AAT3183
300mA Inductorless Step-Down Converter
Capacitor Selection
Applications Information
The AAT3183 requires three external capacitors;
CIN, CFLY and COUT. The capacitor size and type
Input Voltage Headroom
can have a significant impact on charge pump per-
formance, including input and output ripple, stabili-
ty and operating efficiency.
The input voltage headroom is the required mini-
mum input voltage in excess of 2x the output volt-
age. The following equation can be used to calcu-
late the required input voltage headroom:
Surface-mount X5R multi-layer ceramic (MLC)
capacitors are a suitable choice due to their small
size and 15% capacitance tolerance over the
operating temperature range -55ºC to +85ºC. X7R
MLC capacitors provide similar performance over
the extended temperature range -55ºC to +125ºC.
Initial tolerance of 10% is recommended. MLC
capacitors offer superior size (high energy density),
low equivalent series resistance (ESR), and low
equivalent series inductance (ESL) when com-
pared to tantalum and aluminum electrolytic capac-
itor varieties. In addition, MLC capacitors are not
polarized which simplifies placement on the printed
circuit board.
(IOUT · ROUT
M
)
VHR
=
VHR: Input Voltage Headroom
IOUT: Output Current
ROUT:Output Impedance (see “Output Impedance
vs. Input Voltage” performance graph in the
“Typical Characteristics” section of this
datasheet)
M:
Charge Pump Gain
[AAT3183: ½]
Negligible circuit losses and fast charge/discharge
rates are possible with MLC capacitors due to their
low ESR, which is typically less than 10mΩ.
Switching noise is minimized due to their low ESL
which produces voltage spikes due to the fast
switching current events in charge pump converters.
ESL is typically less than 1nH in MLC capacitors.
Design Example:
AAT3183 Application Conditions:
IOUT = 200mA (max)
VOUT = 1.5V
MLC capacitance is reduced with an increasing DC
bias voltage. Capacitance derating varies with
case size, voltage rating and vendor. It is recom-
mended that circuit performance, including output
current capability and input/output voltage ripple,
be verified under worst-case operating conditions.
What is the required minimum input voltage?
Analysis:
Minimum Input Voltage: VIN(MIN) = VHR + 2 · VOUT
The capacitor combinations listed in Table 1 are suit-
able for output currents up to 220mA and 300mA.
Smaller capacitors may be considered for applica-
tions requiring less than 300mA output current.
Smaller solution size can be achieved at the cost of
increased input and output voltage ripple and
decreased output current capability.
(IOUT · ROUT
M
)
Input Voltage Headroom:
VHR
=
=
(0.2A · 1)
½
= 0.4V
Output Voltage: VOUT = 1.5V
Minimum Input Voltage:
CIN, CFLY and COUT should be located close to the
AAT3183 device in order to minimize stray para-
sitics; specifically ESR and ESL due to PCB layout
traces. See the “PCB Layout Guidelines” section
of this datasheet for details.
VIN(MIN) = 0.4V + 2 · 1.5V = 3.4V
Solution:
The required minimum input voltage is 3.4V.
3183.2007.07.1.2
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