LTC3406B-1.2 查看數據表（PDF） - Linear Technology

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LTC3406B-1.2

1.5MHz, 600mA Synchronous Step-Down Regulator in ThinSOT

Linear Technology 

LTC3406B-1.2

APPLICATIO S I FOR ATIO

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:

∆VOUT

≅

∆IL

⎛

⎝⎜

ESR

+

1⎞

8fCOUT ⎠⎟

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

LTC3406B-1.2’s control loop does not depend on the

output capacitor’s ESR for stable operation, ceramic ca-

pacitors 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

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 + ...)

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 LTC3406B-1.2 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 consequence as illustrated in

Figure 2.

1

0.1

0.01

0.001

0.0001

0.1

VIN = 2.7V

VIN = 3.6V

VIN = 4.2V

1

10

100

LOAD CURRENT (mA)

1000

3406B12 F02

Figure 2. Power Loss vs Load Current

8

sn3406b12 3406b12fs

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