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NCV4276DS18

400 mA Low−Drop Voltage Regulator

ON Semiconductor 

NCV4276, NCV4276A

Circuit Description

The NCV4276 is an integrated low dropout regulator that

provides a regulated voltage at 400 mA to the output. It is

enabled with an input to the inhibit pin. The regulator

voltage is provided by a PNP pass transistor controlled by

an error amplifier with a bandgap reference, which gives it

the lowest possible dropout voltage. The output current

capability is 400 mA, and the base drive quiescent current

is controlled to prevent oversaturation when the input

voltage is low or when the output is overloaded. The

regulator is protected by both current limit and thermal

shutdown. Thermal shutdown occurs above 150°C to

protect the IC during overloads and extreme ambient

temperatures.

Regulator

The error amplifier compares the reference voltage to a

sample of the output voltage (VQ) and drives the base of a

PNP series pass transistor via a buffer. The reference is a

bandgap design to give it a temperature−stable output.

Saturation control of the PNP is a function of the load

current and input voltage. Oversaturation of the output

power device is prevented, and quiescent current in the

ground pin is minimized. See Figure 5, Test Circuit, for

circuit element nomenclature illustration.

Regulator Stability Considerations

The input capacitors (CI1 and CI2) are necessary to

stabilize the input impedance to avoid voltage line

influences. Using a resistor of approximately 1.0 W in

series with CI2 can stop potential oscillations caused by

stray inductance and capacitance.

The output capacitor helps determine three main

characteristics of a linear regulator: startup delay, load

transient response and loop stability. The capacitor value

and type should be based on cost, availability, size and

temperature constraints. The aluminum electrolytic

capacitor is the least expensive solution, but, if the circuit

operates at low temperatures (−25°C to −40°C), both the

value and ESR of the capacitor will vary considerably. The

capacitor manufacturer’s data sheet usually provides this

information.

The value for the output capacitor CQ, shown in Figure 3,

should work for most applications; however, it is not

necessarily the optimized solution. Stability is guaranteed

for CQ w 22 mF and an ESR v 2.5 W for the 5.0 V and

Adjustable regulator and CQ w 10 mF and an ESR v 1.8 W

for the 1.8 V, 2.5 V, and 3.3 V regulators. See Figures 5 and

6 for output stability at various load and capacitive ESR

conditions.

Inhibit Input

The inhibit pin is used to turn the regulator on or off. By

holding the pin down to a voltage less than 0.5 V, the output

of the regulator will be turned off. When the voltage on the

Inhibit pin is greater than 3.5 V, the output of the regulator

will be enabled to power its output to the regulated output

voltage. The inhibit pin may be connected directly to the

input pin to give constant enable to the output regulator.

Setting the Output Voltage (Adjustable Version)

The output voltage range of the adjustable version can be

set between 2.5 V and 20 V (Figure ). This is accomplished

with an external resistor divider feeding back the voltage

to the IC back to the error amplifier by the voltage adjust

pin VA. The internal reference voltage is set to a

temperature stable reference of 2.5 V.

The output voltage is calculated from the following

formula. Ignoring the bias current into the VA pin:

VQ + [(R1 ) R2) * Vref]ńR2

Use R2 < 50 k to avoid significant voltage output errors

due to VA bias current.

Connecting VA directly to Q without R1 and R2 creates

an output voltage of 2.5 V.

Designers should consider the tolerance of R1 and R2

during the design phase.

The input voltage range for operation (pin 1) of the

adjustable version is between (VQ + 0.5 V) and 40 V.

Internal bias requirements dictate a minimum input voltage

of 4.5 V. The dropout voltage for output voltages less than

4.0 V is (4.5 V − VQ).

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