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The working principle of LDO and its application examples

21/12/2021

LDO stands for low dropout regulator, which is a low dropout linear regulator. This is relative to the traditional linear regulator. Traditional linear regulators, such as 78XX series chips, require the input voltage to be at least 2V~3V higher than the output voltage, otherwise it will not work properly. But in some cases, such conditions are obviously too harsh, such as 5V to 3.3V, the voltage difference between input and output is only 1.7v, which obviously does not meet the working conditions of traditional linear regulators. In response to this situation, chip manufacturers have developed LDO-type voltage conversion chips.

The most important of LDO is the two characteristics of startup and shutdown

The startup time of the LDO is also called the initial load response or power-up time. In high-performance systems, this requirement is very strict (for example, strict power-up sequence), which requires fast power-up. The ON state of the linear regulator is when the output voltage reaches 95% of the rated output. The start-up time is affected by the gain, bandwidth, output current, and the voltage drop of Vin and Vout of the feedback loop. The schematic diagram of the LDO is as follows:


To get a fast start-up, the MOS transistor must be turned on as soon as possible. The MOS tube is controlled by the error amplifier, and the bandgap reference source (bandgap) is the input of the error amplifier, so the bandgap reference is required to start quickly. Fast start-up will cause the output to overshoot, and the rapid voltage rise will cause a large charging current of the output capacitor, which may cause reliability problems, so the start-up time can meet the needs. In some occasions (such as RF power amplifier), LDO is required to reduce overshoot and start slowly. This requires adding a capacitor to the bandgap reference, which can not only reduce overshoot but also reduce noise. For example, TPS74201 and TPS74401 have a soft start function, which can be realized by adding a Css capacitor externally.

There are two types of LDO shutdown: overheat shutdown and active shutdown. Overheat shutdown is a protective measure to avoid damage to the LDO device. Speaking of the heating of the LDO, let’s first introduce the efficiency of the LDO. When the current to ground is small relative to Iout, the efficiency of the LDO=Vout/Vin. Power consumption consumed by LDO

P=Pin*(1-Vout/Vin)=Iout*(Vin-Vout).

It can be seen that the greater the voltage difference between Vin and Vout, the lower the efficiency of the LDO, the greater the power consumed by itself, and the greater the heat generated. When the heat causes the temperature to reach the overheating shutdown threshold, the protection circuit of the LDO is activated and the output of the LDO is turned off.

The above figure is to short-circuit the output of the LDO to quickly test the LDO overheating shutdown function. The X axis is the time and the Y axis is the current. After the LDO output is short-circuited, the LDO outputs the maximum current and consumes the most power. It can be seen from the figure that the circuit suddenly increases after a short circuit. After about 40ms, the over-temperature threshold is exceeded, the LDO is turned off, and the current is zero. After about 5 milliseconds, the temperature is lower than the over-temperature threshold, and the LDO resumes its output. Since the short-circuit condition is not eliminated, this process is repeated until the short-circuit condition is removed (the current is 0). Over-temperature recovery has a hysteresis threshold to avoid too frequent oscillations.

Therefore, in order to prevent the LDO from shutting down due to temperature within the normal operating current range, it is necessary to dissipate the heat of the LDO. The thermal resistance of the LDO can be viewed in the manual. For example, the thermal resistance description of LP2950 should be based on the selected package to see the thermal resistance parameters. For example, when Vin=12V, Vout=5V, and the current output is 150mA, the LDO consumes power (12-5)*0.15=1.05W. When the ambient temperature is 30 degrees, select the D-type package, and calculate the PN junction temperature of the silicon wafer through the thermal resistance = 30 + 1.05 * 97 = 131 degrees. If it is greater than the maximum allowable temperature Tj=125 degrees, an over-temperature shutdown will occur. So in this case, choose a package with a smaller thermal resistance, or add a fan or heat sink to reduce the thermal resistance, or reduce the voltage difference between Vin and Vout.

The active shutdown of the LDO is a method of power control, such as controlling the power-off sequence or controlling a certain part of the circuit to enter low power consumption, and the active shutdown is realized through the EN pin. It should be noted that because the output of the LDO has a filter capacitor, and the power pin of the subsequent chip is connected with a capacitor, the output voltage will slowly drop due to the charge stored in the capacitor after it is turned off. The slow voltage drop causes the subsequent circuit to stay on the low voltage for a long time. For digital circuits, the circuit may enter an error state under an abnormal power supply voltage, and these error states may trigger certain dangerous operations. Therefore, in order to avoid these situations, the power needs to be powered off quickly, that is, the capacitor needs to be discharged after the LDO is turned off. As shown below:

The LDO integrates a MOS tube at the output, which discharges the capacitor when the output is turned off. For example, LP3995 and TLV711 dual-output series power chips integrate this function.

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