Design of overvoltage protection circuit in TI chip Buck-Boost circuit

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Design of overvoltage protection circuit in TI chip Buck-Boost circuit [Copy link]

TPS92692-Q1 is an LED driver chip for automotive lighting. It is widely used in automotive LED lights because it can be designed into various topologies such as Boost, Buck-Boost, SEPIC, etc.
When designed into Buck-Boost topology, since the LED-end is connected to the input voltage VIN, the voltage across the LED is the voltage difference between the Buck-Boost circuit output and the input voltage VIN, which brings problems to the design of the OVP circuit.

Figure 1 TPS92692-Q1 Buck-Boost Circuit
From the data sheet 1, we can see that the action voltage VOV (THR) of the OV pin of TPS92692-Q1 is 1.228V. This voltage is the voltage of the OV pin to GND. In fact, what we need is OVP protection for the voltage at both ends of the LED string. Therefore, we need to convert the voltage at both ends of the LED into a voltage signal to GND.

Table 1 TPS92692-Q1 output voltage protection parameters
As shown in the red box in Figure 1, the above function can be achieved by adding a simple PNP transistor circuit.
Let us first briefly analyze the working principle of this circuit. Assuming that the voltage between the emitter and base of the transistor is VEB, the voltage across the LED lamp is VLED, and RCS and voltage drop QDIM are ignored, the voltage on the resistor ROV2 is:

Therefore, the current flowing through the emitter of the transistor is:

Since the base resistance of the transistor is generally large, the base current is much smaller than the emitter current and can be ignored. In this way, it can be considered that the current flowing through the collector is equal to the emitter current, that is,

From this, we can get the voltage of the transistor collector, that is, the voltage of the OV pin of TPS92692-Q1:

Usually, the VEB value of the transistor is relatively fixed, usually around 0.7V. Therefore, when the OVP protection is triggered, the output voltage of the Buck-Boost circuit is:

Through the above calculation formula, the required ROV1 and ROV2 can be calculated according to the set OVP protection voltage. To facilitate quick design, TI provides a calculation tool TPS92692-Q1 Buck-Boost Design Calculator for the TPS92692-Q1 Buck-Boost topology on its website.