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LT1587CM Datasheet(PDF) 11 Page - Linear Technology |
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LT1587CM Datasheet(HTML) 11 Page - Linear Technology |
11 / 16 page 11 LT1584/LT1585/LT1587 158457a APPLICATIONS INFORMATION Overload Recovery The LT1584 devices have safe-area protection similar to the LT1083/LT1084/LT1085. The safe-area protection de- creases current limit as input-to-output voltage increases. This behavior keeps the power transistor inside a safe operating region for all values of input-to-output voltage. The LT1584 protection circuitry provides some output current at all values of input-to-output voltage up to the 7V maximum supply voltage. When power is first applied, the input voltage rises and the output voltage follows the input. The input-to-output voltage remains small and the regula- tor can supply large output currents. This action permits the regulator to start-up into very heavy loads. With higher input voltages, a problem can occur where the removal of an output short does not permit the output voltage to recover. This problem is not unique to the LT1584 devices and is present on the LT1083/LT1084/ LT1085 family and older generation linear regulators. The problem occurs with a heavy output load, a high input voltage, and a low output voltage. An example is immedi- ately after the removal of a short circuit. The load line of such a load may intersect the output current curve at two points. If this happens, two stable output operating points exist for the regulator. With this double intersection, the power supply may require cycling down to zero and back up again to make the output recover. This situation does not occur with the LT1585/LT1587 because no foldback cir- cuitry is required to provide safe-area protection. Ripple Rejection The typical curve for ripple rejection reflects values for the LT1584/LT1585/LT1587 fixed output voltage parts be- tween 3.3V and 3.6V. In applications that require improved ripple rejection, use the adjustable devices. A bypass capacitor from the adjust pin to ground reduces the output ripple by the ratio of VOUT/1.25V. The impedance of the adjust pin capacitor at the ripple frequency should be less than the value of R1 (typically in the range of 100 Ω to 120 Ω) in the feedback divider network in Figure 2. There- fore, the value of the required adjust pin capacitor is a function of the input ripple frequency. For example, if R1 equals 100 Ω and the ripple frequency equals 120Hz, the adjust pin capacitor should be 22 µF. At 10kHz, only 0.22µF is needed. Output Voltage The LT1584/LT1585/LT1587 adjustable regulators develop a 1.25V reference voltage between the output pin and the adjust pin (see Figure 3). Placing a resistor R1 between these two terminals causes a constant current to flow through R1 and down through R2 to set the overall output voltage. Normally, this current is the specified minimum load current of 10mA. The current out of the adjust pin adds to the current from R1 and is typically 55 µA. Its output voltage contribution is small and only needs consideration when very precise output voltage setting is required. LT1584 ADJ VIN VOUT IN R1 VREF R2 VOUT = VREF (1 + R2/R1) + IADJ (R2) OUT IADJ 55 µA LT1585 • F03 C1 10 µF C2 22 µF + + Figure 3. Basic Adjustable Regulator Load Regulation It is not possible to provide true remote load sensing because the LT1584/LT1585/LT1587 are three-terminal devices. Load regulation is limited by the resistance of the wire connecting the regulators to the load. Load regulation per the data sheet specification is measured at the bottom of the package. For fixed voltage devices, negative side sensing is a true Kelvin connection with the ground pin of the device re- turned to the negative side of the load. This is illustrated in Figure 4. LT1584-3.3 RP PARASITIC LINE RESISTANCE GND VIN RL IN OUT LT1585 • F04 Figure 4. Connection for Best Load Regulation |
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