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LTC1753

1753fa

The RDS(ON) required for a given conduction loss can now

be calculated by rearranging the relation P = I2R.

DC Q

DS ON Q

MAX Q

MAX

MAX Q

OUT

MAX

DS ON Q

MAX Q

MAX

MAX Q

OUT

MAX

()

[]()

()







()( )

()

[]()

()







−

()( )

PMAX should be calculated based primarily on required

efficiency or allowable thermal dissipation. A typical high

efficiency circuit designed with a 5V input and a 2.8V,

11.2A output might allow no more than 4% efficiency loss

at full load for each MOSFET. Assuming roughly 90%

efficiency at this current level, this gives a PMAX value of:

[(2.8)(11.2A/0.9)(0.04)] = 1.39W per FET

and a required RDS(ON) of:

DS ON Q

()

= ()(

)

()( )

()(

)

−

()( )

51 39

2 8

11 2

0 019

51 39

52 8

11 2

0 025

Ω

Note also that while the required RDS(ON) values suggest

large MOSFETs, the dissipation numbers are only 1.39W

per device or less––large TO-220 packages and heat sinks

are not necessarily required in high efficiency applica-

tions. Siliconix Si4410DY or International Rectifier IRF7413

(both in SO-8) or Siliconix SUD50N03 or Motorola

MTD20N03HDL (both in D PAK) are small footprint sur-

face mount devices with RDS(ON) values below 0.03Ω at 5V

of gate drive that work well in LTC1753 circuits. With

higher output voltages, the RDS(ON) of Q1 may need to be

significantly lower than that for Q2. These conditions can

often be met by paralleling two MOSFETs for Q1 and using

a single device for Q2. Note that using a higher PMAX value

Note: Please refer to the manufacturer’s data sheet for testing conditions

and detail information.

Table 4. Recommended MOSFETs for LTC1753 Applications

TYPICAL INPUT

RDS(ON)

CAPACITANCE

PARTS

AT 25

°C (mΩ)

RATED CURRENT (A)

CISS (pF)

θJC (°C/W)

TJMAX (°C)

Siliconix SUD50N03-10

15 at 25

°C

3200

1.8

175

D-PAK

10 at 100

°C

Siliconix Si4410DY

10 at 25

°C

2700

—

150

SO-8

8 at 75

°C

ON Semiconductor MTD20N03HDL

20 at 25

°C

880

1.67

150

D PAK

16 at 100

°C

Fairchild FDS6670A

13 at 25

°C

3200

150

SO-8

Fairchild FDS6680

11.5 at 25

°C

2070

150

SO-8

ON Semiconductor MTB75N03HDL

7.5

75 at 25

°C

4025

1.0

150

DD PAK

59 at 100

°C

IR IRL3103S

56 at 25

°C

1600

1.8

175

DD PAK

40 at 100

°C

IR IRLZ44

50 at 25

°C

3300

1.0

175

TO-220

36 at 100

°C

Fuji 2SK1388

35 at 25

°C

1750

2.08

150

TO-220

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