11

LTC3409

3409f

voltage, the output ripple is highest at maximum input

electrolytic and dry tantalum capacitors are both available

in surface mount configurations. In the case of tantalum,

it is critical that the capacitors are surge tested for use in

switching power supplies. An excellent choice is the AVX

TPS series of surface mount tantalum. These are specially

constructed and tested for low ESR so they give the lowest

ESR for a given volume. Other capacitor types include

Sanyo POSCAP, Kemet T510 and T495 series, and Sprague

593D and 595D series. Consult the manufacturer for other

specific recommendations.

Using Ceramic Input and Output Capacitors

Higher value, lower cost ceramic capacitors are now

available in smaller case sizes. Their high ripple current,

high voltage rating and low ESR make them ideal for

switching regulator applications. Because the LTC3409’s

control loop does not depend on the output capacitor’s

ESR for stable operation, ceramic capacitors can be used

to achieve very low output ripple and small circuit size.

However, care must be taken when these 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.

Output Voltage Programming

The output voltage is set by a resistive divider according

to the following formula:

VV

R

R

⎛

⎝

⎜

⎞

⎠

⎟

0 613

1

1

2

.

The external resistive divider is connected to the output,

allowing remote voltage sensing as shown in Figure 1.

APPLICATIO S I FOR ATIO

R1

R2

3409 F01

GND

LTC3409

Figure 1

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 no consequence as illustrated in Figure 2.