SC3303 datasheet(20/26 Pages) SEMTECH

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SC3303

Applications Information (continued)

COUT

Low-

side

High-

side

Figure 10 — Signal Injection Circuit

The values of R

, C

, and C

are dependent on the condi-

tions of the specific application such as V

, V

OUT

, f

and

OUT

Select a value for C

, like 10nF. Using C

, calculate R

shown in the following equation:

DCR

Where L is the inductor value and DCR is the resistance of

the inductor.

The value for C

can be between C

C_MIN

and C

C_MAX

MIN

MAX

Where T = 1/f

and R

is represented by the following

equation.

TOP

BOTTOM

TOP

BOTTOM

It is beneficial to use the smallest value of C

that provides

stability and enough voltage ripple at feedback. Larger

values of C

may negatively affect the load regulation

performance.

Output Voltage Dropout

The output voltage adjustable range for continuous-con-

duction operation is limited by the fixed 320ns (typical)

minimum off-time. When working with low input volt-

ages, the duty-factor limit must be calculated using worst-

case values for on and off times. The duty-factor limitation

is shown by the next equation.

)

MAX

(

OFF

)

MIN

(

)

MIN

(

DUTY

The inductor resistance and MOSFET on-state voltage

drops must be included when performing worst-case

dropout duty-factor calculations.

System DC Accuracy — V

OUT Controller

Three factors affect V

OUT

accuracy: the trip point of the FB

error comparator, the ripple voltage variation with line

and load, and the external resistor tolerance. The error

comparator offset is trimmed so that under static condi-

tions it trips when the feedback pin is 750mV, +1%.

The on-time pulse from the SC3303 in the design example

is calculated to give a pseudo-fixed frequency of 500kHz.

Some frequency variation with line and load is expected.

This variation changes the output ripple voltage. Because

adaptive on-time converters regulate to the valley of the

output ripple, ½ of the output ripple appears as a DC regu-

lation error.

To compensate for valley regulation, it may be desirable to

use passive droop. Take the feedback directly from the

output side of the inductor and place a small amount of

trace resistance between the inductor and output capaci-

tor. This trace resistance should be optimized so that at

full load the output droops to near the lower regulation

limit. Passive droop minimizes the required output capaci-

tance because the voltage excursions due to load steps

are reduced as seen at the load.

The use of 1% feedback resistors may result in up to an

additional 1% error. If tighter DC accuracy is required,

resistors with lower tolerances should be used.

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