PRODUCT DATASHEET
AAT2504178
SystemPowerTM
Adjustable 3-Channel Regulator
increase the impedance of the feedback node, making it
more sensitive to external noise and interference. Table
2 summarizes the resistor values for various output volt-
ages with R6 set to either 59kΩ for good noise immu-
nity or 221kΩ for reduced no load input current.
IQ is the step-down converter quiescent current. The
term tsw 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:
With enhanced transient response for extreme pulsed
load application, an external feed-forward capacitor (C1
in Fig.3) can be added.
PTOTAL = IO2 · RDSON(HS) + IQ · VIN
Since RDS(ON), quiescent current, and switching losses all
vary with input voltage, the total losses should be inves-
tigated 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
24.3
48.7
73.2
97.6
124
147
221
232
274
392
464
511
590
Given the total losses, the maximum junction tempera-
ture can be derived from the θJA for the QFN34-20 pack-
age which is 50°C/W.
TJ(MAX) = PTOTAL · ΘJA + 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; however, if the
AAT2504 is physically located more than three centime-
ters from an input power source, a CIN capacitor will be
needed for stable operation. CIN should be located as
closely to the device VLDO pins 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.
124
137
158
Table 2: Step-Down Converter Resistor Values for
Various Output Voltages.
Thermal Calculations
There are three types of losses associated with the
AAT2504 step-down converter: switching losses, con-
duction 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 con-
duction mode (CCM), a simplified form of the LDO losses
is given by:
Ceramic, tantalum, or aluminum electrolytic capacitors
may be selected for CIN. There is no specific capacitor ESR
requirement for CIN; however, ceramic capacitors are rec-
ommended for CIN due to their inherent capability over
tantalum capacitors to withstand input current surges
from low impedance sources such as batteries in portable
devices.
Output Capacitor
IO2 · (RDSON(HS) · VO + RDSON(LS) · [VIN - VO])
For proper load voltage regulation and operational sta-
bility, 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 practi-
cally possible for maximum device performance. The
AAT2504 has been specifically designed to function with
very low ESR ceramic capacitors. For best performance,
ceramic capacitors are recommended.
PTOTAL
=
VIN
+ (tsw · F · IO + IQ) · VIN
* For the 0.9V output, R6 is open.
w w w . a n a l o g i c t e c h . c o m
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2504.2008.02.1.2
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
