AAT2503
Adjustable 3-Channel Regulator
Since RDS(ON), quiescent current, and switching
losses all vary with input voltage, the total losses
should be investigated over the complete input
voltage range.
R6 = 59kΩ
R2 (kΩ)
R6 = 221kΩ
R2 (kΩ)
VOUT (V)
0.9*
1.0
1.1
1.2
1.3
1.4
1.5
1.8
1.85
2.0
2.5
2.8
3.0
3.3
0
0
6.65
13.3
19.6
26.1
32.4
39.2
59.0
61.9
71.5
105
124
137
158
24.3
48.7
73.2
97.6
124
147
221
232
274
392
464
511
590
Given the total losses, the maximum junction tem-
perature can be derived from the θJA for the
QFN34-20 package which is 50°C/W.
TJ(MAX)
=
PTOTAL
·
Θ
JA + TAMB
LDO Linear Regulator Input Capacitor
A 1µF or larger capacitor is typically recommended
for CIN in most applications. A CIN capacitor is not
required for basic LDO regulator operation; howev-
er, if the AAT2503 is physically located more than
three centimeters from an input power source, a
CIN capacitor will be needed for stable operation.
CIN should be located as closely to the device VIN
pin as practically possible. CIN values greater than
1µF will offer superior input line transient response
and will assist in maximizing the highest possible
power supply ripple rejection.
Table 2: Step-Down Converter Resistor Values
for Various Output Voltages.
Thermal Calculations
There are three types of losses associated with the
AAT2503 step-down converter: switching losses,
conduction losses, and quiescent current losses.
Conduction losses are associated with the RDS(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 conduction mode (CCM),
a simplified form of the LDO losses is given by:
Ceramic, tantalum, or aluminum electrolytic capac-
itors may be selected for CIN. There is no specific
capacitor ESR requirement for CIN; however, for
150mA LDO regulator output operation, ceramic
capacitors are recommended for CIN due to their
inherent capability over tantalum capacitors to with-
stand input current surges from low impedance
sources such as batteries in portable devices.
IO2 · (RDSON(HS) · VO + RDSON(LS) · [VIN - VO])
Output Capacitor
PTOTAL
=
For proper load voltage regulation and operational
stability, a capacitor is required between pins OUTA,
OUTB, and GND. The COUT capacitor connection to
the LDO regulator ground pin should be made as
direct as practically possible for maximum device
performance. The AAT2503 has been specifically
designed to function with very low ESR ceramic
capacitors. For best performance, ceramic capaci-
tors are recommended.
VIN
+ (tsw · F · IO + IQ) · VIN
IQ is the step-down converter quiescent current.
The term tsw is used to estimate the full load step-
down converter switching losses.
For the condition where the step-down converter is
in dropout at 100ꢀ duty cycle, the total device dis-
sipation reduces to:
Typical output capacitor values for maximum out-
put current conditions range from 1µF to 10µF.
PTOTAL = IO2 · RDSON(HS) + IQ · VIN
* For the 0.9V output, R6 is open.
16
2503.2007.04.1.1
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
