AAT1155IKS-1.8-T1(2004) Ver la hoja de datos (PDF) - Analog Technology Inc
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AAT1155IKS-1.8-T1 Datasheet PDF : 16 Pages
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Diode
IDIODE= IO · 1-
VO
VIN
=
2.5A·
1
-
3.3V
5.25V
=
0.93A
VFW = 0.35V
PDIODE =VFW · IDIODE =
0.35V ·0.93A = 0.33A
Given a case to ambient thermal resistance of
120°C/W from the manufacturer's data sheet,
TJ(MAX) of the diode is
TJ(MAX)= TAMB + ΘJA · P =
70°C +120°C /W · 0.33W =
109°C
Output Capacitor
The output capacitor value required for sufficient
loop phase margin depends on the type of capaci-
tor selected. For a low ESR ceramic capacitor a
minimum value of 200µF is required. For a low
ESR tantalum capacitor lower values are accept-
able. While the relatively higher ESR associated
with the tantalum capacitor will give more phase
margin and a more damped transient response, the
output voltage ripple will be higher.
The 120µF Vishay 594D tantalum capacitor has an
ESR of 85 mΩ and a ripple current rating of 1.48
Arms in a C case size. Although smaller case sizes
are sufficiently rated for this ripple current, their
ESR level would result in excessive output ripple.
AAT1155
1MHz 2.5A Buck DC/DC Converter
The ESR requirement for a tantalum capacitor can
be estimated by
ESR ≤VRIPPLE=100mV= 111mΩ
∆I 0.9A
IRMS =
1
2·
3
· (VOUT+ VFWD)· (VIN
L · F · VIN
-
VOUT) =
1 · 3.65V·1.7V = 240mArms
2· 3 1.5µH· 1MHz·5V
Two or three 1812 X5R 100uF 6.3V ceramic
capacitors in parallel also provide sufficient phase
margin. The low ESR and ESL associated with
ceramic capacitors also reduces output ripple sig-
nificantly over that seen with tantalum capacitors.
Temperature rise due to ESR ripple current dissi-
pation is also reduced.
Input Capacitor
The input capacitor ripple is:
IRMS = IO ·
VO · 1- VO = 1.82Arms
VIN VIN
In the examples shown C1 is a ceramic capacitor
located as close to the IC as possible. C1 provides
the low impedance path for the sharp edges asso-
ciated with the input current. C4 may or may not be
required depending upon the impedance charac-
teristics looking back into the source. It serves to
dampen out any input oscillations that may arise
from a source that is highly inductive. For most
applications where the source has sufficient bulk
capacitance and is fed directly to the AA1155
through large PCB traces or planes it is not
required. When operating the AAT1155 evaluation
board on the bench C4 is required due to the induc-
tance of the wires running from the laboratory
power supply to the evaluation board.
1155.2004.08.1.2
13
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