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Rev.1.6.Nov.14, 2019

AX6646/A

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APPLICATION INFORMATION

Like any low-dropout regulator, the AX6646/A requires input and output decoupling

capacitors. The device is specifically designed for portable applications requiring minimum

board space and smallest components. These capacitors must be correctly selected for

good performance (see Capacitor Characteristics Section). Please note that linear regulators

with a low dropout voltage have high internal loop gains which require care in guarding

against oscillation caused by insufficient decoupling capacitance.

Capacitor Selection

Normally, use a 1µF capacitor on the input and a 1µF capacitor on the output of the

AX6646/A. Larger input capacitor values and lower ESR (X5R, X7R) provide better

supply-noise rejection and transient response. A higher- value output capacitor (4.7µF) may

be necessary if large, fast transients are anticipated and the device is located several inches

from the power source.

Input-Output (Dropout) Voltage

A regulator's minimum input-to-output voltage differential (dropout voltage) determines

the lowest usable supply voltage. In battery-powered systems, this determines the useful

end-of-life battery voltage. Because the device uses a PMOS, its dropout voltage is a

function of drain-to source on-resistance, RDS (ON), multiplied by the load current:

VDROPOUT = VIN -VOUT = RDS (ON) x IOUT

Current Limit and Short-Circuit Protection

The AX6646/A uses a current sense-resistor to monitor the output current. A portion of

the PMOS output transistor’s current is mirrored to a resistor such that the voltage across

this resistor is proportional to the output current. Once the output current exceeds limit

threshold, AX6646/A would be protected with a limited output current. Further more, when

the output is short to ground, the output current would be folded-back to a less limit.

Thermal Considerations

The AX6646/A series can deliver a current of up to 300mA over the full operating

junction temperature range. However, the maximum output current must be dated at higher

ambient temperature to ensure the junction temperature does not exceed 125°C. With all

possible conditions, the junction temperature must be within the range specified under

operating conditions. Power dissipation can be calculated based on the output current and

the voltage drop across regulator.

PD = (VIN - VOUT) IOUT