MD3221N 查看數據表（PDF） - Unspecified

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MD3221N

DC-DC Converter Power IC

Unspecified 

The OSC pin is used in setting the oscillation

frequency. The 300kHz frequency is selected by

connecting the OSC pin to the Vref pin, and 100kHz

by connecting it to the GND pin.

● Frequency Switching

Vref 2

OSC 3

MD3221N

GND 4

300kHz

100kHz

◆The Soft Start Function

The MD3221N incorporates a soft start function to

prevent overshoot at start-up and to reduce

electrical stress on the device. As shown in the

diagram below, the condenser (C2) is connected

across the S / S and GND pins. As the duty ratio is

limited by the voltage at the S / S pin, gradual

charging of this condenser allows a fixed rate of

increase in the output voltage.

● Soft Start Circuit

Is/s

32

S/S

C2

MD3221N

● Relationship Between Soft Start Voltage and Input/Output Voltage

Vi

2.9V

Vss

Vo

It is necessary to monitor output voltage VO is

always started up in advance to the S / S terminal

voltage. (Refer to the chart above)

As shown in the graph below, the capacitance of the

condenser connected to the S / S pin determines the

start-up time.

● Relationship Between S/S Condenser and Output Voltage Start-up Time

100

Vo/Vi=0.1

10

Vo/Vi=0.3

Vo/Vi=0.5

Vo/Vi=0.7

1

0.1

0.001

0.010

0.100

S/S condenser capacitance (µF)

1.000

Select a high value for capacitance of the condenser

connected to the S / S pin if latch is halted at start-up.

As shown in the diagram, the S / S pin is charged at a

fixed current. When the voltage at this pin reaches 0.5V,

main switch oscillation begins and output voltage

increases. As the voltage at the S / S pin increases to

a constant value of 2.9V the output voltage must

reach the set voltage during that interval. If the

capacitance of the output condenser is too high, the

increase in the output voltage does not follow the

increase at the S / S pin, thus preventing start-up in some

cases.

◆The Remote ON / OFF Function (R/C)

An external signal may be used for ON / OFF control

of the MD3221N. The MD3221N is switched ON when

the R / C pin is set to L (0.7V or lower), and switched

OFF when it is set to H (2V or higher), or is open.

Current consumption at OFF is approximately 25μA.

As the R / C pin is pulled up internally it may be used

as an open collector, thus eliminating the need for

application of an external voltage. Any external

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