AAT1154IAS-1.8-T1 View Datasheet(PDF) - Analog Technology Inc
Part Name
Description
MFG CO.
'AAT1154IAS-1.8-T1' PDF : 16 Pages View PDF
SwitchRegTM
The ESR requirement for a tantalum capacitor can be
estimated by:
ESR ≤VRIPPLE=100mV= 121mΩ
ΔI 0.82A
IRMS =
1
2·
3
·
(VOUT
+ VF) ·
L · FS
(VIN -
· VIN
VOUT)
=
1
2·
· 3.65V·1.7V
3 1.5μH· 1MHz·5V
= 240mArms
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 significantly over that seen with
tantalum capacitors. Temperature rise due to ESR ripple
current dissipation is also reduced.
Input Capacitor
The input capacitor ripple is:
IRMS = IO ·
VO · ⎛1- VO ⎞ = 1.42Arms
VIN ⎝ VIN ⎠
In the examples shown, C1 is a ceramic capacitor locat-
ed as closely to the IC as possible. C1 provides the low
PRODUCT DATASHEET
AAT1154
1MHz 3A Step-Down DC/DC Converter
impedance path for the sharp edges associated with the
input current. C4 may or may not be required, depend-
ing upon the impedance characteristics looking back into
the source. It serves to dampen out any input oscilla-
tions that may arise from a source that is highly induc-
tive. For most applications, where the source has suffi-
cient bulk capacitance and is fed directly to the AAT1154
through large PCB traces or planes, it is not required.
When operating the AAT1154 evaluation board on the
bench, C4 is required due to the inductance of the wires
running from the laboratory power supply to the evalua-
tion board.
Adjustable Output
For applications requiring an output other than the fixed
outputs available, the 1V version can be externally pro-
grammed. Resistors R3 and R4 of Figure 5 force the
output to regulate higher than 1V. For accurate results
(less than 1% error for all outputs), select R4 to be
10kΩ. Once R4 has been selected, R3 can be calculated.
For a 1.25V output with R4 set to 10kΩ, R3 is 2.5kΩ.
R3 = (VO - 1) · R4 = 0.25 · 10kΩ = 2.5kΩ
Figures 6 and 7 display the suggested PCB layout for the
adjustable output AAT1154.
VIN 2.7V to 5.5V
VOUT 1.25V @3A
C4
100µF
R1 R2
100 100k
R3
2.55k
C1
10µF
C3
0.1µF
R4
10.0k
U1
AAT1154-1.0
FB VP
GND LX
EN LX
VCC VP
L1
1.5µH
D1
C2
B340LA 120µF
rtn
C1 Murata 10µF 6.3V X5R GRM42-6X5R106K6.3
C2 Vishay 120µF 6.3V 594D127X96R6R3C2T
C3 0.1µF 0603ZD104M AVX
C4 Vishay Sprague 100µF 16V 595D107X0016C 100µF 16V
D1 B340LA Diodes Inc.
L1 CDRH6D28-1.5µH Sumida
Options
C2 Murata 100µF 6. 3V GRM43-2 X5R 107M 100µF 6.3V (two or three in parallel)
C2 TDK 100µF 6.3V C3325X5R0J107M 100µF 6.3V (two or three in parallel)
Figure 5: AAT1154 Evaluation Board With Adjustable Output.
14
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1154.2007.11.1.7
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