Function Description(Cont.)

Low Supply Operation

The FTD6821 will operate with input supply voltages as

low as 2.5V, but the maximum allowable output current

is reduced at this low voltage. Figure 2 shows the

reduction in the maximum output current as a function of

input voltage for various output voltages.

Slope Compensation and Inductor Peak

Current

Slope compensation provides stability in constant

frequency architectures by preventing subharmonic

oscillations at high duty cycles. It is accomplished

internally by adding a compensating ramp to the inductor

current signal at duty cycles in excess of 40%. Normally,

this results in a reduction of maximum inductor peak

current for duty cycles >40%. However, the FTD6821

uses a patent-pending scheme that counteracts this

compensating ramp, which allows the maximum inductor

peak current to remain unaffected throughout all duty

cycles.

Maximum Output Current vs Input Voltag

The basic FTD6821 application circuit is shown in Figure

3. External component selection is driven by the load

requirement and begins with the selection of L followed

by CIN and COUT.

Inductor Selection

For most applications, the value of the inductor will fall in

the range of 1mH to 4.7mH. Its value is chosen based on

the desired ripple current. Large value inductors lower

ripple current and small value inductors result in higher

ripple currents. Higher VIN or VOUT also increases the

ripple current as shown in equation 1. A reasonable

starting point for setting ripple current is DIL = 480mA

(40% of 1200mA).

The DC current rating of the inductor should be at least

equal to the maximum load current plus half the ripple

current to prevent core saturation. Thus, a 1320mA

rated inductor should be enough for most applications

(1200mA + 120mA). For better efficiency, choose a low

DC-resistance

inductor.

The inductor value also has an effect on Burst Mode

operation. The transition to low current operation begins

when the inductor current peaks fall to approximately

200mA. Lower inductor values (higher DIL) will cause

this to occur at lower load currents, which can cause a

dip in efficiency in the upper range of low current

operation. In Burst Mode operation, lower inductance

values will cause the burst frequency to increase.

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SEMICONDUCTOR

TECHNICAL D ATA

FTD6821

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