TYPICAL APPLICATION CIRCUIT

APPLICATION INFORMATION

Selection of Input/ Output Capacitors

In general, all the capacitors need to be low leakage. Any leakage the capacitors have will

reduce efficiency, increase the quiescent current.

A recent trend in the design of portable devices has been to use ceramic capacitors to filter

DC-DC converter inputs. Ceramic capacitors are often chosen because of their small size,

low equivalent series resistance (ESR) and high RMS current capability. Also, recently,

designers have been looking to ceramic capacitors due to shortages of tantalum capacitors.

Unfortunately, using ceramic capacitors for input filtering can cause problems. Applying a

voltage step to a ceramic capacitor causes a large current surge that stores energy in the

inductances of the power leads. A large voltage spike is created when the stored energy is

transferred from these inductances into the ceramic capacitor. These voltage spikes can

easily be twice the amplitude of the input voltage step. (See “Ceramic Input Capacitors Can

Cause Overvoltage Transients”——Linear Technology application note 88, March 2001)

Many types of capacitors can be used for input bypassing, however, caution must be

exercised when using multilayer ceramic capacitors (MLCC). Because of the self-resonant

and high Q characteristics of some types of ceramic capacitors, high voltage transients can

be generated under some start-up conditions, such as connecting the LDO input to a live

power source. Adding a 3

resistor in series with an X5R ceramic capacitor will minimize

start-up voltage transients.

The LDO also requires an output capacitor for loop stability. Connect a 1 F tantalum

capacitor from OUT to GND close to the pins. For improved transient response, this output

capacitor may be ceramic.

LESHAN RADIO COMPANY, LTD.

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LR6302A/D

LR6302B