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TCL1585 Datasheet(PDF) 6 Page - TelCom Semiconductor, Inc |
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TCL1585 Datasheet(HTML) 6 Page - TelCom Semiconductor, Inc |
6 / 13 page TCL1584 TCL1585 TCL1587 7A / 5A / 4.6A / 3A, FAST RESPONSE, LOW DROPOUT POSITIVE LINEAR VOLTAGE REGULATORS PRELIMINARY INFORMATION 6 TCL1584/1585/1587-04 6/6/97 Figure 3. Overload Recovery IOUT Current Limit Curve Load Line Start Up Point Unrecoverable I-V Point VIN – VOUT (Constant VIN) parameters will allow it. Typically, increasing the output capacitance helps reduce the oscillation. NOTE: The TCL1585 and TCL1584 have fixed current limit over the entire voltage range and are not susceptible to this phenom- enon. Protection Diodes The TCL158x family of devices do not normally require any external current limiting circuitry such as protection diodes, frequently used with older LDO regulators. A diode is internally present between the output and input which is capable of handling short-duration surge currents of up to 100A. This capability typically ensures safe operation ex- cept for the case where output capacitance is exceedingly large (>1000 µF) and the input is suddenly shorted to ground. This situation can produce excessive reverse current in the device - enough to cause damage. An external high current diode should be used as shown in Figure 2. The ADJ pin does not normally need protection diodes either. It can handle ±7V without any performance degrada- tion or device damage. Current at this pin is internally limited by a series resistor so the bypass capacitors do not present any danger. Of course, exceeding 7V differential between any two pins will cause catastrophic junction breakdowns and possible damage to the device. Overload Recovery The built-in Short Circuit and Safe-Operating-Area (SOA) protection function of the TCL158x family can cause sec- ondary effects which must be considered for robust system design. The behavior of the regulator under heavy loads (short circuit) at start-up is such that the output voltage will remain low while sourcing maximum current until the load is removed or reduced. Normally, the output voltage will rise as the load is reduced and trace a line I-V relationship accord- ing to the SOA limit. If the load line intersects this output curve at two points the output voltage may not recover from the heavy load/short-circuit condition. This condition is illus- trated in Figure 3. The current limit constraint does not allow any load point above it and the load line is defined absolutely by the I-V characteristics of the load (a resistor, in this case). Under these conditions it may be necessary to cycle the power supply off and then on again. This phenomenon is common for LDO’s with fold-back current limiting schemes. NOTE: Overload recovery is always guaranteed on the TCL1585 and TCL1587 because of the constant current limit characteristic. Ripple Rejection A typical ripple rejection curve for the fixed output voltage devices is shown in Figure 3. It is possible to obtain improved performance in ripple rejection by using the ad- justable output TCL158x with a bypass capacitor (CADJ) shown in Figure 2. This capacitor should be chosen to have a value such that its impedance at the ripple frequency is less than R1 (see Figure 2.). Usually this is on the order of 100 Ω. Example: If R1 = 100Ω and fRIPPLE = 120Hz, the bypass capacitor should be chosen to be 22 µF or greater. If these conditions are met the ripple rejection will be improved by a factor of VOUT/1.25 when compared to the performance of the fixed output devices. Figure 2. Protection Diodes and CADJ + + VOUT C1 10 µF C2 22 µF OUT IN TCL1584-3.3 D1 1N4002 (OPTIONAL) GND VIN + + + VOUT R1 R2 C1 10 µF C2 22 µF C ADJ OUT IN TCL1584 D1 1N4002 (OPTIONAL) ADJ VIN |
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