SC411EVB SEMTECH [Semtech Corporation], SC411EVB Datasheet - Page 12

• Manufacturer Part Number: SC411EVB
• Description: Synchronous Buck Pseudo-Fixed Frequency Power Supply Controller
• Manufacturer: SEMTECH [Semtech Corporation]

Dropout Performance
The output voltage adjust range for continuous-conduction
operation is limited by the fi xed 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 signifi cantly with supply
and temperature. Thus, the error comparator contributes
1.2% or less to DC system inaccuracy. Board components
and layout also infl uence 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- fi xed 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 fi ve, 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.
© 2007 Semtech Corp.
POWER MANAGEMENT
Application Information (Cont.)
DUTY
=
t
ON
(
MIN
t
ON
)
+
(
MIN
t
OFF
)
(
MAX
)
12
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 specifi cations. For purposes of
demonstrating the procedure the output for the schemat-
ic in
The maximum input voltage (V
highest AC adaptor voltage. The minimum input voltage
(V
accounting for voltage drops due to connectors, fuses and
battery selector switches. For the purposes of this design
example we will use a V
Four parameters are needed for the output:
1) nominal output voltage, V
2) static (or DC) tolerance, TOL
3) transient tolerance, TOL
use +/-8% and 6A for purposes of this demonstration).
4) maximum output current, I
Switching frequency determines the trade-off between
size and effi ciency. Increased frequency increases the
switching losses in the MOSFETs, since losses are a func-
tion of VIN
budget for MOSFET switches usually dictates where the
design ends up. A default R
as a starting point, but this is not set in stone. The fi rst
thing to do is to calculate the on-time, t
V
For VOUT < 3.3V:
t
ON_VBAT(MIN)
BAT(MAX)
BAT(MIN)
Figure 4 on Page 17
, since this depends only upon V
) is determined by the lowest battery voltage after
= 3.3 10
2
. Knowing the maximum input voltage and
-12
Rt
BAT
ON
will be designed.
range of 8V to 20V.
TR
+ 37 10
tON
OUT
and size of transient (we will
OUT
BAT(MAX)
ST
value of 1MΩ is suggested
(we will use 1.2V).
(we will design for 6A).
(we will use +/-4%).
3
) is determined by the
V
BAT
BAT(MIN)
V
ON
OUT
, V
www.semtech.com
, at V
OUT
SC411
+ 50 10
and R
BAT(MIN)
tON
and
-9
.
s