Skip to main content Skip to first level navigation

RICOH imagine. change.


Skip to main content First level navigation Menu
Main content

FAQ: LDO Regulators (Linear Regulators)

Q. How can I figure out the dropout voltage of a product when the condition of output current is different from that of Electrical Characteristics on the datasheet?

How to use/ LDO: 001

Fig1. Dropout Voltage vs. Output Current


Dropout voltage (Vdif) refers to the necessary difference between the input and the output voltage to gain fixed output current. Basically, dropout voltage can be calculated at any output current referring to the Dropout Voltage specification written in the datasheet of each product.

Take an example of an LDO regulator whose  electrical characteristics are specified in its datasheet as follows: 300 mA, 2.6 V output with dropout voltage of typically 0.23 V.

Fig. 1 shows the Dropout Voltage vs. Output Current of the regulator on its datasheet. According to Fig. 1, in order to acquire 200 mA when the temperature is 25°C, the necessary dropout voltage is about 0.15 V. Therefore, the typical minimum input voltage should be about 2.75 V for normal output regulation. You can also figure out the maximum dropout voltage by proportional calculation. The maximum dropout voltage is about 0.23 V, so the minimum input voltage is about 2.83 V.

See datasheets of each product for more details.

Q. What happens to LDO regulators when the input voltage is lower than the output voltage?

How to use/ LDO: 002


The output capacitor sometimes holds electric charge after the whole system is turned off. In that occasion, the input voltage may be lower than the output voltage.

In this case, reverse current may flow from the output to the input. If the resistance in the input circuit is high and the reverse current is limited to 30 mA or around, the IC will not be damaged. However, if large reverse current (several hundred mA or more) flows for a few seconds, a schottky barrier diode (SBD) must be inserted between the input and output (Fig. 1).

Moreover, if the voltages applied from another source to the Vout exceeds the input voltage by 0.3 V or more, reverse current flows inside the IC via the parasitic diode, which may activate parasitic elements as electrical paths for large current, and the LDO regulator may be damaged. In order to protect regulators from reverse currents, insert an SBD between the input and output (Fig. 2).

Our LDO regulators including reverse current protection do not need external components such as an SBD.

Reverse Current Protection

Q. Is there any problem if the CE is at high impedance?

How to use/ LDO: 003


If the CE is pulled up or down by a constant current circuit or an internal resistor inside the IC shown in Fig. 1 or Fig. 2, the IC can be set to standby state even though the input voltage is at high impedance.

However, if the CE is not pulled up nor down inside the IC shown in Fig. 3, the IC status may be indefinite, and therefore the operation may be unstable. In this case, pull up or down the CE with an external resistor.

Block diagram example

Q. How should external components be mounted on PCBs?

How to use/ LDO: 004


Connect external components close to the regulator so that the wiring distance becomes as short as possible. Especially, the capacitor between the VIN and the GND must be connected with the shortest wiring. See datasheets and design guides of each product for details.

Q. Is there any problem if a voltage is applied to the CE prior to the VIN?

How to use/ LDO: 005


It depends on the internal circuit of the products. Some Regulators with a current path between the CE and the VIN may be damaged if a voltage is applied to the CE prior to the VIN.

In the circuit structure shown in Fig. 1, input sequence to the CE and the VIN is free, because there is no current path between them. However, some exceptional LDOs which reset their internal circuits by the CE signal may not operate properly.

In the case of Fig. 2, the IC may be damaged if a large current flows from the CE by input sequence problem. Do not apply voltages to the CE before the VIN.

When the circuit structure between the VIN and CE is uncertain, please contact us.

CE pin

Q. Are the LDO regulators damaged by inrush current over their absolute maximum ratings?

How to use/ LDO: 006


LDO regulators are not damaged by momentary inrush current. Current at or over the absolute maximum ratings may affect the reliability of LDO regulators if the current is kept at the DC level.

Q. How to calculate power dissipation of LDO regulators?

How to use/ LDO: 007


Power dissipation (PD) means maximum power consumption to maintain its performance.

Here is the formula to calculate PD:

[PD] = (Maximum Power Consumption [Tjmax] - Ambient Temperature [Ta]) / Junction-to-Ambient Thermal Resistance [θja]

For example, described on its datasheet as:

Tjmax = 125°C, Ta = 85°C, θja = 40°C/W

PD = (125-85) / 40 = 1.0 W

Therefore, the PD of this example is 1.0 W.

When the Ta is 25°C, its PD becomes 2.5 W by the formula.
In this case, supposed that the output current is 1.5 A, the maximum dropout voltage can be estimated as:

2.5 W/1.5 A = 1.68 V

Therefore, supposed that the output voltage is fixed at 1.2 V, the input voltage must be kept under 2.88 V.

Some datasheets refer to θjc instead of θja. Replace the θja in the formula with the θjc and calculate PD like this:

[PD] = ([Tjmax] - Package Surface Temperature [Tc]) / Junction-to-Case Thermal Resistance [θjc]

Attention: These values are derived from measurements* under certain conditions. These values depend on the actual PCB.

  • * The measurements are conducted according to the conditions regulated by JEDEC (ex. the chip mounted in the center of the 3 inch x 3 inch four-layer board).

Q. Is the auto-discharge valid at the state of VDD = CE?

How to use/ LDO: 013


The auto-discharge circuit turns the gate of an N-Channel transistor high when the CE signal turns low. Therefore, it cannot function sufficiently if the VDD and the CE turns low at the same time.

Q. Which types of output capacitors are recommended for LDO regulators, in the perspective of equivalent series resistance (ESR)?

How to use/ LDO: 014

ESR vs. Output Current


Ceramic capacitors are recommended for the latest LDO regulators.

Select a capacitor whose ESR characteristic is within the hatched area in the figure titled ESR vs. Output Current on datasheets, even when you select a capacitor except a ceramic type.

Q. What is the alternative product for RE5RA series?

How to use/ LDO: 015


The Rx5RL is.

Q. What is the constant slope?

Function, Term/ LDO: 008

RP110x25xB/D Inrush Current Characteristics


A constant slope circuit is what is called a soft-start circuit that makes output voltage start up gradually when a regulator is turned on. A capacitor for the start-up slope is built in the IC, which does not require any external components. Instead, the start-up time and the start-up slope angle are fixed inside the IC.

If the capacitance of the external output capacitor (COUT) becomes more than the certain capacitance, the start-up time becomes longer and the start-up slope angle becomes more moderate.

According to the characteristics of the RP110x25xB/D, if the COUT is less than 4.7 µF, the constant slope circuit is effective at the startup. On the contrary, if the COUT is over 10 µF, the output current limit circuit is dominant at the start-up. The boundary point of using these two circuits is inversely proportional to the output voltage. The point is different depending on the product.

Q. What is the difference between DC/DC converters (switching regulators) and LDO regulators (linear regulators)?

Function, Term/ LDO: 009 | DC/DC: 001


Both of DC/DC converters and LDO regulators convert an input direct current into a different direct current. The way of converting direct currents differs from each other. DC/DC converters regulate electric power by turning on and off switching elements (FETs, etc.). On the other hand, LDO regulators regulate power supply by controlling on-resistance of FETs.

DC/DC converters are highly efficient in converting electricity by the switching control. Depending on conditions such as output current, the efficiency can reach about 95%. Instead, there are some disadvantages. First, output voltages from this type of regulators contain ripples and switching noise, not suitable for noise-sensitive devices. Second, due to their complex control method, many external components are necessary compared with LDO regulators. Designing PCBs using DC/DC converters tends to be more difficult and more cost-consuming than those using LDO regulators.

The control method of LDO regulators is simple, so designing PCBs is easy. However, the conversion efficiency is inferior to that of DC/DC converters. In order to convert 5 V into 3 V, the efficiency by LDO regulators is 60%, while DC/DC converters can achieve 90% or more. Besides, if dropout voltage and output current are large, heat dissipation must be considered.

  DC/DC converters (switching regulators) LDO regulators (linear regulators)
Efficiency High Low
Price High Low
Noise Loud Low
Design Difficult Easy
Components Many Few
Load Current Large Small

Q. What is a short circuit protection?

Function, Term/ LDO: 010


A short circuit protection prevents ICs from being damaged by overcurrent when the output is short to the GND. Circled in the blue line in Fig. 1 is the circuit named Short Current Limit.

In Fig. 2, the overcurrent protection functions in the area indicated by the blue line, and the short current limit functions in the blue circle. The value of overcurrent protection is not defined in Electrical Characteristics. Refer to Typical Characteristics graphs.

The value of short circuit current limit is defined as Isc in Electrical Characteristics, and the value is different depending on the product.

Short Circuit Protection and Typical Characteristics graphs

Q. What does "Output Current min." on datasheets mean?

Function, Term/ LDO: 011


The amount of output current capability of regulators depends on input and output voltages.

The Output Current min. refers to the lowest value guaranteed as the maximum output current at a specified condition. The regulator can supply a lower current than the “Output current min.” in Electrical Characteristics.

If a current limit circuit is built-in, the value of current limit is higher than “Output Current min”.

Q. What is the auto-discharge?

Function, Term/ LDO: 012


If a large-capacity capacitor is used on the output, the VOUT voltage cannot drop quickly when a turn-off signal of CE is asserted. Auto-discharge discharges the external output capacitor quickly by a built-in discharge transistor.

Ricoh LDO regulators (Low Dropout regulators, or linear regulators) achieve low supply current, high accuracy, low noise, high noise immunity and miniaturization by CMOS analog technology and by elaborate design skills. Our products can meet various needs of customers.

We have a wide lineup of LDO regulators that can meet customer’s different requirements, featuring operating input voltages from 1.0 V to 60 V, output currents from 25 mA to 3,000 mA, and low supply currents from 200 nA.

For small devices, products with the world's smallest WLCSP series contribute to reducing the mounting area. Only one small capacitor is required for phase compensation, which enables your products to be both smaller and safer. For large current LDO regulators, the HSOP series supports high heat dissipation.

Using the highest level of CMOS analog technology, we provide high quality LDO regulators that will enhance the values of your products.