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CONFIGURE

更新时间： 2024-11-23 23:43:07

品牌	Logo	应用领域
其他 - ETC		控制器
页数	文件大小	规格书
3页	25K
描述
Configure Low-Cost Buck Controller for Boost Operation (21k)

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CONFIGURE 数据手册

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:

Vout = 1.25 × (1+ R1÷ R5)

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:

Fz(s) = 1− (sL ÷ R)× (Vo²÷ Vi²)

Where R is primarily the effective output load resistor:

Vout Iout

The double pole introduced by the output inductor and the output capacitor is used for rolling off the gain

and is calculated as follows:

W_o= (1− D) ÷ LC

Where D is the converter duty cycle and is calculated as follows:

12 3

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CONG1A-A2	ETC	获取价格	General Accessories Plugs Type CONG
CONG1A-A5	ETC	获取价格	General Accessories Plugs Type CONG
CONG1A-S2	ETC	获取价格	General Accessories Plugs Type CONG
CONG1A-S5	ETC	获取价格	General Accessories Plugs Type CONG
CONG1O-A2	ETC	获取价格	General Accessories Plugs Type CONG
CONG1O-A5	ETC	获取价格	General Accessories Plugs Type CONG
CONG1O-S2	ETC	获取价格	General Accessories Plugs Type CONG
CONG1O-S5	ETC	获取价格	General Accessories Plugs Type CONG
CONG5A-A2	ETC	获取价格	General Accessories Plugs Type CONG

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