SC461 datasheet(26/31 Pages) SEMTECH

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SC461

Applications Information (continued)

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 following

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 SC461 in the design example

is calculated to give a pseudo-fixed frequency of 220kHz.

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 = 25V, 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.

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 contributes 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 Variations

The switching frequency will vary depending on line and

load conditions. The line variations are a result of fixed

propagation delays in the on-time one-shot, as well as

unavoidable delays in the external MOSFET switching. As

IN increases, these factors make the actual DH on-time

slightly longer than the ideal on-time. The net effect is

that frequency tends to falls slightly with increasing input

voltage.

The switching frequency also varies with load current as a

result of the power losses in the MOSFETs and the induc-

tor. For a conventional PWM constant-frequency con-

verter, 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). Because the on-time is

essentially constant for a given V

OUT/VIN combination, to

offset the losses the off-time will reduce slightly as load

increases. The net effect is that switching frequency

increases slightly with increasing load.

PCB Layout Guidelines

A switch-mode converter requires good PCB layout which

is essential to achieving high performance. The following

guidelines will provide an optimum PCB layout.

The device layout recommendations consist of four parts.

Grounding for PGND and AGND

Power components

Low-noise analog circuits

Bypass capacitors

Grounding for PGND and AGND

A ground plane layer for PGND is recommended

to minimize the effects of switching noise, resis-

tive losses, and to maximize heat removal from

the power components.

•

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