FTD6821 datasheet(14/17 Pages) FS

14 / 17 page

Function Description(Cont.)

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 FTD6821’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 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

characteristics of all the ceramics for a given value and

size.

Output Voltage Programming

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

resistive divider according to the following formula:

The external resistive divider is connected to the output,

allowing remote voltage sensing as Figure4.

Figure 4:Setting the output Voltage

Vout

1.2V

150K

1.5V

160K

240K

1.8V

150K

300K

2.5V

150K

470K

3.3V

150K

680K

Table 2. Vout VS. R1, R2, Cf Select Table

Ef�iciency 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 FTD6821 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 5.

2013.01. 28

14/17

SEMICONDUCTOR

TECHNICAL D ATA

FTD6821

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