LTC3549

10

3549f

tions do not offer much relief. Note that the capacitor

manufacturer’s ripple current ratings are often based

on 2000 hours of life. This makes it advisable to further

derate the capacitor, or choose a capacitor rated at a

higher temperature than required. Always consult the

manufacturer if there is any question. The selection of

been met, the RMS current rating generally far exceeds

is determined by:

∆= ∆

+

⎛

⎝⎜

⎞

⎠⎟

V

I ESR

fC

OUT

L

OUT

1

8• •

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

The LTC3549 typically will require an output capacitor

in the 4.7µF to 10µF range for optimum stability. 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 LTC3549’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

⎛

⎝⎜

⎞

⎠⎟

0 611

1

1

2

.

(2)

The external resistive divider is connected to the output,

allowing remote voltage sensing as shown in Figure 1

Table 2 gives 1% resistor values for selected output

voltages.

APPLICATIO S I FOR ATIO

Figure 1. Setting the LTC3549 Output Voltage

R1

R2

3549 F01

GND

LTC3549

Table 2. Resistor Values for Selected Output Voltages

R1

R2

0.85V

53.6k

137k

1.2V

133k

137k

1.5V

200k

137k

1.8V

267k

137k