LTC3545/LTC3545-1

13

35451fb

series resistance (ESR). Typically, once the ESR require-

output ripple

ΔΔ

V

I

ESR

C

OUT

L

OUT

≅+

⎛

⎝⎜

⎞

⎠⎟

1

8 ••

ƒ

and

voltage, the output ripple is highest at maximum input

voltage since

Using Ceramic Input and Output Capacitors

Higher value, lower cost, ceramic capacitors are now

widely available in smaller case sizes. Their high ripple

current, high voltage rating and low ESR make them

ideal for switching regulator applications. Because the

LTC3545/LTC3545-1’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.

Output Voltage Programming

according to the following formula:

VV

R

R

⎛

⎝⎜

⎞

⎠⎟

06

1

2

1

.

The external resistive divider is connected to the output

allowing remote voltage sensing as shown in Figure 2.

APPLICATIONS INFORMATION

Figure 2. Setting the LTC3545 Output Voltage

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 on the

front page of the data sheet.

GND

LTC3545

R2

R1

3545 F02