When the input and output voltages of the LDO are reversed, the reverse current flows from the output pin to the input pin into the IC chip. An anti-current protection diode is usually connected to the outside of the IC to prevent damage to the IC. This application note explains guidelines on whether a protection diode is required and how to select a reverse current protection diode. 1.A guide on whether a diode needs to be protected is listed in Table 1 for specific cases as to whether a diode needs to be protected. Also, note that the guidelines vary depending on the type of output transistor. For details of each item, please refer to the references in the table.
table 1.A decision table for whether a diode needs to be protected.
2.Selection of protection circuits There are two types of protection circuits: the method of inserting a diode on the input side of the LDO to block the reverse current (Figure 1), and the method of placing a diode between the input and output to bypass the reverse current (Figure 2). The pros and cons of each protection circuit are listed in Table 2, so use the one that is appropriate for your application.
table 2.Advantages and disadvantages of the type of protection circuit.
3.Diode Selection The types of diodes used in the 2 types of protection circuits described earlier and the requirements for electrical characteristics are listed in Table 3.
table 3.Requirements for the type and electrical characteristics of the diode.
3-1.The case of reverse current blocking circuit Regarding the types of diodes, although rectifier diodes, Schottky barrier diodes, fast recovery diodes, and switching diodes can be used, Schottky barrier diodes or rectifier diodes are generally used due to the electrical characteristics and cost. Switching diodes can also be used in low-current applications up to 100mA. Fast recovery diodes can also be used, but there will be some overperformance in this use. When there is sufficient voltage drop voltage allowance between input and output, a rectifier diode can be used. If there is no margin for the voltage drop, a Schottky barrier diode with a low forward voltage is used, but note that the reverse current increases exponentially at high temperatures.
3-2.In the case of reverse current bypass circuits, Schottky barrier diodes with low forward voltage are used because the bypass diode must be on (on) than the parasitic elements inside the IC. The reverse current IR of Schottky barrier diodes is large, and as the temperature rises, the current increases exponentially, and some products will reach tens of mA.
Figure 3 is an example of an LDO with a shut-off switching function, even if the output of the LDO is turned off, the applied input voltage is still there, and the diode's reverse current IR causes the current to flow to the load.
As a result, malfunctions of the backing circuit may occur. In addition, the voltage regulation characteristics may deteriorate due to the fact that the leakage current always flows from the input to the output during normal operation. To sum up, it is necessary to choose a product with a low reverse current IR. The recommended Schottky barrier diodes are shown in Table 4. ldo in cin gnd cout vout vin ir en out shutdown rload figure 3.Even if the LDO output is turned off, the current will flow to the load through the reverse current IR of the diode.
How to select an LDO reverse current protection diode.
4.Output Transistor Form LDO has two types of output transistors, bipolar and MOSFET, each of which has a different name. ROHM's bipolar products start with "BA" and MOSFET products with "BD", "BU", and "BH".
4-1.Bipolar Figure 5 is a block diagram of a bipolar LDO, and looking at the input side from the output side, there may be a PN junction acting as a parasitic diode on the fabrication structure of the silicon wafer. The parasitic diode is not designed as a circuit, and the flow of current may cause damage to the device or malfunction of the circuit. Therefore, it is important to take measures to prevent the flow of bipolar reverse currents.
figure 5.Block diagram of a bipolar LDO with an example of a PNP transistor with output.
4-2.MOSFET Type Figure 6 is a block diagram of a MOSFET-type LDO, where a body diode is present in the output MOSFET through which reverse current flows. Figure 7 is a cross-sectional view of a P-channel MOSFET with the body diode as a parasitic element in the PN junction between the drain and gate-substrate. This body diode is the same device size as the output transistor, so it allows a certain amount of current to flow through. vref + in out body diode
figure 6.Block diagram of a MOSFET-type LDO, the output is an example of a P-channel MOSFET.
figure 7.Cross-sectional view of a P-channel MOSFET.
5.Case Guidelines.
Depending on each case, it is determined whether a diode protection is necessary.
5-1.This is a common example of a sudden drop in voltage on the input side when the power supply drops, and the voltage of the power supply circuit in the LDO preamplifier drops rapidly, and the voltage between the input and output is temporarily reversed due to residual charge on the capacitor connected to the output side of the LDO, and the reverse current flows in reverse. Figure 8 shows the circuit when the voltage on the input side drops rapidly, and Figure 9 shows the relationship between the input and output voltages and the reverse current. The time that this reverse current flows is a temporary phenomenon before the discharge of the output capacitor ends. The value and time of the reverse current vary depending on the circuit as the impedance of the power supply drops from the front power supply, the total charge of the capacitor on the output side, and the load current. ldo in out gnd cin cout vin vout r
figure 8.A circuit when the voltage on the input side drops rapidly.
figure 9.The input/output voltage and reverse current when the voltage on the input side drops rapidly.
Basically, the reverse voltage is at 0A protection diode is required above 6V, but not when all of the following conditions are met. Diode not required (all met) - The peak value of the reverse current must be below the maximum output current specified in the recommended operating range of the datasheet. -Do not turn the power on and off frequently. - The final product does not require high reliability and safety.
5-2.When there is always a reverse current flow Since the power supply is composed of multiple system circuits, when the LDO power supply is down, there is always a situation where a reverse current flows from other circuit modules, or in the process of product factory inspection, by applying a voltage to the LDO output from other power supplies, there is a situation where a reverse current flows to the LDO, regardless of the current value, the diode needs to be protected.
5-3.When the input becomes open, the circuit diagram is shown in Figure 10, and the relationship between the input and output voltages is shown in Figure 11. When the input becomes open when the power supply drops, there is no difference in the input and output voltage, so no reverse current is generated. So there is no need for a protection diode.
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