SC411 datasheet(12/27 Pages) SEMTECH

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SC411

POWER MANAGEMENT

Dropout Performance

The output voltage adjust range for continuous-conduction

operation is limited by the ﬁxed 550ns (maximum) mini-

mum off-time one-shot. For best dropout performance,

use the slowest on-time setting of 200kHz. When work-

ing with low input voltages, the duty-factor limit must be

calculated using worst-case values for on and off times.

The IC duty-factor limitation is given by:

Be sure to include inductor resistance and MOSFET on-

state voltage drops when performing worst-case dropout

duty-factor calculations.

SC411 System DC Accuracy

Two IC parameters affect system DC accuracy, the error

comparator threshold voltage variation and the switching

frequency variation with line and load. The error com-

parator threshold does not drift signiﬁcantly with supply

and temperature. Thus, the error comparator contributes

1.2% or less to DC system inaccuracy. Board components

and layout also inﬂuence DC accuracy. The use of 1%

feedback resistors contribute 1%. If tighter DC accuracy

is required use 0.1% feedback resistors.

The on-pulse in the SC411 is calculated to give a pseu-

do- ﬁxed frequency. Nevertheless, some frequency varia-

tion with line and load can be expected. This variation

changes the output ripple voltage. Because constant-on

regulators regulate to the valley of the output ripple, ½ of

the output ripple appears as a DC regulation error. For

example, if the feedback resistors are chosen to divide

down the output by a factor of ﬁve, the valley of the output

ripple will be VOUT. For example: if VOUT is 2.5V and the

ripple is 50mV with VBAT = 6V, then the measured DC

output will be 2.525V. If the ripple increases to 80mV

with VBAT = 25V, then the measured DC output will be

2.540V.

The output inductor value may change with current. This

will change the output ripple and thus the DC output volt-

age but it will not change the frequency.

)

MAX

(

OFF

)

MIN

(

)

MIN

(

DUTY

Switching frequency variation with load can be minimized

by choosing MOSFETs with lower RDS(ON). High RDS(ON)

MOSFETs will cause the switching frequency to increase

as the load current increases. This will reduce the ripple

and thus the DC output voltage.

Design Procedure

Prior to designing an output and making component selec-

tions, it is necessary to determine the input voltage range

and the output voltage speciﬁcations. For purposes of

demonstrating the procedure the output for the schemat-

ic in Figure 4 on Page 17 will be designed.

The maximum input voltage (V

BAT(MAX)) is determined by the

highest AC adaptor voltage. The minimum input voltage

BAT(MIN)) is determined by the lowest battery voltage after

accounting for voltage drops due to connectors, fuses and

battery selector switches. For the purposes of this design

example we will use a V

BAT range of 8V to 20V.

Four parameters are needed for the output:

1) nominal output voltage, V

OUT (we will use 1.2V).

2) static (or DC) tolerance, TOL

ST (we will use +/-4%).

3) transient tolerance, TOL

TR and size of transient (we will

use +/-8% and 6A for purposes of this demonstration).

4) maximum output current, I

OUT (we will design for 6A).

Switching frequency determines the trade-off between

size and efﬁciency. Increased frequency increases the

switching losses in the MOSFETs, since losses are a func-

tion of VIN2. Knowing the maximum input voltage and

budget for MOSFET switches usually dictates where the

design ends up. A default R

tON value of 1MΩ is suggested

as a starting point, but this is not set in stone. The ﬁrst

thing to do is to calculate the on-time, t

ON, at VBAT(MIN) and

BAT(MAX), since this depends only upon VBAT, VOUT and RtON.

For VOUT < 3.3V:

Application Information (Cont.)

tON_VBAT(MIN) = 3.3 10-12

RtON + 37 103

VOUT

VBAT(MIN) + 50 10

-9

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