How Overcurrent Protection Works in the MAX77812
Abstract
The MAX77812 is a quad-phase, high-current, step-down (buck) converter for high-end gaming consoles, VR/AR headsets, DSLR cameras, drones, network switches and routers, and FPGA systems that use multi-core processors. This application note explains the MAX77812 overcurrent protection scheme and provides I_{PLIM}/I_{VLIM} selection guidelines for a given maximum load current.
Introduction
The MAX77812 provides cycle-by-cycle peak and valley current limit protection by monitoring the current through high- and low-side MOSFETs. The programmable peak current limit (I_{PLIM}) and valley current limit (I_{VLIM}) allows the customer to set the inductor current limit based on the application.
Overcurrent Protection Scheme
When the output is a short to ground, the output voltage collapses and the average inductor current increases rapidly, which results in hitting the I_{PLIM} threshold. When the fault condition persists, the inductor current can increase the staircase beyond the I_{PLIM} threshold due to the minimum on-time requirement.
In order to address this issue, the MAX77812 introduces the I_{VLIM} threshold. When the inductor current reaches the I_{PLIM} threshold, the high-side MOSFET turns off immediately, allowing the inductor current to discharge its energy through the low-side MOSFET. Until the inductor current falls down to the I_{VLIM} threshold, the high-side MOSFET is not allowed to turn on. Thus, the short circuit current is limited way below the I_{PLIM} threshold.
In the event of a short circuit, the MAX77812 does not terminate its operation. When the output is a short to ground, the output voltage collapses, and a POK interrupt is generated by the MAX77812. Then, an application processor or a microcontroller unit can handle this interrupt signal to recover the system.
Figure 1 shows the overcurrent protection scheme.
I_{PLIM} and I_{VLIM} Selection
The MAX77812 supports the programmable I_{PLIM} and I_{VLIM} thresholds. Each master has its own configuration registers, and there are eight options of I_{PLIM}/I_{VLIM} pairs, as shown in Table 1.
Mx_ILIM[2:0] in Mx_CFG Registers | I_{PLIM} | I_{VLIM} | I_{LOADMAX} |
000b | 3.0A | 2.0A | 2.5A |
001b | 3.6A | 2.4A | 3.0A |
010b | 4.2A | 2.8A | 3.5A |
011b | 4.8A | 3.2A | 4.0A |
100b | 5.4A | 3.6A | 4.5A |
101b* | 6.0A | 4.0A | 5.0A |
110b | 6.6A | 4.4A | 5.5A |
111b | 7.2A | 4.8A | 6.0A |
*POR Default |
The following equations and examples are guides to choosing the optimal I_{PLIM}/I_{VLIM} setting. Consider the maximum required load current to determine the I_{PLIM} thresholds. The following equation shows how to decide a proper I_{PLIM}:
where I_{LOADMAX} is the maximum load current and ?I_{L} is the inductor current ripple.
Derive the inductor current ripple by using the following equation:
For example, a device has the following operating conditions:
The calculated inductor current ripple is 1.675A. If the maximum required load current is 5.0A, I_{PLIM} should be higher than 5.84A, as shown by the following calculation:
Then, the I_{PLIM}/I_{VLIM} pair can be set to 6.6A/4.4A with some margin.
When the output short occurs, the maximum output current is limited by the I_{PLIM}/I_{VLIM} thresholds, as expressed by the following equation:
For example, when the I_{PLIM}/I_{VLIM} thresholds are set to 6.6A/4.4A, the maximum output current is limited to 5.5A.