SC402B Datasheet, PDF(27/32 Page) Semtech Corporation – 10A EcoSpeed® Integrated FET Regulator with Programmable LDO

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SC402B Datasheet, PDF (27/32 Pages) Semtech Corporation – 10A EcoSpeed® Integrated FET Regulator with Programmable LDO

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SC402B

Applications Information (continued)

SC402B regulates to the valley of the ripple voltage at the

FB pin, a high ripple magnitude is undesirable as it signifi-

cantly impacts the output voltage regulation. As a result,

it is desirable to select a corner frequency for (R1// R2) x CC

to achieve enough, but not excessive, ripple magnitude

and phase margin. The component values for R1, R2, and

CC should be calculated using the following procedure.

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

time constant using the following equation.

DCR u CL

Select CC by using the following equation.

R1 // R2

2 u S u fsw

The resistor values (R1 and R2) in the voltage divider circuit

set the VOUT for the switcher. The typical value for CC is

from 10pF to 1nF.

The on-time pulse from the SC402B 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, Â½ of the output ripple appears as a DC regu-

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

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

above the comparator trip point. If the ripple increases to

80mV with VIN = 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.

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.

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.

DUTY

TON(MIN)

T T ON(MIN)

OFF(MAX)

The inductor resistance and MOSFET on-state voltage

drops must be included when performing worst-case

dropout duty-factor calculations.

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

tially constant for a given VOUT/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.

System DC Accuracy (VOUT Controller)

Three factors affect VOUT 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%.

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