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SPX1587U-1.5 Datasheet(PDF) 3 Page - Sipex Corporation |
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SPX1587U-1.5 Datasheet(HTML) 3 Page - Sipex Corporation |
3 / 7 page Rev. 10/30/00 SPX1587 APPLICATION HINTS The SPX1587 incorporates protection against over-current faults, reversed load insertion, over temperature operation, and positive and negative transient voltage. However, the use of an output capacitor is required in order to improve the stability and the performances. Stability The output capacitor is part of the regulator’s frequency compensation system. Either a 22 µF aluminum electrolytic capacitor or a 10 µF solid tantalum capacitor between the output terminal and ground guarantees stable operation for all operating conditions. However, in order to minimize overshoot and undershoot, and therefore optimize the design, please refer to the section ‘Ripple Rejection’. Ripple Rejection Ripple rejection can be improved by adding a capacitor between the ADJ pin and ground as shown in figure 6. When ADJ pin bypassing is used, the value of the output capacitor required increases to its maximum (22 µF for an aluminum electrolytic capacitor, or 10 µF for a solid tantalum capacitor). If the ADJ pin is not bypass, the value of the output capacitor can be lowered to 10 µF for an electrolytic aluminum capacitor or 4.7 µF for a solid tantalum capacitor. However the value of the ADJ-bypass capacitor should be chosen with respect to the following equation: C = 1 / (6.28 * FR * R1) Where C = value of the capacitor in Farads (select an equal or larger standard value), FR = ripple frequency in Hz, R1 = value of resistor R1 in Ohms. If an ADJ-bypass capacitor is use, the amplitude of the output ripple will be independent of the output voltage. If an ADJ- bypass capacitor is not used, the output ripple will be proportional to the ratio of the output voltage to the reference voltage: M = VOUT / VREF Where M = multiplier for the ripple seen when the ADJ pin is optimally bypassed. VREF = Reference Voltage Reducing parasitic resistance and inductance One solution to minimize parasitic resistance and inductance is to connect in parallel capacitors. This arrangement will improve the transient response of the power supply if your system requires rapidly changing current load condition. Thermal Consideration Although the SPX1587 offers some limiting circuitry for overload conditions, it is necessary not to exceed the maximum junction temperature, and therefore to be careful about thermal resistance. The heat flow will follow the lowest resistance path, which is the Junction-to-case thermal resistance. In order to insure the best thermal flow of the component, a proper mounting is required. Note that the case of the device is electrically connected to the output. In case the case has to be electrically isolated, a thermally conductive spacer can be used. However do not forget to consider its contribution to thermal resistance. Assuming: VIN = 10V, VOUT = 5V, IOUT = 1.5A, TA = 50 °C/W, θ Heatsink Case = 6°C/W, θ Heatsink Case = 0.5°C/W, θ JC = 3°C/W Power dissipation under this condition PD = (VIN – VOUT) * IOUT = 7.5W Junction Temperature TJ = TA + PD * ( θ Case - HS+ θ HS + θ JC) For the Control Sections TJ = 50 + 7.5*(0.5 +6=3) = 121.25 °C 121.25 °C < TJ (max) for the Control & Power Sections. In both case reliable operation is insured by adequate junction temperature. |
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