UC2844BDR2(2008) データシートの表示（PDF） - ON Semiconductor

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UC2844BDR2
(Rev.:2008)

High Performance Current Mode Controllers

ON Semiconductor 

UC3844B, UC3845B, UC2844B, UC2845B

VCC

Vin

7(12)

8(14)

4(7)

R2

2(3)

1(1)

R1

VClamp ≈

R

Bias

5.0V Ref

+

-

R

Osc

+

1.0 mA

+

-

VClamp

EA

2R

R

1.0V

T

S

Q

R

Comp/Latch

7(11)

Q1

6(10)

5(8)

3(5)

RS

5(9)

1.67

ǒ Ǔ R2

ǒ Ǔ R1

)

1

+ 0.33x10-3

R1R2

R1 ) R2

Where: 0 ≤ VClamp ≤ 1.0 V

Ipk(max) 

[ 

VClamp

RS

Figure 20. Adjustable Reduction of Clamp Level

8(14)

R

Bias

5.0V Ref

R

+

-

Osc

4(7)

+

T

1.0mA

S

2(3)

Q

2R

R

1.0M

EA

R

1.0V

1(1)

C

5(9)

tSoft-Start ≈ 3600C in mF

Figure 21. Soft−Start Circuit

VCC

Vin

7(12)

8(14)

R

Bias

5.0V Ref

+

-

R

+

-

Osc

4(7)

+

VClamp

1.0 mA

2(3)

EA

R2

1(1)

R1 MPSA63

2R

R

1.0V

5(9)

T

S

Q

R

Comp/Latch

7(11)

Q1

6(10)

5(8)

3(5)

RS

1.67

ǒ Ǔ VClamp ≈

R2

R1

)

1

Where: 0 ≤ VClamp ≤ 1.0 V

ƪ ƫ tSoftStart + * In

1

* 

VC

3VClamp

 C

R1 R2

R1 ) R2

Ipk(max) 

[ 

VClamp

RS

VCC

Vin

(12)

VPin 5 

[ 

RS Ipk rDS(on)

rDM(on)  )  RS

5.0V Ref

+

-

+

-

T

S

Q

R

If: SENSEFET = MTP10N10M

RS = 200

Then :  VPin 5  [  0.075 Ipk

(11)

G

(10)

D SENSEFET

S

K

M

(8)

Comp/Latch

(5)

RS

1/4 W

Power Ground:

To Input Source

Return

Control Circuitry Ground:

To Pin (9)

Virtually lossless current sensing can be achieved with the implementation

of a SENSEFETt power switch. For proper operation during over-current

conditions, a reduction of the Ipk(max) clamp level must be implemented.

Refer to Figures 20 and 22.

Figure 22. Adjustable Buffered Reduction of

Clamp Level with Soft−Start

Figure 23. Current Sensing Power MOSFET

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