AAT1123 데이터 시트보기 (PDF) - Advanced Analogic Technologies

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AAT1123

1MHz Step-Down Converter

Advanced Analogic Technologies 

The maximum input capacitor RMS current is:

IRMS = IO ·

VO

VIN

·

⎛⎝1 -

VO ⎞

VIN ⎠

The input capacitor RMS ripple current varies with

the input and output voltage and will always be less

than or equal to half of the total DC load current.

VO · ⎛1 - VO ⎞ = D · (1 - D) = 0.52 = 1

VIN ⎝ VIN ⎠

2

for VIN = 2 x VO

I = RMS(MAX)

IO

2

AAT1123

1MHz Step-Down Converter

The term

VO

VIN

·

⎛⎝1 -

VO ⎞

VIN ⎠

appears in both the input volt-

age ripple and input capacitor RMS current equations

and is a maximum when VO is twice VIN. This is why

the input voltage ripple and the input capacitor RMS

current ripple are a maximum at 50% duty cycle.

The input capacitor provides a low impedance loop

for the edges of pulsed current drawn by the

AAT1123. Low ESR/ESL X7R and X5R ceramic

capacitors are ideal for this function. To minimize

stray inductance, the capacitor should be placed

as closely as possible to the IC. This keeps the

high frequency content of the input current local-

ized, minimizing EMI and input voltage ripple.

The proper placement of the input capacitor (C2) can

be seen in the evaluation board layout in Figure 2.

Figure 2: AAT1123 Evaluation Board

Top Side.

Figure 3: Exploded View of Evaluation

Board Top Side Layout.

Figure 4: AAT1123 Evaluation Board

Bottom Side.

1123.2006.05.1.5

11

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