SC410 SEMTECH [Semtech Corporation], SC410 Datasheet - Page 20

• Manufacturer Part Number: SC410
• Description: 3A EcoSpeed TM Step-Down Regulator with LDO and Ultrasonic Power Save
• Manufacturer: SEMTECH [Semtech Corporation]

Applications Information (continued)
The values of R
tions of the specifi c application such as V
I
Select a value for C
shown in the following equation:
Where L is the inductor value and DCR is the resistance of
the inductor.
The value for C
Where T = 1/f
equation.
It is benefi cial to use the smallest value of C
stability and enough voltage ripple at feedback. Larger
values of C
performance.
OUT
.
R
R
C
C
EQ
L
C
C
_
_
High-
Low-
side
MIN
MAX
side
C
Figure 10 — Signal Injection Circuit
R
L
C
BOTTOM
may negatively affect the load regulation
R
L
t
DCR
R
SW
ON
EQ
L
T
EQ
, C
and R
C
L
, and C
R
can be between C
L
R
C
, like 10nF. Using C
L
BOTTOM
C
EQ
is represented by the following
R
L
C
FB
pin
TOP
C
are dependent on the condi-
L
R
TOP
R1
R2
C_MIN
L
, calculate R
C
IN
C
, V
that provides
OUT
and C
OUT
, f
SW
C_MAX
and
L
as
.
Output Voltage Dropout
The output voltage adjustable range for continuous-con-
duction operation is limited by the fi xed 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.
The inductor resistance and MOSFET on-state voltage
drops must be included when performing worst-case
dropout duty-factor calculations.
System DC Accuracy — V
Three factors aff ect V
error comparator, the ripple voltage variation with line
and load, and the external resistor tolerance. The error
comparator off set is trimmed so that under static condi-
tions it trips when the feedback pin is 750mV,
The on-time pulse from the SC410 in the design example
is calculated to give a pseudo-fi xed 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.
DUTY
t
ON
(
MIN
t
ON
)
(
OUT
MIN
t
OFF
)
accuracy: the trip point of the FB
(
MAX
OUT
)
Controller
+
SC410
1%.
20