Linear Regulators Part 7 - Schematic Design Checklist

LDO Selection Guide

1. Input and Output Capacitors
2. Adjustable regulator – resistor value selection
3.  Special purpose pin connection – (Enable/PowerGood/Reset)
4. Thermal Management

Input and Output Capacitors

Input Capacitors:

Low-dropout linear regulators need capacitors on both their input and output. The input capacitor provides bypassing of the internal op amp used in the voltage regulation loop. The output capacitor improves regulator response to sudden load changes and provides loop compensation that allows stable operation.

The input capacitor for monolithic regulators should feature low inductance and generally good

high frequency performance. Capacitance is not too critical except for systems where excessive input ripple voltage is present. The capacitor must, as a minimum, maintain the input voltage minimum value above the dropout point. Otherwise, the regulator ceases regulation and becomes merely a saturated switch.

A 0.1µF ceramic directly adjacent to the regulator is always a good choice, however. If the

regulator is farther away from the filter capacitor, local bypassing is mandatory

Output Capacitors:

Linear regulators require a certain minimum value of output capacitance for operation—below

this minimum value, the output may exhibit oscillation. The output capacitor is inside the voltage control loop and is necessary for loop stabilization. Minimum recommended values are listed on each device data sheet.

Excellent response to high frequency load changes (load current transient recovery) demands

low inductance, low ESR, high frequency filter capacitors. Stringent requirements are solved by paralleling multiple medium sized capacitors.

Capacitors should be chosen by comparing their lead inductance,ESR, and dissipation factor. Multiple small or medium sized capacitors provide better high frequency characteristics than a single capacitor of the same total capacity since the lead inductance and ESR of the multiple

capacitors is reduced by paralleling.

Adjustable output regulator – Resistor Value Selection:

For Adjustable regulators, the output also depends upon the accuracy of two programming resistors. Some systems require supply voltage accuracies better than ±2.5%—including noise and transients. Adjustable regulators use the ratio of two resistors to multiply the reference voltage to produce the desired output voltage. The formula for output voltage from two resistors is presented as

Since the output voltage is proportional to the product of the reference voltage and the ratio of the programming resistors, at high output voltage, the error contribution of the programming resistors is the sum of each resistor’s tolerance. Two standard ±1% resistors contribute as much as 2% to output voltage error. At lower voltages, the error is less significant. The more expensive, tighter accuracy resistors provide improved tolerance, but it is still limited by the adjustable regulator’s internal reference.

Special purpose pin connection – (Enable/Power Good/Reset)

Enable:

Check the pin is active low or active high. Connect to GND if it is active low enable or pull it up to VCC if it is active high enable.

Power Good/Reset:

Check the pin is active low or active high output. If voltage level meet the required thersold the power good pin will go low or high based on the output type. Connect the required pull up to voltage if it is open drain output.

Thermal management:

TO-220 package applications at moderate (room) temperatures may not require heat sinking if

the power dissipation is less than 2 watts. Otherwise,heat sinks are necessary. Use the minimum practical lead length so heat may travel more directly to the board, and use the board itself as a heat sink.

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