National Instruments LM1085 Series manual Application Note Continued, Regulator, Overload Recovery

LM1085

Application Note (Continued)

Figure 3 shows a typical application using a fixed output

regulator. Rt1 and Rt2 are the line resistances. VLOAD is less than the VOUT by the sum of the voltage drops along the line resistances. In this case, the load regulation seen at the

RLOAD would be degraded from the data sheet specification. To improve this, the load should be tied directly to the output terminal on the positive side and directly tied to the ground terminal on the negative side.

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FIGURE 3. Typical Application using Fixed Output

Regulator

When the adjustable regulator is used (Figure 4), the best performance is obtained with the positive side of the resistor R1 tied directly to the output terminal of the regulator rather than near the load. This eliminates line drops from appearing effectively in series with the reference and degrading regu- lation. For example, a 5V regulator with 0.05Ω resistance between the regulator and load will have a load regulation due to line resistance of 0.05Ω x IL. If R1 (= 125Ω) is connected near the load the effective line resistance will be 0.05Ω (1 + R2/R1) or in this case, it is 4 times worse. In addition, the ground side of the resistor R2 can be returned near the ground of the load to provide remote ground sens- ing and improve load regulation.

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FIGURE 4. Best Load Regulation using Adjustable

Output Regulator

PROTECTION DIODES

Under normal operation, the LM1085 regulator does not need any protection diode. With the adjustable device, the internal resistance between the adjustment and output ter- minals limits the current. No diode is needed to divert the current around the regulator even with a capacitor on the

adjustment terminal. The adjust pin can take a transient signal of ±25V with respect to the output voltage without damaging the device.

When an output capacitor is connected to a regulator and the input is shorted, the output capacitor will discharge into the output of the regulator. The discharge current depends on the value of the capacitor, the output voltage of the regulator, and rate of decrease of VIN. In the LM1085 regu- lator, the internal diode between the output and input pins can withstand microsecond surge currents of 10A to 20A. With an extremely large output capacitor (≥1000 µf), and with input instantaneously shorted to ground, the regulator could be damaged. In this case, an external diode is recom- mended between the output and input pins to protect the regulator, shown in Figure 5.

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FIGURE 5. Regulator with Protection Diode

OVERLOAD RECOVERY

Overload recovery refers to regulator’s ability to recover from a short circuited output. A key factor in the recovery process is the current limiting used to protect the output from drawing too much power. The current limiting circuit reduces the output current as the input to output differential increases. Refer to short circuit curve in the curve section.

During normal start-up, the input to output differential is small since the output follows the input. But, if the output is shorted, then the recovery involves a large input to output differential. Sometimes during this condition the current lim- iting circuit is slow in recovering. If the limited current is too low to develop a voltage at the output, the voltage will stabilize at a lower level. Under these conditions it may be necessary to recycle the power of the regulator in order to get the smaller differential voltage and thus adequate start up conditions. Refer to curve section for the short circuit current vs. input differential voltage.

THERMAL CONSIDERATIONS

ICs heats up when in operation, and power consumption is one factor in how hot it gets. The other factor is how well the heat is dissipated. Heat dissipation is predictable by knowing the thermal resistance between the IC and ambient (θJA). Thermal resistance has units of temperature per power (C/ W). The higher the thermal resistance, the hotter the IC.

The LM1085 specifies the thermal resistance for each pack- age as junction to case (θJC). In order to get the total resistance to ambient (θJA), two other thermal resistance

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