AN557 datasheet(45/52 Pages) STMICROELECTRONICS

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AN557 APPLICATION NOTE

Infact we have:

therefore:

The inductor can be calculated using the usual for-

mula, that is:

with

∆IL = 10% Iomax, L = 30µH

When one operates with input voltage below of 5V,

it’ very difficult to obtain a good efficiency. In our

case having the conduction losses and switching

losses of the internal power transistor fixed by both

external operating electrical condition and the

electrical characteristics of the itself power transis-

tor, since integrate, it’s necessary to optimize the

losses of the catch diode, using new type at lower

forward voltage drop, as soon as available on the

market or by appropriately over-dimensioning. In

some case a Power MOS used as a synchro-nou-

os switch can contribute to elevate the overall effi-

ciency of the system.

Following are citated the principle results obtained

by using our evaluation board: The same solution,

obviously can be applied also to the other types of

the family, adjusting if needed the compensation

network and the coil.

Figure 62. Efficiency vs. Input Voltage

Efficiency vs. Input Voltage

Figure 63. Load Transient Response

Figure 64. Load Transient Response

Output voltage ripple vs inductor value.

Co = 1000µF/10V - EKR; ESR = 10mΩ

---------

------ T

–

() V

⋅

∆

fsw

⋅⋅

----------------------------------

Vi(V)

(%)

Io=5A

Io=7A

Io=10A

Vo=3.3V

fsw=100KHz

D93IN002

Vi (V)

Io = 10A

η% Io = 7A η% Io = 4A η%

70.9

76.2

72.1

76.7

72.9

77.3

81.2

72.9

77.2

80.6

71.9

76.4

79.6

68.3

75.9

78.5

L = 30

µHL = 50µHL = 60µH L = 100µH

∆VOmax

80mV

60mV

40mV

25mV

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