1 Introduction: Anomalous Frequency Drop in the LM5190

The LM5190 is a widely adopted Texas Instruments synchronous buck controller featuring a wide input voltage range. Distinguished by its high-precision constant-voltage/constant-current (CV-CC) control loop, it is extensively deployed in applications such as supercapacitor energy backup, energy storage systems, USB Power Delivery, server battery backup units (BBUs), and power supplies for audio amplifiers. A typical system design of LM5190 cooperating with TAS6754 for Class-H operation is shown in Figure 1-1.

The LM5190 selects whether to work in the CC loop or the CV loop through the IMIN Selector. When the load current does not exceed the limit and is far from the set ISET, the LM5190 will work in the CV loop. At this time, when the load current is significantly greater than the DCM boundary value, the LM5190 should theoretically work in PWM mode, and the output voltage frequency is constant at the set value set by RT. However, during actual testing, it was noted that when the load current is not large enough, the LM5190 output voltage exhibits significant frequency reduction, as shown in Figure 1-2. At this time, the LM5190's resistor is the EVM default valueof 54.9k, and the set output frequency is 433kHz, but the output frequency in Figure 1-2 is 148.5kHz, far lower than the set value.