11

LTC3403

3403f

Using Ceramic Input and Output Capacitors

Higher values, lower cost ceramic capacitors are now

becoming available in smaller case sizes. Their high ripple

current, high voltage rating and low ESR make them ideal

for switching regulator applications. Because the

LTC3403’s control loop does not depend on the output

capacitor’s ESR for stable operation, ceramic capacitors

can be used freely to achieve very low output ripple and

small circuit size.

However, care must be taken when ceramic capacitors are

used at the input and the output. When a ceramic capacitor

is used at the input and the power is supplied by a wall

adapter through long wires, a load step at the output can

couple to the output and be mistaken as loop instability. At

worst, a sudden inrush of current through the long wires

to damage the part.

When choosing the input and output ceramic capacitors,

choose the X5R or X7R dielectric formulations. These

dielectrics have the best temperature and voltage charac-

teristics of all the ceramics for a given value and size.

Ceramic capacitors of Y5V material are not recommended

because normal operating voltages cause their bulk ca-

pacitance to become much less than the nominal value.

Programming the Output Voltage With a DAC

The output voltage can be dynamically programmed to any

voltage from 0.3V to 3.5V with an external DAC driving the

REF pin. When the output is commanded low, the output

voltage descends quickly in forced continuous mode

pulling current from the output and transferring it to the

input. If the input is not connected to a low impedance

source capable of absorbing the energy, the input voltage

could rise above the absolute maximum voltage of the part

higher is the voltage spike at the input. For best results,

ramp the REF pin from high to low as slow as the

application will allow. Avoid abrupt changes in voltage of

>0.2V/

µs. If ramp control is unavailable, an RC filter with

a time constant of 10

µs can be inserted between the REF

pin and the DAC as shown in Figure 3.

Efficiency Considerations

The efficiency of a switching regulator is equal to the

output power divided by the input power times 100%. It is

often useful to analyze individual losses to determine what

is limiting the efficiency and which change would produce

the most improvement. Efficiency can be expressed as:

Efficiency = 100% – (L1 + L2 + L3 + ...)

where L1, L2, etc. are the individual losses as a percentage

of input power.

Although all dissipative elements in the circuit produce

losses, two main sources usually account for most of the

loss dominates the efficiency loss at medium to high load

currents. In a typical efficiency plot, the efficiency curve at

very low load currents can be misleading since the actual

power lost is of little consequence as illustrated in

Figure 4.

APPLICATIO S I FOR ATIO

LTC3403

REF

GND

DAC

10k

1000pF

Figure 3. Filtering the REF Pin

Figure 4. Power Lost vs Load Current

LOAD CURRENT (mA)

0.1

1

0.00001

0.001

1

10

100

1000

3406 F04

0.0001

0.01

0.1