2 Analysis of LM5190 Operating Mechanism

The LM5190 utilizes a peak current mode control architecture. By directly sensing the peak inductor current, this control scheme of LM5190 enables precise regulation of the power switch, serving as the foundation for the device's stable operation. The core control logic is connecting external sampling resistor Rs through ISNS+ and VOUT pins to achieve real-time sampling of inductor current. After the sampling signal is amplified by the internal amplifier (typical gain is 10V/V), it is compared with the compensation voltage output from the COMP pin. When the sampled current reaches the current peak corresponding to the COMP voltage, the PWM comparator controls the turn-off of the high-side switch and the turn-on of the low-side switch, completing the control of this switching cycle. The control loop is shown in the section highlighted by the red box in Figure 2-1.

The LM5190 configures two different operating modes through the FPWM/SYNC pin to adapt to working scenarios with different loads. The LM5190 data sheet describes that when its FPWM/SYNC pin is connected to VCC, the LM5190 will operate in FPWM mode; and when the FPWM/SYNC pin is connected to AGND, the LM5190 will operate in PFM mode. The working principle of PFM is as follows: the COMP voltage is proportional to the load current and the peak inductor current. When the load current decreases, causing the COMP voltage to drop below a certain value, the device clamps the COMP voltage and simultaneously turns off both the high-side and low-side switches, entering an inactive state. At this time, the output capacitor supplies power to the load. When the output voltage drops to a certain value, the device detects this drop through the FB pin and automatically raises the COMP value to reactivate the high-side switch. Once the COMP voltage drops back to the clamped value corresponding to the current load current, both switches are turned off again, completing one switching cycle.

The PFM mode will greatly improve the efficiency of the LM5190 when working at light load. The efficiency of the LM5190 when working in FPWM and PFM respectively is shown in Figure 2-2.

In theory, the control logic of eco mode is not complex. From the aforementioned content, it is known that the peak inductor current of the LM5190 is proportional to the COMP voltage. When the peak inductor current is less than a certain value, that is, when the COMP voltage is lower than a certain value, the LM5190 will enter eco-mode. The measured relationship between the LM5190 peak inductor current and COMP voltage versus load current is shown in Figure 2-3. As you can see,

1. when the load current decreases, both Vcomp and ILpeak will decrease. For the LM5190, when Vcomp is less than about 0.3V, it will clamp Vcomp at 0.3V and enter the working state of eco-mode.
2. As the load decreases, there will be several turning points in ILpeak. The more obvious ones are when the load current is 2.251A, 1.849A, and 1.608A. At these three turning points, as the load current decreases, Vcomp will decrease accordingly, but ILpeak will first decrease and then increase. The reason is that the LM5190 has a Minimum peak current limit threshold. When the peak current of the LM5190 is less than the minimum value, the LM5190 will increase the charging period of the upper tube by reducing the CLK frequency to raise the ILpeak value. The minimum peak inductor current can be calculated by Equation 1.

As shown in the figure, in the actual test of the LM5190 EVM, the minimum peak current constrained by the LM5190 is about 2.7A. The actual inductor current ripple can be calculated by Equation 2. From this equation, it is known that with the output frequency unchanged, as Iload decreases, Ilpeak will decrease. Combining with Figure 2-3, it is known that when Ilpeak is about to be lower than the minimum peak current, i.e., 2.7A, the LM5190 will increase Ilpeak by reducing the CLK frequency, keeping Ilpeak clamped at 2.7A. Only after Vcomp is clamped at 0.3V and completely enters PFM, will it stop this compensation for Ilpeak and reduce the peak of its peak current control. This compensation mechanism is also the essential reason for the LM5190 frequency reduction, PFM/PWM hybrid operation, and multi-peak current control at critical points described later.

In the following chapters, several typical transition point phenomena will be presented and analyzed in order of load current from large to small. Between the first and second transition points, the LM5190 exhibits a typical frequency reduction operating state, which will be introduced in Chapter 3; between the second and third transition points, it also operates in this state. After the third transition point and before fully entering PFM, the LM5190 works in a hybrid PFM/PWM operating state, which will be introduced in Chapter 4. Finally, after Vcomp is clamped at 0.3V, it fully enters the PFM operating state, which will be introduced in Chapter 5.