K

˜

˜

IN

OUT

OUT

IN

V

I

V

I

29

LM53602, LM53603

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SNVSAR0 – NOVEMBER 2016

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Copyright © 2016, Texas Instruments Incorporated

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 6, where η is the efficiency.

(6)

If the regulator is connected to the input supply through long wires or PCB traces, take special care 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. This circuit may cause overvoltage transients at the VIN

pin, each time the input supply is cycled on and off. The parasitic resistance causes the voltage at the VIN pin to

dip when the load on the regulator is switched on, or exhibits a transient. If the regulator is operating close to the

minimum input voltage, this dip may cause the device to shutdown or reset. The best way to solve these kinds of

issues is to reduce the distance from the input supply to the regulator or use an aluminum or tantalum input

capacitor in parallel with the ceramics. The moderate ESR of these types of capacitors helps to damp the input

resonant circuit and reduce any voltage 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 Simple

Success with Conducted EMI for DC-DC Converters (SNVA489), 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 V-I characteristic (thyristor type). The use of a device with this type of characteristic is not

recommend. When the TVS fires, the clamping voltage drops to a very low value. If this holding voltage is less

than the output voltage of the regulator, the output capacitors is discharged through the regulator back to the

input. This uncontrolled current flow could damage the regulator.