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| 型號： | CONFIGURING |
| 英文描述： | Configuring the ?lanSC300 Device＇s Internal CGA Controller for a Specific LCD Panel Application Note? 277KB (PDF) |
| 中文描述： | 配置？lanSC300設備的內部海巡署控制器為特定的液晶面板應用說明？ 277KB（PDF格式） |
| 文件頁數： | 1/3頁 |
| 文件大小： | 21K |
| 代理商： | CONFIGURING |
Configure Low Cost Buck Converter for Boost Operation
Buck converters are inherently different from boost converters in that the power switch is referenced to the
output voltage (Buck mode) rather than ground (Boost mode). The Semtech SC1101 low cost , Buck
controller has a floating output drive section allowing it to be configured as a Boost or Buck controller.
The circuit shown in figure 1 configures the SC1101 for a +5 to +/-12V conversion with +/-500 mA output
current.
Some pin connections are changed to use the SC1101 for step-up applications. The BST pin, which is
normally connected to a high side drive supply in a buck converter, is now connected to the Vcc pin to
drive the ground referenced MOSFET. Also, the low side of the current sense resistor, CS-, is connected to
PGND pin. The feedback voltage will also need to reflect the new ratio of output to the 1.25V internal
reference. In the schematic shown, the output is calculated as follows:
The sense resistor, R4, serves two purposes. First, it assures current limiting in the event of an overload or
short circuit condition. It also guarantees proper start up upon application of power with full load by
limiting the maximum duty cycle. With the output capacitors fully discharged, application of a full load
demands high peak inductor currents to both charge the output capacitor and to supply the load current.
However, if the duty cycle exceeds a maximum limit, the inductor will not have sufficient time to discharge
and will saturate. By limiting the peak the inductor/switch current and thus the duty cycle, the output
capacitors will be allowed to charge in several cycles upon start-up without inductor saturation. In this
application, a .012 ohm resistor performs peak current limiting while allowing the required output current
of +/- 500ma to be delivered. A 1K/100pf capacitor is added to filter the leading edge spike. In other
cases a voltage divider may be used to tailor the current sense requirements to the 70mv internal
comparator threshold.
While most boost converters require external frequency compensation components, the SC1101 voltage
mode converter requires no additional parts in Boost mode. It is required that the output capacitor be
selected with attention paid to the RHPZ (Right Half Plane Zero) introduced in all Boost (and Flyback)
converters. From a practical view, at RHPZ frequency, the loop gain starts increasing at a 20db/decade rate
but the loop phase decreases by –45 degrees (in a normal, Left Half Plane Zero, the loop phase will
increase by +45 degrees). This imposes the restriction that the gain be rolled off to 0db before
encountering the RHPZ. The RHPZ frequency is calculated as follows:
Where R is primarily the effective output load resistor:
The double pole introduced by the output inductor and the output capacitor is used for rolling off the gain
and is calculated as follows:
Where D is the converter duty cycle and is calculated as follows:
)
5
R
1
R
1
×
25
.
Vout
÷
+
=
)
Vi
Vo
(
)
R
sL
(
1
)
Fz
2
2
÷
×
÷
=
Iout
Vout
R
÷
=
LC
)
D
1
W
o
÷
=
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