LMZ20501 Datasheet, PDF (23/34 Pages) Texas Instruments – 1.0 A SIMPLE SWITCHER® Nano Module

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LMZ20501
SNVS874C – AUGUST 2012 – REVISED APRIL 2015
9.3 Do's and Don'ts
• Don't: Exceed the Absolute Maximum Ratings.
• Don't: Exceed the ESD Ratings .
• Don't: Exceed the Recommended Operating Conditions.
• Don't: Allow the EN or MODE input to float.
• Don't: Allow the voltage on the EN or MODE input to exceed the voltage on the VIN pin.
• Don't: Allow the output voltage to exceed the input voltage.
• Don't: Use the thermal data given in the Thermal Information table to design your application.
• Do: Follow all of the guidelines and/or suggestions found in this data sheet, before committing your design to
production. TI Application Engineers are ready to help critique your design and PCB layout to help make your
project a success.
• Do: Refer to the helpful documents found in Table 3 and Table 4
.
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 4
I_IN

V_OUT b I_OUT
V_IN b 
(4)
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. This circuit may cause over-voltage transients at the VIN
pin, each time the input supply is cycled on and off. The parasitic resistance will cause 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 and/or reset. The best way to solve these
kinds 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 will help 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 module. This can lead
to instability, as well as some of the effects mentioned above, unless it is designed carefully. The following user
guide provides helpful suggestions when designing an input filter for any switching regulator: SNVA489.
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 will be discharged through the regulator back to the
input. This uncontrolled current flow could damage the regulator.
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