AAT3183_08 Datasheet, PDF (11/15 Pages) Advanced Analogic Technologies – 300mA Inductorless Step-Down Converter

ChargePump^TM
PRODUCT DATASHEET
AAT3183
300mA Inductorless Step-Down Converter
Applications Information
Input Voltage Headroom
The input voltage headroom is the required minimum
input voltage in excess of 2x the output voltage. The
following equation can be used to calculate the required
input voltage headroom:
V_HR
=
(I_OUT · R_OUT)
M
V_HR: Input Voltage Headroom
I_OUT: Output Current
R_OUT: Output Impedance (see “Output Impedance vs.
Input Voltage” performance graph in the “Typical
Characteristics” section of this datasheet)
M: Charge Pump Gain [AAT3183: ½]
Design Example:
AAT3183 Application Conditions:
I_OUT = 200mA (max)
V_OUT = 1.5V
What is the required minimum input voltage?
Analysis:
Minimum Input Voltage: V_IN(MIN) = V_HR + 2 · V_OUT
Input
Voltage
Headroom:
V_HR
=
(I_OUT
· R_OUT)
M
=
(0.2A · 1)
½
= 0.4V
Output Voltage: V_OUT = 1.5V
Minimum Input Voltage: V_IN(MIN) = 0.4V + 2 · 1.5V = 3.4V
Solution:
The required minimum input voltage is 3.4V.
Capacitor Selection
The AAT3183 requires three external capacitors; C_IN, C_FLY
and C_OUT. The capacitor size and type can have a signifi-
cant impact on charge pump performance, including input
and output ripple, stability and operating efficiency.
Surface-mount X5R multi-layer ceramic (MLC) capaci-
tors are a suitable choice due to their small size and
±15% capacitance tolerance over the -55°C to +85°C
operating temperature range. X7R MLC capacitors pro-
vide similar performance over the extended temperature
range of -55°C to +125°C. 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 capacitor
varieties. In addition, MLC capacitors are not polarized,
which simplifies placement on the printed circuit board.
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.
MLC capacitance is reduced with an increasing DC bias
voltage. Capacitance derating varies with case size, volt-
age rating and vendor. It is recommended that circuit
performance, including output current capability and
input/output voltage ripple, be verified under worst-case
operating conditions.
The capacitor combinations listed in Table 1 are suitable
for output currents up to 220mA and 300mA. Smaller
capacitors may be considered for applications 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.
C_IN, C_FLY and C_OUT should be located close to the AAT3183
device in order to minimize stray parasitics, specifically
ESR and ESL due to PCB layout traces. See the “PCB
Layout Guidelines” section of this datasheet for details.
An input capacitor (C_IN) is required to maintain low input
voltage ripple as well as minimize noise coupling to
nearby circuitry. The size of the required input capacitor
can vary, and depends on the source impedance of the
input voltage source. A small 1µF to 2.2µF MLC input
capacitor is suitable in most applications. MLC capacitors
sized as small as 0402 are available which meet these
requirements.
The flying capacitor (C_FLY) transfers energy to the output
during both ‘charge’ and ‘discharge’ intervals. C_FLY is
sized to maintain the maximum output load and main-
tain acceptable output voltage ripple at the minimum
input voltage.
The ratio C_OUT to C_FLY is determined by the input to output
voltage ratio and should be maintained near 5:1 for best
performance across the operating range.
3183.2008.02.1.3
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