AMS1117 （AMS1117 3.3, AMS1117 5.0, AMS1117 1.8） GENERAL DESCRIPTION

The adjustable and fixed voltage regulators of the AMS1117 series are designed to provide an output current of up to 1 A and to operate with an input-output voltage difference of up to 1 V. The voltage drop of the device is guaranteed to a maximum of 1.3 V, decreasing at lower load currents. On-chip trim adjusts the reference voltage up to 1.5%. The current limit is set to minimize voltage under overload conditions at both the regulator and the power supply circuit. The AMS1117 （AMS1117 3.3, AMS1117 5.0, AMS1117 1.8）is compatible with other 3-pin SCSI regulators and is available in SOT-223 SMD, SOIC 8L and TO-252 Plastic (DPAK) packages.

* When soldering the chassis to the rear ground plane or internal power rail, ϕ JA can vary from 46 ° C / W to> 90 ° C / W depending on the mounting method and the size of the copper heatsink

ELECTRICAL SPECIFICATIONS AMS1117

Electrical specifications at I OUT = 0 mA and T J = + 25 ° C unless otherwise noted.

Ams1117 datasheet

You can download here (Ams1117 datasheet)

APPLICATION TIPS FOR AMS1117

The AMS1117 series stabilizers with fixed and variable output voltage are easy to use and are protected against short circuit and thermal overload. The thermal protection circuit will disable the regulator if the crystal temperature exceeds 165 ° C at the measuring point. Pin-compatible with older 3-pin regulators, these devices have the advantage of lower voltage drop, more accurate reference tolerance, and better temperature stability.

Stability

The circuitry used in the AMS1117 series requires the use of an output capacitor as part of the frequency compensation device. Connecting a 22uF tantalum capacitor at the output will ensure stability for all operating conditions. When the control terminal is bypassed without a capacitor to improve ripple suppression, the requirements on the output capacitor increase. A value of 22 μF covers all such uses. If a capacitor connected to the regulation pin is used, then the capacitance of the capacitor at the output can be reduced. To further improve the stability and transient response of these devices, larger values ​​of the output capacitor can be used.

Protective diodes

Unlike older regulators, the AMS1117 family does not need any protection diodes between the regulating pin and the output and between the output and the input to prevent overvolting the circuit. Internal resistors limit the internal current paths at the control terminal of the AMS1117, so even with capacitors at the control terminal, no protection diode is required to ensure the safety of the device in short circuit conditions.

Diodes between input and output are usually not needed. Microsecond pulse currents from 50 A to 100 A can be processed by an internal diode connected between the input and output. Under normal conditions, it is difficult to obtain these impulse currents even when using large output capacitances. When using high-capacity output capacitors, such as 1000 to 5000 μF, if the output is momentarily shorted to ground, damage to the device may occur. A diode between input and output is recommended when a so-called crowbar circuit is used at the input of the AMS1117 (pic 1)

Output voltage

The AMS1117 series provides a 1.25 V reference voltage between the output and the regulation pin. Connecting a resistor between these two terminals causes a DC current to flow through R1 and then to ground through R2 to set the overall output voltage. This current is usually equal to the specified minimum load current of 10 mA. Since I ADJ is very small and constant, it is a small error and can therefore be neglected.

Thermal performance

The AMS1117 series has an internal power and temperature limiting circuit designed to protect the device under overload conditions. However, the maximum junction temperatures of 125 ° C must not be exceeded under continuous normal load conditions. Much attention must be paid to all sources of thermal resistance from the crystal to the environment. For the SOT-223 surface mount enclosure, consider any additional heat sources installed near the unit. The heat dissipation capacity of the PCB and its copper tracks should be used as a heat sink for the device. The thermal resistance from the junction to the connection plane with the heatsink for the AMS1117 is 15 ° C / W. The thermal resistance from the plane of the connection to the radiator to the environment can be as low as 30 ° C / W.

The total thermal resistance of the crystal to the environment can be as low as 45 ° C / W. This will require a reasonably sized PCB with a small copper polygon to distribute heat around the PCB and dissipate it to the environment.

Experiments have shown that the heat-spreading copper layer does not have to be electrically connected to the case terminal. The PCB material can be very effective in transferring heat between the area of ​​the pad attached to the plane of the device's enclosure and the ground on the inside or on the opposite side of the board. Although the actual thermal resistance of the PCB material is high, the length-to-area ratio of thermal resistance between layers is small. The data was taken using a 1/16 inch (1.6 mm) FR-4 board with a 1 oz (35 µm) copper layer. It can be used as a rough guideline for evaluating thermal resistance.