit, an oscillation circuit, a maximum duty cycle limiting circuit, a light-load comparator, a heavy-load monitoring and overload mode switching circuit, and a pulse width modulation circuit. 2.1 Startup Circuit Before the switching power supply circuit starts working, LM5021 has no PWM output, and the auxiliary winding cannot charge the startup capacitor CIN. It must rely on an external circuit to trigger the startup capacitor first. The high voltage HV obtained after AC rectification charges capacitor CIN through the startup resistor Rs. When the voltage on CIN reaches 20V, the internal voltage regulator starts working and begins to charge capacitor CVcc. When the voltage on CVcc reaches 7V, the internal bias circuit allows PWM output, completing the energy transfer from the primary side to the secondary side of the transformer. At the same time, the auxiliary winding charges CIN, thus completing the circuit startup process. If the energy stored in the auxiliary winding is sufficient to support the VCC voltage (greater than 5.8V), the circuit can enter normal operating mode. The value of the starting resistor affects the startup time and power supply efficiency. The extremely low startup current of the LM5021 allows for a large starting resistor, thus ensuring a reasonable startup time and high efficiency for the startup circuit. 2.2 The signal representing the error voltage is input from the COMP terminal, first biased by 1.2V, then divided by a 3:1 resistor, and finally input to the inverting input of the PWM comparator. The current ramp signal is input to the non-inverting input of the comparator. The PWM comparator compares these two signals and outputs a pulse width modulation signal. Simultaneously, the PWM control logic and OR gate reset the flip-flop and output a turn-off signal. Then, the clock pulse sets the flip-flop to output an on signal, thus completing the pulse width modulation function. This circuit is called a dual-pulse suppression logic circuit, which ensures that the pulse width modulator outputs only one pulse width modulation signal within one clock cycle. When the error controller input is 0V, the controller outputs a 0 duty cycle signal. 2.3 Overcurrent Comparison Circuit: The LM5021 provides overcurrent protection. This function is accomplished by an internal current comparator. When the input of the current comparator exceeds 0.5V, the output pulse is immediately blocked. The CS pin will remain low for 50ns before the MOSFET is turned on again to release the low-pass filter. 2.4 Oscillator: Connecting a resistor between RT and GND of the LM5021 sets the circuit's oscillation frequency. For the LM5021-1, the oscillation frequency f = 10¹²/RT * 158, while for the LM5021-2, the oscillation frequency f = 10¹²/RT * 316. The LM5021 can also be synchronized by an external clock; a 100pF capacitor must be coupled between RT and GND, and a peak voltage greater than 3.8V is required as a synchronization head. Because the crystal oscillator is very sensitive, the timing resistor must be placed close to RT and GND during PCB layout. 2.5 Soft Start and Gate Drive The purpose of soft start is to allow the power conversion circuit to slowly reach its steady-state operating point, thus reducing startup impact and inrush current. Its working principle is that an internal 20μA current source charges the capacitor connected to the SS terminal, causing the voltage ramp on the capacitor to rise, thereby limiting the rate of rise of the COMP voltage and ultimately limiting the rise amplitude of the output pulse duty cycle, thus achieving the soft start function. When the output load is too large, the soft start capacitor is also used to generate a hiccup mode delay. The LM5021 outputs a PWM signal at OUT. When the output is high, the current draw can reach 0.35A, and when the output is low, the current sink can reach 0.7A. The maximum duty cycle of the LM5021-1 is 80%, and the maximum duty cycle of the LM5021-2 is 50%. 3 Application of LM5021 A portable charger is designed using the LM5021-1. The main circuit topology is a single-ended flyback configuration, and its working principle is shown in Figure 3. The input parameters are 50Hz AC mains power, ranging from 85V to 265V, and the output is 12V/2A. The voltage regulation is ±0.1%, the load regulation is ±0.14%, the output voltage ripple is 120mV, and the output power is 24W. The high-frequency transformer used in this power supply is an Epcos E25/13/7, made of N27 steel, with a vertical frame.