Trying Out the ROHM Solution Simulator (2)

In succession to “(1) Simulation of the Frequency Characteristics of a Step-down Switching Regulator”, in this part (2) two examples of simulations of transient response characteristic waveforms are presented. In these examples, the Solution Circuits provided by ROHM are modified somewhat. Persons wanting to quickly learn about simulator operation should consult the “Hands-On User’s Manual（PDF）“.

Trying Out the ROHM Solution Simulator
(2)： Simulation of the Transient Response Characteristic Waveform of a Step-down Switching Regulator

As in Part (1), proceed to the ROHM Solution Simulator web page, open “Switching Regulators” in the “ICs Solution Circuit” category, and click on the “Time Domain” “Simulation” button for the BD90640EFJ, shown in Fig. 1.

Fig. 1. “Time Domain” “Simulation” button for the BD90640EFJ

Upon doing so, similarly to part (1), a simulation circuit is displayed; when the Run symbol (▶) in the center is clicked, the simulation screen appears (Fig. 2).

Fig. 2. Time Domain simulation screen for the BD90640EFJ

This simulation circuit is the same as the basic circuit in part (1); in the simulation, the switching node voltage and output voltage startup waveforms at the time the input voltage is applied are monitored. This is modified and the load transient response is simulated to confirm the output current and output voltage waveforms. To do so, the following modifications are made.

• 1) In order to impart a transient load, a pulsed current source (Current Source – Pulsed) component is added to the output node, and the conditions of the pulsed current source are set.
• 2) To monitor the load current, a current monitor (Current Monitor) component is added to the output node.
• 3) The load resistance Rload is changed according to the simulation.
• 4) The simulation time is changed according to the simulation.

These modifications involve component additions and other changes to the provided Solution Circuit; however, in the ROHM Solution Simulator, only the component parameters can be changed. Hence in order to make these modifications, it is necessary to migrate to “PartQuest™ Explore”, which is the platform of the ROHM Solution Simulator. The migration is simple, and merely involves clicking the “Edit in PartQuest Explore” button surrounded by the red oval in the lower-right corner of the simulation screen in Fig. 2. Upon migration, a screen like that in Fig. 3 appears. Component selection options appear in the left side menu; select those that are necessary and expand them on the screen to modify the circuit diagram.

Fig. 3. Screen after migration to “PartQuest™ Explore”, which is the ROHM Solution Simulator platform

Go ahead and perform the modifications 1) to 4) required for this simulation. Below, details are explained separately in advance, but begin by executing the simulation according to the example.

In Fig. 3, a pulsed current source (Current Source – Pulsed) component and a current monitor (Current Monitor) component have already been dragged from the menu on the left and placed in the circuit diagram. These components are in Generic Components > Analog Electronics in the menu on the left.

Fig. 4 is a circuit diagram in which the pulsed current source is connected so as to be the load, the current monitor is inserted into the output line, and the blue probe is connected to monitor the output voltage and the red probe is connected to the current monitor component to monitor the output current. Please refer to 1) to 4) below to modify the circuit.

Fig. 4. Simulation circuit example modified for simulation of output transient response

1) Add the pulsed current source to the circuit. The current must be sunk, so the arrow indicating the current direction is reversed so as to point downward (↓). Upon clicking on the component, a reverse/rotate function icon appears. When the cursor is applied to existing wiring, the cursor changes to “+”; wiring can be drawn by left-clicking and moving the cursor. After addition, double-click on the pulsed current source to set the conditions as in Fig. 5. In the numerical values, “m” represents milli- and “u” stands for micro-.

2) The current monitor is inserted simply by dragging to the insertion position. Reverse the right-level direction so that the icon arrow indicates left (←). Upon dragging the red probe to the current monitor, a small window opens; select the topmost item “i” (Fig. 6).

3) The value of Rload is changed as in Fig. 7.

4) Change the simulation time. Click on the settings button on the left side of the Run button (▶ ) in the upper part of the screen, and set the time as in Fig. 8.

This concludes the simulation changes and condition settings. Click the Run button (▶) to begin the simulation. When a window opens asking if you want to save the simulation, click the checkbox (?) for now. Some time is required to run the simulation; the simulation progress is displayed in percent.

When the simulation ends, results such as those in Fig. 9 are displayed. The simulation results are for a total of 4 ms in which, after input voltage turn-on, the output rises to 5 V, a pulse load is added to the steady load of Rload 2 ms after startup, and 1 ms later the pulse load ends and the load returns to the steady load. It should be clear that the results are concordant with the settings used for the pulsed current source, Rload, and the simulation.

A slight amount of fluctuation in the load transient response of the output voltage can be seen, but to improve clarity, we will change the graph range. Right-click around the scale on the horizontal axis and drag to specify the range to display. The output voltage range can similarly be specified using the vertical axis (Fig. 10). As a result a graph like that in Fig. 4 is obtained, and the output voltage response can be studied. Functions are displayed by right-clicking on the graph; feel free to experiment. The simulation guide(PDF） for the IC will prove useful.

Fig. 10. Changing the range of the simulation results graph to improve clarity

The present example was provided in order to enable the reader to try using the simulator. Various functions and other methods of operation will be explained separately.