K

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˜

IN

OUT

OUT

IN

V

I

V

I

25

LM60440, LM60430

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SNVSBN5A – FEBRUARY 2020 – REVISED JUNE 2020

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10 Power Supply Recommendations

The characteristics of the input supply must be compatible with the Absolute Maximum Ratings and

Recommended Operating Conditions found in this data sheet. In addition, the input supply must be capable of

delivering the required input current to the loaded regulator. The average input current can be estimated with

Equation 11, where η is the efficiency.

(11)

If the regulator is connected to the input supply through long wires or PCB traces, special care is required to

achieve good performance. The parasitic inductance and resistance of the input cables can have an adverse

effect on the operation of the regulator. The parasitic inductance, in combination with the low-ESR, ceramic input

capacitors, can form an under damped resonant circuit, resulting in overvoltage transients at the input to the

regulator. The parasitic resistance can cause the voltage at the VIN pin to dip whenever a load transient is

applied to the output. If the application is operating close to the minimum input voltage, this dip can cause the

regulator to momentarily shutdown and reset. The best way to solve these kind of issues is to reduce the

distance from the input supply to the regulator and/or use an aluminum or tantalum input capacitor in parallel with

the ceramics. The moderate ESR of these types of capacitors help damp the input resonant circuit and reduce

any overshoots. A value in the range of 20 µF to 100 µF is usually sufficient to provide input damping and help to

hold the input voltage steady during large load transients.

Sometimes, for other system considerations, an input filter is used in front of the regulator. This can lead to

instability, as well as some of the effects mentioned above, unless it is designed carefully. The user guide AN-

2162 Simple Success With Conducted EMI From DCDC Converters provides helpful suggestions when

designing an input filter for any switching regulator.

In some cases, a transient voltage suppressor (TVS) is used on the input of regulators. One class of this device

has a snap-back characteristic (thyristor type). The use of a device with this type of characteristic is not

recommended. When the TVS fires, the clamping voltage falls to a very low value. If this voltage is less than the

output voltage of the regulator, the output capacitors discharge through the device back to the input. This

uncontrolled current flow can damage the device.

The input voltage must not be allowed to fall below the output voltage. In this scenario, such as a shorted input

test, the output capacitors discharges through the internal parasitic diode found between the VIN and SW pins of

the device. During this condition, the current can become uncontrolled, possibly causing damage to the device. If

this scenario is considered likely, then a Schottky diode between the input supply and the output should be used.