AAT1156 查看數據表（PDF） - Advanced Analogic Technologies

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AAT1156

1MHz 700mA Step-Down DC-DC Converter

Advanced Analogic Technologies 

AAT1156

1MHz 700mA Step-Down DC-DC Converter

Thermal Calculations

There are three types of losses associated with the AAT1156: MOSFET switching losses, conduction losses,

and quiescent current losses. The conduction losses are due to the RDS(ON) characteristics of the internal P-

and N-channel MOSFET power devices. At full load, assuming continuous conduction mode (CCM), a simpli-

fied form of the total losses is given by:

P=

IO2 ⋅ (RDS(ON)H ⋅ VO + RDS(ON)L ⋅

VIN

(VIN - VO))

+ (tsw ⋅ FS ⋅ IO ⋅ VIN + IQ) ⋅ VIN

where IQ is the AAT1156 quiescent current.

Once the total losses have been determined, the junction temperature can be derived from the θJA for the

QFN33-16 package.

TJ = P · ΘJA + TAMB

Adjustable Output

Resistors R3 and R4 of Figure 1 force the output to regulate higher than 0.6V. The optimum value for R4 is

59kΩ. Values higher than this may cause problems with stability, while lower values can degrade light load

efficiency. For a 2.5V output with R4 set to 59kΩ, R3 is 187kΩ.

R3

=

⎛ VO

⎝VREF

-1⎞⎠

·

R4

=

⎛ 2.5V

⎝ 0.6V

-

1 ⎞⎠

·

59kΩ

=

187kΩ

500

450 R4=59kΩ

400

350

300

250

200

150

100

50

0

1 1.5

2 2.5

3

3.5

4 4.5

5

5.5

Output Voltage (V)

Figure 4: R3 vs. VOUT for Adjustable Output Using the AAT1156.

1156.2007.01.1.4

11

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