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NCV4266

150 mA Low-Dropout Voltage Regulator with Enable

ON Semiconductor 

NCV4266

Circuit Description

The NCV4266 is an integrated low dropout regulator that

provides a regulated voltage at 150 mA to the output. It is

enabled with an input to the enable 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 150 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 2, 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 2,

should work for most applications; see also Figure 3 for

output stability at various load and Output Capacitor ESR

conditions. Stable region of ESR in Figure 3 shows ESR

values at which the LDO output voltage does not have any

permanent oscillations at any dynamic changes of output

load current. Marginal ESR is the value at which the output

voltage waving is fully damped during four periods after

the load change and no oscillation is further observable.

ESR characteristics were measured with ceramic

capacitors and additional series resistors to emulate ESR.

Low duty cycle pulse load current technique has been used

to maintain junction temperature close to ambient

temperature.

Enable Input

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

holding the pin down to a voltage less than 1.8 V, the output

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

enable pin is greater than 2.8 V, the output of the regulator

will be enabled to power its output to the regulated output

voltage. The enable pin may be connected directly to the

input pin to give constant enable to the output regulator.

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