PRODUCT DATASHEET
AAT1218
TM
SwitchReg
1A, 1.2MHz Synchronous Boost Converter
Fit the proper L into the equations to meet the following
criteria:
Output Capacitor
The output capacitor limits the output ripple and pro-
vides holdup during large load transitions. When choos-
ing the output capacitor, first consider the acceptable
output ripple level (ΔVtoatal) and solve for COUT from the
equations below; assume that output current remains
constant in steady state:
1. The DC current rating of the inductor must be high-
er than IPEAK to avoid magnetic saturation. Cored
inductor devalues when core temperature increases,
so verify the inductor of choice with its temperature
characteristics in mind.
2. The inductor ripple current ΔI is typically set for
20% to 40% of the peak inductor current.
ΔVTOTAL = ΔV + ΔVESR = ΔV + IOUT · RESR
Higher inductance means less inductor ripple current;
larger size inductors can handle more power. However,
larger size inductors also have higher profiles and are
more expensive. For optimum load transient and effi-
ciency performance, low DCR inductors should be select-
ed. Table 2 lists some surface mount inductors which are
suitable for typical AAT1218 applications.
I
OUT · D
ΔV =
C
OUT · FSW
A 10ꢀF to 47ꢀF X5R or X7R ceramic capacitor typically
provides sufficient bulk capacitance to stabilize the out-
put during large load transitions and has the ESR and
ESL characteristics necessary for low output ripple.
In addition, the output voltage droop during load tran-
sient is related to the capacitance of the ceramic output
capacitor. A larger output capacitor helps to reduce volt-
age droop.
Input Capacitor
Select a 4.7ꢀF to 10ꢀF X7R or X5R ceramic capacitor for
the input. The input capacitor provides a low impedance
loop for the edges of pulsed current drawn by the
AAT1218. Low ESR/ESL X7R and X5R ceramic capaci-
tors are ideal for this function. To minimize stray induc-
tance, the capacitor should be placed as close as pos-
sible to the IC. This keeps the high frequency content of
the input current localized, minimizing EMI and input
voltage ripple.
Rectifier Diode Selection
A rectifier diode must be added (D1 in Figure 1) when the
output voltage is greater than 4.5V. The Schottky diode
is optional for output voltages less than 4.5V, but can
improve efficiency by about 2% to 3%. A low forward
voltage Schottky diode is recommended. Its voltage rat-
ing should be higher than the output voltage. Thus,
diodes with 10V or more reverse voltage are recom-
mended. The diode rated current can be slightly less than
the peak inductor current to save cost and board space.
Always examine the ceramic capacitor DC voltage coef-
ficient characteristics to derive the proper value. For
example, the capacitance of a 10ꢀF, 6.3V, X5R ceramic
capacitor with 5.0V DC applied is actually about 6ꢀF.
L
Max DCR
(mΩ)
Rated DC Current
(A)
Size
WxLxH (mm)
Manufacturer
Part Number
(μH)
2.2
3.3
4.7
2.2
3.3
5.0
2.2
3.5
4.7
2.0
3.3
5.1
94
1.50
1.2
1.0
3.84
3.20
2.60
3.2
2.5
2.2
3.2
2.6
2D14
125
169
23.4
28.6
44.2
44.3
65.1
82.6
57
3.2x3.2x1.55
5.6x6.1x4.85
5.0x5.0x2.4
6.0x6.3x1.5
CR54
Sumida
CDRH4D22/HP
CDRH5D14/HP
96
140
2.0
Table 2: AAT1218 Typical Surface Mount Inductors.
w w w . a n a l o g i c t e c h . c o m
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1218.2008.10.1.1
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