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參數資料
| 型號： | SC411EVB |
| 廠商： | Semtech Corporation |
| 英文描述： | Synchronous Buck Pseudo-Fixed Frequency Power Supply Controller |
| 中文描述： | 同步降壓偽固定頻率電源控制器 |
| 文件頁數： | 12/27頁 |
| 文件大?。?/td> | 488K |
| 代理商： | SC411EVB |
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12
2007 Semtech Corp.
www.semtech.com
SC411
POWER MANAGEMENT
Application Information (Cont.)
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 signi
fi
cantly with supply
and temperature. Thus, the error comparator contributes
1.2% or less to DC system inaccuracy. Board components
and layout also in
fl
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, 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.
)
MAX
(
OFF
t
)
MIN
(
ON
t
)
MIN
(
ON
t
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 speci
fi
cations. 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
) is determined by the
highest AC adaptor voltage. The minimum input voltage
(V
) 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
(we will use 1.2V).
2) static (or DC) tolerance, TOL
(we will use +/-4%).
3) transient tolerance, TOL
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 ef
fi
ciency. Increased frequency increases the
switching losses in the MOSFETs, since losses are a func-
tion of VIN
2
. Knowing the maximum input voltage and
budget for MOSFET switches usually dictates where the
design ends up. A default R
value of 1M
Ω
is suggested
as a starting point, but this is not set in stone. The
fi
rst
thing to do is
to calculate the on-time, t
ON
, at V
and
V
BAT(MAX)
, since this depends only upon V
BAT
, V
OUT
and R
tON
.
For VOUT < 3.3V:
t
ON_VBAT(MIN)
= 3.3 10
-12
Rt
ON
+ 37 10
3
V
OUT
V
BAT(MIN)
+ 50 10
-9
s
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相關代理商/技術參數 |
參數描述 |
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| SC411MLTRT | 制造商:Semtech Corporation 功能描述:Synchronous Buck Pseudo-Fixed Frequency Power Supply Controller |
| SC412A | 制造商:SEMTECH 制造商全稱:Semtech Corporation 功能描述:Synchronous Buck Controller |
| SC412A_1 | 制造商:SEMTECH 制造商全稱:Semtech Corporation 功能描述:Synchronous Buck Controller |
| SC412AEVB | 制造商:SEMTECH 制造商全稱:Semtech Corporation 功能描述:Synchronous Buck Controller |
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