"point"

1. Supply current to the load as a current source that uses the properties of an inductor (when current flows, it tries to maintain its state)
2. Square wave output from power supply IC is smoothed and converted to DC.

(1) Confirmation of inductor current

First, check the effect of the L value on the inductor current by simulation.
The circuit to be simulated is shown in Figure 1. For this circuit configuration, 2.2 μH is the proper value for L1, but here we will compare the following two values, which are extremely different, in order to clarify the difference.
Compare the inductor current with a relatively large value of 4.7 µH for inductor L1 and a relatively small value of 1.0 µH.

The simulation results are as follows.

The ripple current (ΔIL)​ ​is about 0.32A for 4.7µH and about 1.50A for 1.0µH.
It can be confirmed that the relationship between the L value and the ripple current (ΔIL) is inversely proportional.

(2) Confirmation of output voltage status

Next, change the L value and check the state of the output voltage.
The circuit to be simulated is shown in Figure 2. The circuit configuration is the same as in Fig. 1, but the comparison target is the output voltage. Again, compare inductor L1 values of 4.7​ ​µH and 1.0​ ​µH.

The simulation results are as follows.

​The ripple voltage is about 3.5mV for 4.7µH and about 19.8mV for 1.0µH.
It can be seen that a large inductor current affects the ripple voltage of the output voltage. To reduce the ripple voltage when the inductor value cannot be changed, the capacitance of the capacitor must be increased.

(3) Confirmation of transient response state

Now let's change the L value and check the transient response condition. The load fluctuation current conditions are simulated under the conditions of increasing from 0.5A to 1.5A in 0.1us, flowing 1.5A for 1ms, and decreasing from 1.5A to 0.5A in 0.1us.

The circuit to be simulated is shown in Figure 3. The basic circuit configuration is the same as Figures 1 and 2, but a current source is connected as a load to flow the load current. Again, compare the output voltages with inductor L1 values of 4.7​ ​µH and 1.0​ ​µH.

The simulation results are as follows.

As mentioned in the lamp current speed column of Table 1, a large inductor results in poor transient response (slow response speed). Comparing the output voltages for 4.7​ ​μH and 1.0​ ​μH during periods when the load current fluctuates abruptly, it can be seen that the 4.7​ ​μH case has larger fluctuations. From this we can see that the inductor value affects the state of the output voltage during transient response.

(4) Confirmation of increased load current

What if the load current is higher? Let's use a simulation to check what kind of phenomenon occurs when the load current is 2.8A, which is larger than the 1.5A mentioned earlier.

The circuit to be simulated is shown in Figure 4. Increase the current from 1A to 2.8A for a period of time using Load2 as the load. Again, compare the output voltages with inductor L1 values of 4.7​ ​µH and 1.0​ ​µH.

Before starting the simulation, as preliminary knowledge, we will explain the over current protection (OCP​) function when the output current of the power supply IC increases.
The overcurrent protection function is a function that stops the output of the power supply IC when the output current becomes abnormally large due to some accident such as an output short circuit. This prevents deterioration of power supply IC characteristics, malfunction, and damage due to excessive current. The method of detecting overcurrent and the method of stopping the output differ depending on the product.
In this simulation, the load current is large, so the overcurrent protection function of the power supply IC (this IC is a frequency foldback type: a method that limits the output current by lowering the oscillation frequency and reducing the minimum ON duty). may work.

For details on the overcurrent protection function, refer to the manuals (manuals, etc.) of each product.

(a) Inductor - simulation results for L1 = 4.7μH​

Immediately after the load current increases, the change in the load current is fast and the correction speed of the IC 's output voltage cannot keep up with it, so the voltage drops immediately after the load current increases rapidly, but the change in the inductor peak current is suppressed. , does not exceed the overcurrent protection threshold of the power supply IC. Therefore, the overcurrent protection function does not operate and normal operation continues.

(b) Inductor - simulation results for L1 = 1.0μH​

If the L value of the inductor is small, the ripple current increases, so if the load current suddenly increases, the inductor peak current also increases, the overcurrent protection function of the power supply IC is activated, and the output voltage drops. .

"point"

1. The relationship between L value and inductor ripple current (ΔIL) is inversely proportional.
2. A large inductor current affects the ripple voltage of the output voltage.
3. The magnitude of the inductor current affects the state of the output voltage during transient response.
4. If the L value of the inductor is small, the ripple current will increase, so if the load current increases, the inductor peak current will also increase, and the overcurrent protection function of the power supply IC will operate, resulting in an intermittent state with a low duty and a drop in the output voltage. do.