Advanced Power
Electronics Corp. APE8975/A
APPLICATION INFORMATION
Capacitor Selection
Input-Output (Dropout) Voltage
Current Limit and Short-Circuit Protection
Thermal Considerations
5
The APE8975/A series can deliver a current of up to 3A over the full operating
unction 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
Where TJ (MAX) is the maximum junction temperature of the die (125oC) and TAis
the maximum ambient temperature. The junction to ambient thermal resistance (θJA)
for SOT-223 package at recommended minimum footprint is 75°C/W that is connect 1
in2 PCB copper area to tap pin.
Like any low-dropout regulator, the APE8975/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.
Normally, use a 10µF capacitor on the input and a 10µF capacitor on the output
of the APE8975/A. Larger input capacitor values and lower ESR (X5R, X7R) provide
better supply-noise rejection and transient response. A higher- value output capacito
may be necessary if large, fast transients are anticipated and the device is located
several inches from the power source.
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
The APE8975/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, AX6639/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.