Introduction: DIY DC-DC Over-Current Protection

About: GreenPAK™ is a broad family of cost-effective NVM programmable devices that enable innovators to integrate many system functions into a single custom circuit.

This Instructable is a user guide, which explains how to use the SLG46585 DC/DC OCP feature if output voltage is 3 V or 3.3 V.

Below we described steps needed understand how the solution has been programmed to create the DC-DC over current protection. However, if you just want to get the result of programming, download GreenPAK software to view the already completed GreenPAK Design File. Plug the GreenPAK Development Kit to your computer and hit program to create the DC-DC over current protection.

Step 1: Background

When output voltage of DC/DC is 3 V or 3.3 V and the over current protection feature is enabled, there is a specific application circuit that is recommended. During operation, if the inductor current exceeds the OCP threshold, then internally, a high side MOSFET will turn off for 2 us, and the INT output becomes HIGH during the same 2 us.

2us after the high-side MOSFET is turned off and if the VOUT is below the target output voltage, the high side MOSFET will turn on again. At this point, If the inductor current is still greater than the OCP threshold, the high side MOSFET will turn off. This loop will repeat until the OCP event is removed (when the inductor current drops below the OCP threshold), then DC/DC output voltage will finally recover.

However, if DC/DC output voltage is set 3 V or 3.3 V, the DC/DC cannot recover. This happens because of the internal low-side MOSFET. During operation, the low side MOSFET is opened during INT and a capacitor discharges through this MOSFET, meaning the inductor current becomes negative (the inductor’s current flows from the output capacitor, C3 and C4, to the power supply). See Figure 2 channel 2 for an example. Due to this, the DC/DC can’t recover even if there is no output load. For a given input of 5V, the output will become stuck at 2.5 V and the DC/DC consumes approximately 200 mA.

The recommended workaround is to use the typical application circuit shown in Figure 1 and the internal design circuitry shown in Figure 3.

In the Figure 2:

Channel 1 (yellow/top line) – V_OUT

Channel 2 (light blue/2nd line) – PIN#11 (DC_SW)

Channel 3 (magenta /3rd line) – PIN#7 (DC_INT)

If DC/DC output voltage is set to 3 V or 3.3 V, it is recommended to use the application circuit shown in Figure 1 and Figure 3. When the over current condition occurs, CNT1 will start to detect INT edges. If CNT1 counts more than eight interrupts (CNT1 counter data + 2) within a period of time set by CNT 2, the CNT1 will generate a pulse which launches DLY3. DLY3 will generate a one-shot pulse, which turns off the DC/DC. This one-shot pulse also resets CNT2, which, in turn, resets the CNT1. The purpose of CNT2 is to create a window. It will periodically reset CNT1.

When the DC/DC is turned on or a load transient occurs, there can be a few interrupt pulses (usually 1 or 2). Therefore, CNT1 should have a count of at least 3 to filter out false over current conditions. The time of one-shot (DLY3), periodical reset (CNT2) and number of counted interrupts (CNT1) can be adjusted.

Since PIN#7(DC_INT) is open drain, add a pull up resistor. In the design PIN#5(INT) is configured with 10k pull up resistor and the two pins PIN#7(DC_INT) and PIN#5(INT) are tied together.

In the Figure 5:

Channel 1 (yellow/top line) – PIN#11 (DC_SW)

Channel 2 (light blue/2nd line) – CNT3 OUT/DC-DC ENABLE

Channel 3 (magenta /3rd line) – PIN#7 (DC_INT)


This Instructable explains how to use the OCP in SLG46585's DC/DC, when DC/DC output voltage is set to 3 V or 3.3 V. Here is a typical circuit which is recommended to use with the DC/DC.