ACE5019A

18V Low Current Consumption 350mA CMOS Voltage Regulator

VER 1.1

8

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 3Ω 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 1μF tantalum capacitor from OUT

to GND close to the pins. For improved transient response, this output capacitor may be ceramic.