AAT2503 Datasheet, PDF (16/22 Pages) Advanced Analogic Technologies – Adjustable 3-Channel Regulator

Thermal Calculations
There are three types of losses associated with the
AAT2503 step-down converter: switching losses, con-
duction losses, and quiescent current losses. Conduction
losses are associated with the R_DS(ON) characteristics of
the power output switching devices. Switching losses are
dominated by the gate charge of the power output
switching devices. At full load, assuming continuous con-
duction mode (CCM), a simplified form of the LDO losses
is given by:
P_TOTAL
=
I_O2
·
(R_DSON(HS)
·
V_O
+ R_DSON(LS)
V_IN
·
[V_IN
-
V_O])
+ (t_sw · F · I_O + I_Q) · V_IN
I_Q is the step-down converter quiescent current. The
term t_sw is used to estimate the full load step-down con-
verter switching losses.
For the condition where the step-down converter is in
dropout at 100% duty cycle, the total device dissipation
reduces to:
P_TOTAL = I_O2 · R_DSON(HS) + I_Q · V_IN
Since R_DS(ON), quiescent current, and switching losses all
vary with input voltage, the total losses should be inves-
tigated over the complete input voltage range.
Given the total losses, the maximum junction tempera-
ture can be derived from the _JA for the QFN34-20 pack-
age which is 50°C/W.
T_J(MAX) = P_TOTAL · Θ_JA + T_AMB
LDO Linear Regulator Input Capacitor
A 1μF or larger capacitor is typically recommended for
C_IN in most applications. A C_IN capacitor is not required
for basic LDO regulator operation; however, if the
AAT2503 is physically located more than three centime-
ters from an input power source, a C_IN capacitor will be
needed for stable operation. C_IN should be located as
closely to the device VIN pin as practically possible. C_IN
values greater than 1μF will offer superior input line
transient response and will assist in maximizing the
highest possible power supply ripple rejection.
Ceramic, tantalum, or aluminum electrolytic capacitors
may be selected for C_IN. There is no specific capacitor
DATA SHEET
AAT2503
Adjustable Three-Channel Regulator
ESR requirement for C_IN; however, for 150mA LDO regu-
lator output operation, ceramic capacitors are recom-
mended for C_IN due to their inherent capability over
tantalum capacitors to withstand input current surges
from low impedance sources such as batteries in porta-
ble devices.
Output Capacitor
For proper load voltage regulation and operational stabil-
ity, a capacitor is required between pins OUTA, OUTB,
and GND. The C_OUT capacitor connection to the LDO
regulator ground pin should be made as direct as practi-
cally possible for maximum device performance. The
AAT2503 has been specifically designed to function with
very low ESR ceramic capacitors. For best performance,
ceramic capacitors are recommended.
Typical output capacitor values for maximum output cur-
rent conditions range from 1μF to 10μF.
Applications utilizing the exceptionally low output noise
and optimum power supply ripple rejection characteris-
tics of the AAT2503 should use 2.2μF or greater for C_OUT.
If desired, C_OUT may be increased without limit. In low
output current applications where output load is less
than 10mA, the minimum value for C_OUT can be as low as
0.47μF.
Capacitor Characteristics
Ceramic composition capacitors are highly recommend-
ed over all other types of capacitors for use with the
AAT2503. Ceramic capacitors offer many advantages
over their tantalum and aluminum electrolytic counter-
parts. A ceramic capacitor typically has very low ESR, is
lower cost, has a smaller PCB footprint, and is non-
polarized. Line and load transient response of the LDO
regulator is improved by using low ESR ceramic capaci-
tors. Since ceramic capacitors are non-polarized, they
are not prone to incorrect connection damage.
Equivalent Series Resistance
ESR is a very important characteristic to consider when
selecting a capacitor. ESR is the internal series resistance
associated with a capacitor that includes lead resistance,
internal connections, size and area, material composi-
tion, and ambient temperature. Typically, capacitor ESR
is measured in milliohms for ceramic capacitors and can
range to more than several ohms for tantalum or alumi-
num electrolytic capacitors.
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