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Advanced Monolithic Systems, Inc. 6680B Sierra Lane, Dublin, CA 94568 Phone (925) 556-9090 Fax (925) 556-9140 AMS1117 APPLICATION HINTS Connected as shown , RP is not multiplied by the divider ratio AMS1117 IN OUT ADJ R L R1* R2* V IN R P PARASITIC LINE RESISTANCE *CONNECT R1 TO CASE CONNECT R2 TO LOAD Figure 3. Connections for Best Load Regulation In the case of fixed voltage devices the top of R1 is connected Kelvin internally, and the ground pin can be used for negative side sensing. Thermal Considerations The AMS1117 series have internal power and thermal limiting circuitry designed to protect the device under overload conditions. However maximum junction temperature ratings of 125 °C should not be exceeded under continuous normal load conditions. Careful consideration must be given to all sources of thermal resistance from junction to ambient. For the surface mount package SOT-223 additional heat sources mounted near the device must be considered. The heat dissipation capability of the PC board and its copper traces is used as a heat sink for the device. The thermal resistance from the junction to the tab for the AMS1117 is 15 °C/W. Thermal resistance from tab to ambient can be as low as 30 °C/W. The total thermal resistance from junction to ambient can be as low as 45 °C/W. This requires a reasonable sized PC board with at least on layer of copper to spread the heat across the board and couple it into the surrounding air. Experiments have shown that the heat spreading copper layer does not need to be electrically connected to the tab of the device. The PC material can be very effective at transmitting heat between the pad area, attached to the pad of the device, and a ground plane layer either inside or on the opposite side of the board. Although the actual thermal resistance of the PC material is high, the Length/Area ratio of the thermal resistance between layers is small. The data in Table 1, was taken using 1/16” FR-4 board with 1 oz. copper foil, and it can be used as a rough guideline for estimating thermal resistance. For each application the thermal resistance will be affected by thermal interactions with other components on the board. To determine the actual value some experimentation will be necessary. The power dissipation of the AMS1117 is equal to: PD = ( VIN - VOUT )( IOUT ) Maximum junction temperature will be equal to: TJ = TA(MAX) + PD(Thermal Resistance (junction-to-ambient)) Maximum junction temperature must not exceed 125 °C. Ripple Rejection The ripple rejection values are measured with the adjustment pin bypassed. The impedance of the adjust pin capacitor at the ripple frequency should be less than the value of R1 (normally 100 Ω to 200 Ω) for a proper bypassing and ripple rejection approaching the values shown. The size of the required adjust pin capacitor is a function of the input ripple frequency. If R1=100 Ω at 120Hz the adjust pin capacitor should be >13µF. At 10kHz only 0.16µF is needed. The ripple rejection will be a function of output voltage, in circuits without an adjust pin bypass capacitor. The output ripple will increase directly as a ratio of the output voltage to the reference voltage (VOUT / VREF ). Table 1. COPPER AREA THERMAL RESISTANCE TOP SIDE* BACK SIDE BOARD AREA (JUNCTION-TO-AMBIENT) 2500 Sq. mm 2500 Sq. mm 2500 Sq. mm 45 °C/W 1000 Sq. mm 2500 Sq. mm 2500 Sq. mm 45 °C/W 225 Sq. mm 2500 Sq. mm 2500 Sq. mm 53 °C/W 100 Sq. mm 2500 Sq. mm 2500 Sq. mm 59 °C/W 1000 Sq. mm 1000 Sq. mm 1000 Sq. mm 52 °C/W 1000 Sq. mm 0 1000 Sq. mm 55 °C/W * Tab of device attached to topside copper. |
