AAT2786 Datasheet, PDF (20/27 Pages) Advanced Analogic Technologies – 1.5A Step-Down Converter and 150mA LDO

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PRODUCT DATASHEET
AAT2786
1.5A Step-Down Converter and 150mA LDO
Capacitor Characteristics
Ceramic composition capacitors are highly recommend-
ed over all other types of capacitors for use with the
AAT2786. 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 resis-
tance associated with a capacitor that includes lead
resistance, internal connections, size and area, material
composition, and ambient temperature. Typically, capac-
itor ESR is measured in milliohms for ceramic capacitors
and can range to more than several ohms for tantalum
or aluminum electrolytic capacitors.
Ceramic Capacitor Materials
Ceramic capacitors less than 0.1µF are typically made
from NPO or C0G materials. NPO and C0G materials
generally have tight tolerance and are very stable over
temperature. Larger capacitor values are usually com-
posed of X7R, X5R, Z5U, or Y5V dielectric materials.
Large ceramic capacitors (i.e., greater than 2.2µF) are
often available in low-cost Y5V and Z5U dielectrics.
These two material types are not recommended for use
with LDO regulators since the capacitor tolerance can
vary more than ±50% over the operating temperature
range of the device. A 2.2µF Y5V capacitor could be
reduced to 1µF over temperature; this could cause prob-
lems for circuit operation. X7R and X5R dielectrics are
much more desirable. The temperature tolerance of X7R
dielectric is better than ±15%.
Capacitor area is another contributor to ESR. Capacitors
that are physically large in size will have a lower ESR
when compared to a smaller sized capacitor of an equiv-
alent material and capacitance value. These larger devic-
es can improve circuit transient response when compared
to an equal value capacitor in a smaller package size.
Consult capacitor vendor datasheets carefully when
selecting capacitors for LDO regulators.
Thermal Calculations
There are three types of losses associated with the
AAT2786 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 step-down
converter and LDO losses is given by:
P_TOTAL
=
I_O2
·
(R_DS(ON)H
·
V_OBUCK
+
R_DS(ON)L ·
V_IN(BUCK)
[V_INBUCK
-
V_OUTBUCK])
+ (t_sw · F_S · I_OBUCK + I_QBUCK) · V_INBUCK + (V_INLDO - V_OUTLDO) · I_OLDO
I_QBUCK and I_QLDO are the step-down converter and LDO qui-
escent currents respectively. The term t_SW is used to esti-
mate the full load step-down converter 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
=
I²
OBUCK
·
R_DS(ON)H
+
(t_SW
·
F_S
·
I_BUCK
+
I_QBUCK)
·
V_INBUCK
+
(V_INLDO
-
V_OUTLDO)
·
I_OLDO
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 θ_JA for the TDFN34-16
package, which is 50°C/W.
T_J(MAX) = P_TOTAL · Θ_JA + T_AMB
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