3 Turning point 1~3: Analysis of LM5190 Frequency Drop Operation

During the experimental test, adjusting the load current to 2.264A and 2.224A respectively yielded the experimental results shown in Figure 3-1 and Figure 3-2.

As shown in Figure 3-1 and Figure 3-2, at the first transition point, that is, when the load current is greater than 2.264A, the LM5190 operates normally in PWM mode. If the load current is adjusted to be lower than 2.264A, combining Equation (1) and Equation (2), it can be known that if the set switching frequency of 433kHz (EVM default) is still maintained, the ILpeak of the LM5190 will be less than its set minimum value of 2.4A (in this experiment, this value is 2.6A). At this time, the LM5190 will compensate the ILpeak value by reducing the CLK frequency, significantly increasing the ILpeak value even when the load current decreases only by a small amount. Furthermore, as can be seen from Figure 2-3, at this time Vcomp continues to decrease as the load current decreases; this compensation is not achieved by increasing Vcomp.

Since the LM5190 adopts peak current control mode, in this control method, the turn-off logic for the device's high-side switch is that the inductor current reaches the peak value, while the turn-on logic is that the CLK signal is at a high level, setting the high-side switch to turn on via an RS flip-flop. Therefore, when the CLK frequency of the LM5190 decreases, both the positive half-cycle and the negative half-cycle of the entire switching period are extended, thereby allowing the IL peak value to be raised so that it is greater than the limited minimum value.

Between the second transition point and the third transition point, the LM5190 also operates in a state where the operating frequency is reduced due to the compensation of the peak value in peak current control. Furthermore, when the load current is located between the load currents corresponding to two peak currents, as shown in Figure 8 for the case where the load current is 1.855A, at this time the load current is located between 1.876A and 1.849A. As known from Figure 2-3, when the load current is 1.876A and 1.849A, the corresponding peak currents are 2.8A and 3A respectively, leading to a phenomenon where, when the load current is 1.855A, the current peak appears to be controlled by alternating uncertainly between the two values. At this time, the frequency of the output voltage becomes more uncertain. As shown in Figure 3-3, it can be clearly seen that at this time the peak current control of the LM5190 exists with two peak values of 2.7A and 3.0A.

When the load current is 1.742A and 1.876A, the operating state of the LM5190 is as shown in Figure 3-4 and Figure 3-5. As can be seen from Figure 3-4 and Figure 3-5, the peak currents of the LM5190 at load currents of 1.742A and 1.876A are 2.7A and 3.0A, respectively. Therefore, it can be known that when the load current is 1.835A, the LM5190 is in an intermediate or indeterminate state between the load currents of 1.742A and 1.876A. Its non-fixed CLK frequency leads to the two peak currents being the peak currents of the LM5190 when the load currents are 1.742A and 1.876A.