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LTC3541EDD-3

High Efficiency Buck + VLDO Regulator

Linear Technology 

LTC3541-3

APPLICATIO S I FOR ATIO

or less is recommended to ensure stability. The LTC3541-3

VLDO is a micropower device and output transient response

will be a function of output capacitance. Larger values

of output capacitance decrease the peak deviations and

provide improved transient response for larger load current

changes. Note that bypass capacitors used to decouple

individual components powered by the LTC3541-3 will

increase the effective output capacitor value. High ESR

tantalum and electrolytic capacitors may be used, but

a low ESR ceramic capacitor must be in parallel at the

output. There is no minimum ESR or maximum capacitor

size requirement.

20

BOTH cAPAcITORS ARE 1µF,

10V, 0603 cASE SIZE

0

X5R

–20

–40

Y5V

–60

–80

–100

0

2

4

6

8

10

Dc BIAS VOLTAGE (V)

35413 F06

Figure 6. Change in Capacitor vs Bias Voltage

20

0

X5R

–20

Y5V

–40

–60

–80

BOTH cAPAcITORS ARE 1µF,

10V, 0603 cASE SIZE

–100

–50 –25 0

25 50

TEMPERATURE (°c)

75

35413 F07

Figure 7. Change in Capacitor vs Temperature

Extra consideration must be given to the use of ceramic

capacitors. Ceramic capacitors are manufactured with a

variety of dielectrics, each with different behavior across

temperature and applied voltage. The most common

dielectrics used are Z5U, Y5V, X5R and X7R. The Z5U

and Y5V dielectrics are good for providing high capaci-

tances in a small package, but exhibit large voltage and

temperature coefficients as shown in Figures 6 and 7.

When used with a 2V regulator, a 1µF Y5V capacitor can

lose as much as 75% of its initial capacitance over the

operating temperature range. The X5R and X7R dielectrics

result in more stable characteristics and are usually more

suitable for use as the output capacitor. The X7R type has

better stability across temperature, while the X5R is less

expensive and is available in higher values. In all cases,

the output capacitance should never drop below 1µF or

instability or degraded performance may occur.

EFFICIENCY CONSIDERATIONS

Generally, the efficiency of a regulator is equal to the out-

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

often useful to analyze individual loss terms to determine

which terms are limiting efficiency and what if any change

would yield the greatest improvement. Efficiency can be

expressed as:

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

where L1, L2, etc. are the individual loss terms as a per-

centage of input power.

Although all dissipative elements in the circuit produce

losses, three main sources typically account for the major-

ity of the losses in the LTC3541-3 circuits: VIN quiescent

current, I2R losses and loss across VLDO output device.

When operating with both the buck and VLDO regulator

active (ENBUCK and ENVLDO equal to logic high), VIN

quiescent current loss and loss across the VLDO output

device dominate the efficiency loss at low load currents,

whereas the I2R loss and loss across the VLDO output

device dominate the efficiency loss at medium to high load

currents. At low load currents with the part operating with

the linear regulator (ENBUCK equal to logic low, ENVLDO

35413fc

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