AAT1157 Ver la hoja de datos (PDF) - Analog Technology Inc

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Fabricante

AAT1157

1MHz 1.2A Buck DC/DC Converter

Analog Technology Inc 

AAT1157

1MHz 1.2A Buck DC/DC Converter

VIN+

C1

10µF

GND

Enable

R1

100

R2

100K

C2

0.1µF

U1

AAT1157

12

VP

4

FB

11

VP

15

LX

10

VP

14

LX

7

EN

13

LX

9

VCC

16

N/C

6

3

N/C PGND

8

2

N/C PGND

5

1

SGND PGND

LX

L1

3.0µH

R4

59.0k

VOUT+

R3

VOUT(V) R3 (kΩ)

0.8

19.6

0.9

29.4

1.0

39.2

1.1

49.9

1.2

59.0

1.3

68.1

1.4

78.7

1.5

88.7

1.8

118

C3-C4

2.0

137

2x 22µF

2.5

3.3

187

267

GND

C1 Murata 10µF 6.3V X5R GRM42-6X5R106K6.3

C3,C4 MuRata 22µF 6.3V GRM21BR60J226ME39L X5R 0805

L1 Sumida CDRH5D28-3R0NC

Figure 1: AAT1157 Evaluation Board Schematic

Lithium-Ion to 2.5V Converter.

Inductor

The output inductor should limit the ripple current to

330mA at the maximum input voltage. This match-

es the inductor current downslope with the fixed

internal slope compensation. For a 2.5V output and

the ripple set to a maximum input voltage of 4.2V,

the inductance value required to limit the ripple cur-

rent to 330mA is 3.0µH. From this calculated value,

a standard value can be selected.

Manufacturer's specifications list both the inductor

DC current rating, which is a thermal limitation, and

the peak current rating, which is determined by the

saturation characteristics. The inductor should not

show any appreciable saturation under normal load

conditions. Some inductors may meet the peak and

average current ratings yet result in excessive loss-

es due to a high DCR. Always consider the losses

associated with the DCR and its effect on the total

converter efficiency when selecting an inductor.

L

=

VOUT

∆IPP ⋅ F

⋅ ⎛⎝1

-

VOUT ⎞

VIN(MAX)⎠

=

2.5V

0.33A ⋅ 1MHz

⋅

⎛⎝1 -

2.5V ⎞

4.2V ⎠

= 3.07µH

For a maximum ripple current of 330mA, the peak

switch and inductor current at 1.2A is 1.365A. A stan-

dard value of 3.0µH can be used in this example. The

3.0µH Sumida series CDRH5D28 inductor has a

24mΩ maximum DCR and a 2.4A DC current rating.

Input Capacitor

The primary function of the input capacitor is to pro-

vide a low impedance loop for the edges of pulsed

current drawn by the AAT1157. A low ESR/ESL

ceramic capacitor is ideal for this function. To mini-

mize stray inductance, the capacitor should be

placed as closely as possible to the IC. This keeps

the high frequency content of the input current

localized, minimizing radiated and conducted EMI

while facilitating optimum performance of the

AAT1157. Ceramic X5R or X7R capacitors are

ideal for this function. The size required will vary

depending on the load, output voltage, and input

voltage source impedance characteristics. Values

range from 1µF to 10µF. The input capacitor RMS

current varies with the input voltage and the output

voltage. The equation for the RMS current in the

input capacitor is:

IRMS = IO ⋅

VO

VIN

⋅ ⎝⎛1 -

VO ⎞

VIN ⎠

The input capacitor RMS ripple current reaches a

maximum when VIN is two times the output volt-

age where it is approximately one half of the load

current. Losses associated with the input ceramic

capacitor are typically minimal and are not an

issue. The proper placement of the input capaci-

tor can be seen in the evaluation board layout (C1

in Figure 2).

8

1157.2005.11.1.4

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