PRODUCT DATASHEET
AAT2120
SwitchRegTM
500mA Low Noise Step-Down Converter
VO
VIN
⎛
VO ⎞
VIN ⎠
1
2
Output Voltage (V)
L1 (μH)
· 1 -
⎝
=
D · (1 - D) = 0.52 =
1.0
1.2
1.5
1.8
2.5
3.0
3.3
1.5
2.2
2.7
3.0
3.9
4.7
5.6
for VIN = 2 · VO
IO
IRMS(MAX)
=
2
VO
VIN
⎛
⎝
VO
VIN
⎞
⎠
·
1 -
The term
appears in both the input voltage
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.
Table 1: Inductor Values.
The 3.0μH CDRH2D09 series inductor selected from
Sumida has a 150mΩ DCR and a 470mA DC current
rating. At full load, the inductor DC loss is 9.375mW
which gives a 2.08% loss in efficiency for a 250mA,
1.8V output.
The input capacitor provides a low impedance loop for
the edges of pulsed current drawn by the AAT2120. Low
ESR/ESL X7R and X5R ceramic capacitors are ideal for
this function. To minimize stray inductance, the capaci-
tor should be placed as closely as possible to the IC. This
keeps the high frequency content of the input current
localized, minimizing EMI and input voltage ripple.
Input Capacitor
Select a 4.7μF to 10μF X7R or X5R ceramic capacitor for
the input. To estimate the required input capacitor size,
determine the acceptable input ripple level (VPP) and solve
for CIN. The calculated value varies with input voltage and
is a maximum when VIN is double the output voltage.
The proper placement of the input capacitor (C1) can be
seen in the evaluation board layout in Figure 2.
A laboratory test set-up typically consists of two long
wires running from the bench power supply to the eval-
uation board input voltage pins. The inductance of these
wires, along with the low-ESR ceramic input capacitor,
can create a high Q network that may affect converter
performance. This problem often becomes apparent in
the form of excessive ringing in the output voltage dur-
ing load transients. Errors in the loop phase and gain
measurements can also result.
VO
VIN
⎛
VO ⎞
VIN ⎠
· 1 -
⎝
CIN =
⎛ VPP
⎝ IO
⎞
- ESR ·FS
⎠
VO
VIN
⎛
VO ⎞
VIN ⎠
1
· 1 -
⎝
=
for VIN = 2 · VO
4
Since the inductance of a short PCB trace feeding the
input voltage is significantly lower than the power leads
from the bench power supply, most applications do not
exhibit this problem.
1
CIN(MIN)
=
⎛ VPP
⎝ IO
⎞
⎠
- ESR · 4 · FS
In applications where the input power source lead induc-
tance cannot be reduced to a level that does not affect
the converter performance, a high ESR tantalum or alu-
minum electrolytic should be placed in parallel with the
low ESR, ESL bypass ceramic. This dampens the high Q
network and stabilizes the system.
Always examine the ceramic capacitor DC voltage coeffi-
cient characteristics when selecting the proper value. For
example, the capacitance of a 10μF, 6.3V, X5R ceramic
capacitor with 5.0V DC applied is actually about 6μF.
The maximum input capacitor RMS current is:
VO
VIN
⎛
VO ⎞
VIN ⎠
Output Capacitor
IRMS = IO ·
· 1 -
⎝
The output capacitor limits the output ripple and pro-
vides holdup during large load transitions. A 4.7μF to
10μF X5R or X7R ceramic capacitor typically provides
sufficient bulk capacitance to stabilize the output during
large load transitions and has the ESR and ESL charac-
teristics necessary for low output ripple. For enhanced
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.
w w w . a n a l o g i c t e c h . c o m
2120.2008.02.1.2
11
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