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Rev.2.3 Nov.12, 2019

AX6630/A

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APPLICATION INFORMATION

Like any low-dropout regulator, the AX6630/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 2.2µF capacitor on the input and a 2.2µF capacitor on the output of the

AX6630/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 AX6630/A used 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, AX6630/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 AX6630/A series can deliver a current of up to 1.2A 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

The final operating junction temperature for any set of conditions can be estimated by the

following thermal equation:

PD (MAX) = (TJ (MAX) - TA) / θJA