LTC3405A-1.5 datasheet(9/16 Pages) LINER

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LTC3405A-1.5/LTC3405A-1.8

sn3405a1518 3405a1518fs

APPLICATIO S I FOR ATIO

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

losses in LTC3405A series parts circuits: VIN quiescent

current and I2R losses. The VIN quiescent current loss

dominates the efficiency loss at very low load currents

whereas the I2R 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 conse-

quence as illustrated in Figure 3.

Aluminum 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 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 LTC3405A

series’ 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.

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 induce

ringing at the input, VIN. At best, this ringing can couple to

the output and be mistaken as loop instability. At worst, a

sudden inrush of current through the long wires can

potentially cause a voltage spike at VIN, large enough 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.

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 + ...)

Figure 3. Power Lost vs Load Current

LOAD CURRENT (mA)

0.1

1000

0.1

0.01

0.001

0.0001

3405A1518 F03

100

VOUT = 1.8V

VIN = 3.6V

VOUT = 1.5V

1. The VIN quiescent current is due to two components:

the DC bias current as given in the electrical character-

istics and the internal main switch and synchronous

switch gate charge currents. The gate charge current

results from switching the gate capacitance of the

internal power MOSFET switches. Each time the gate is

switched from high to low to high again, a packet of

charge, dQ, moves from VIN to ground. The resulting

dQ/dt is the current out of VINthat is typically larger than

the DC bias current. In continuous mode, IGATECHG =

f(QT + QB) where QT and QB are the gate charges of the

internal top and bottom switches. Both the DC bias and

gate charge losses are proportional to VIN and thus

their effects will be more pronounced at higher supply

voltages.

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