LTC3405 datasheet(9/16 Pages) LINER

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LTC3406

LTC3406-1.5/LTC3406-1.8

3406fa

The selection of COUT is driven by the required effective

series resistance (ESR).

Typically, once the ESR requirement for COUT has been

met, the RMS current rating generally far exceeds the

IRIPPLE(P-P) requirement. The output ripple ∆VOUT is deter-

mined by:

∆≅ ∆

⎛

⎝

⎜

⎞

⎠

⎟

I ESR

OUT

where f = operating frequency, COUT = output capacitance

and ∆IL = ripple current in the inductor. For a fixed output

voltage, the output ripple is highest at maximum input

voltage since ∆IL increases with input voltage.

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

LTC3406’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

APPLICATIO S I FOR ATIO

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.

Output Voltage Programming (LTC3406 Only)

In the adjustable version, the output voltage is set by a

resistive divider according to the following formula:

OUT =+

⎛

⎝

⎜

⎞

⎠

⎟

(2)

The external resistive divider is connected to the output,

allowing remote voltage sensing as shown in Figure 3.

Figure 3. Setting the LTC3406 Output Voltage

VFB

GND

LTC3406

0.6V ≤ VOUT ≤ 5.5V

3406 F03

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.

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