AAT2786 Datasheet, PDF (19/27 Pages) Advanced Analogic Technologies – 1.5A Step-Down Converter and 150mA LDO

SystemPower^TM
PRODUCT DATASHEET
AAT2786
1.5A Step-Down Converter and 150mA LDO
V_OUT(V)
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.8
1.85
2.0
2.5
3.0
3.3
R₂ = 59kΩ
R₁(kΩ)
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
124
137
187
237
267
R₂ = 221kΩ
R₁(kΩ)
75
113
150
187
221
261
301
332
442
464
523
715
887
1000
Table 1: AAT2786 Resistor Values for Various
Output Voltages.
Component Selection For LDO
Input Capacitor
Typically, a 1µF or larger capacitor is recommended for
CIN in most applications. A C_IN capacitor is not required
for basic LDO regulator operation. However, if the
AAT2786 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
close to the device V_IN 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
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 por-
table devices.
Output Capacitor
For proper load voltage regulation and operational sta-
bility, a capacitor is required between pins V_OUT and GND.
The C_OUT capacitor connection to the LDO regulator
ground pin should be made as direct as practically pos-
sible for maximum device performance.
The AAT2786 has been specifically designed to function
with very low ESR ceramic capacitors. For best perfor-
mance, 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 characteristics of the
AAT2786 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.
Bypass Capacitor and
Low Noise Applications
A bypass capacitor pin is provided to enhance the low
noise characteristics of the AAT2786 LDO regulator. The
bypass capacitor is not necessary for operation of the
AAT2786. However, for best device performance, a small
ceramic capacitor should be placed between the bypass
pin (BYP) and the device ground pin (GND). The value of
C_BYP may range from 470pF to 10nF. For lowest noise and
best possible power supply ripple rejection performance,
a 10nF capacitor should be used. To practically realize
the highest power supply ripple rejection and lowest out-
put noise performance, it is critical that the capacitor
connection between the BYP pin and GND pin be direct
and PCB traces should be as short as possible. Refer to
the PCB Layout Recommendations section of this
datasheet for examples.
There is a relationship between the bypass capacitor value
and the LDO regulator turn-on time and turn-off time. In
applications where fast device turn-on and turn-off time
are desired, the value of C_BYP should be reduced.
In applications where low noise performance and/ or
ripple rejection are less of a concern, the bypass capac-
itor may be omitted. The fastest device turn on time will
be realized when no bypass capacitor is used.
DC leakage on this pin can affect the LDO regulator output
noise and voltage regulation performance. For this rea-
son, the use of a low leakage, high quality ceramic (NPO
or C0G type) or film capacitor is highly recommended.
2786.2008.04.1.0
www.analogictech.com
19