Application case for 12V to 5V TPS54339DDAR device
Author: Sveinn Jia
TPS54339 is a synchronous rectification BUCK Converter launched by TI in 2013 based on D-CAP control mode, input voltage 4.5V-23V, 3A, and is widely used in low-voltage systems. This article mainly introduces a fault case. Through the analysis of this case, it gives the FCCM mode device in D-CAP control mode. When the backup power supply voltage is higher than the BUCK preset output voltage, the risks, experimental test results, and avoidance of the problem are given. risk approach.
The customer uses the TPS54339DDAR device in FCCM mode for 12V to 5V conversion and has a need for backup power. The backup power is connected to the output bus of BUCK through switch circuit S1. When the TPS54339 input voltage is detected to be lower than 10V, the switch circuit S1 is closed and the system is powered by the backup battery.
Figure 1: System framework diagram
The output voltage set by BUCK is called V_target
The actual output voltage at the BUCK terminal is called V_out;
The backup battery voltage is called V_backup;
According to the TPS54339 manual, on the falling edge, the chip will only shut down when the EN voltage is lower than 0.6V, and the chip will stop working when the UVLO is lower than 3.45V. When the backup power is turned on, both EN and UVLO of the chip are enabled, so the chip is in normal working condition. The rated voltage of the backup battery is 5.4V, and the battery voltage can be charged up to 5.6V. After passing through the switch circuit S1, the voltage reaching the BUCK bus may be as high as 5.3V. This results in V_out > V_target and the TPS54339 is enabled. In the FCCM mode of D-CAP, this will cause the input side voltage to increase, causing overvoltage breakdown and short circuit on the input side of the chip. The fault manifests itself as a short circuit between pins 1/2/3/6 of TPS54339 to ground.
Figure 2: TPS54339 block diagram
root cause analysis
Assuming that VIN can establish a stable voltage, then the entire BUCK converter must meet the volt-second balance and inductor current balance to operate stably.
Since V_out > V_target, the lower side of BUCK will continue to be turned on until the chip's NOC (negative current) protection is triggered.so
Due to stable operation, the following equation can be obtained by combining the above equation:
According to Faraday’s law of electromagnetic induction, under working conditions,
It will be a very small value, so there is a risk of high voltage at the VIN end.
Using TPS54339EVM-056 as the test board, only the R1 resistance was modified from 8.25KΩ to 120KΩ. Add a certain voltage to the output terminal and test the input terminal voltage. At 12V input, the rated no-load output voltage is 5.25V.
Input and output terminal no-load experiment
Adjust the output voltage to 5.26V-5.32V. It can be seen that the voltage at the input terminal is as high as 33V, which has exceeded the maximum withstand voltage of the TPS54339 VIN pin of 25V and may cause permanent damage to the device.
Table 1: No-load test results of BUCK input and output terminals
Input terminal voltage (V)
Backup voltage (V)
Figure 3: When the V_backup voltage is 5.32V, different pin waveforms of TPS54339
Input end no-load experiment, output 0-3A load experiment
The test results are consistent with the input and output terminals being unloaded. When doing actual experiments, you need to pay attention to the line loss voltage, because a voltage drop of 0.01V will have a great impact on the voltage value at the input end.
Input terminal 30mA load experiment, output no-load experiment
Considering that in the actual system, the input end of TPS54339 may also be loaded. Here, take the TPS54339 input terminal with a 30mA load as an example. The test results are as follows:
Table 2: BUCK input end 30mA load, output end no-load test results
Input terminal voltage (V)
Input load current (mA)
Backup voltage (V)
When FCCM devices in D_CAP control mode are used in systems that require backup power, it should be noted that the voltage of the backup power supply cannot be higher than the preset voltage of BUCK, otherwise high voltage may occur on the input side.
For systems where the backup power supply voltage is higher than the BUCK preset voltage, it is recommended to choose a device with light load frequency modulation mode, such as TPS54339EDDAR, or control the EN pin so that when the input power is lost, EN can be quickly turned off to stop the chip. If you are not very concerned about system efficiency, you can also connect a diode in series after the BUCK output to prevent current backflow.
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