SC401B datasheet(27/32 Pages) SEMTECH

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SC401B

Applications Information (continued)

The magnitude of the feedback ripple voltage, which is

dominated by the contribution from C

L , is controlled by

the values of R

1, R2 and CC . If the corner frequency of (R1//

2) x CC is too high, the ripple magnitude at the FB pin will

be smaller, which can lead to double-pulsing. Conversely,

if the corner frequency of (R

1// R2) x CC is too low, the ripple

magnitude at FB pin will be higher. Since the SC401B

regulates to the valley of the ripple voltage at the FB pin,

a high ripple magnitude is undesirable as it significantly

impacts the output voltage regulation. As a result, it is

desirable to select a corner frequency for (R

1// R2) x CC to

achieve enough, but not excessive, ripple magnitude and

phase margin. The component values for R

1, R2, and CC

should be calculated using the following procedure.

Select C

L (typical 10nF) and RL to match with L and DCR

time constant using the following equation.

DCR

Select C

C by using the following equation.

The resistor values (R

1 and R2) in the voltage divider circuit

set the V

OUT for the switcher. The typical value for CC is

from 10pF to 1nF.

Dropout Performance

The output voltage adjustment range for continuous con-

duction operation is limited by the fixed 250ns (typical)

minimum off-time of the one-shot. When working with

low input voltages, the duty-factor limit must be calcu-

lated 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 600mV, 1%.

The on-time pulse from the SC401B in the design example

is calculated to give a pseudo-fixed frequency of 300kHz.

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. For example, if the output ripple is 50mV with

IN = 6 volts, then the measured DC output will be 25mV

above the comparator trip point. If the ripple increases to

80mV with V

IN = 17V, then the measured DC output will be

40mV above the comparator trip. The best way to mini-

mize this effect is to minimize the output ripple.

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

error. If tighter DC accuracy is required, 0.1% resistors

should be used.

The output inductor value may change with current. This

will change the output ripple and therefore will have a

minor effect on the DC output voltage. The output ESR

also affects the output ripple and thus has a minor effect

on the DC output voltage.

Switching Frequency Variation

The switching frequency varies with load current as a

result of the power losses in the MOSFETs and DCR of the

inductor. For a conventional PWM constant-frequency

converter, as load increases the duty cycle also increases

slightly to compensate for IR and switching losses in the

MOSFETs and inductor. An adaptive on-time converter

must also compensate for the same losses by increasing

the effective duty cycle (more time is spent drawing

energy from V

IN as losses increase). The on-time is essen-

tially constant for a given V

OUT/VIN combination, to offset

the losses the off-time will tend to reduce slightly as load

increases. The net effect is that switching frequency

increases slightly with increasing load.

SC401B Datasheet(PDF) 27 Page - Semtech Corporation

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