PRODUCT DATASHEET
AAT2515
SystemPowerTM
Dual 600mA FastTransient High Frequency Buck Converter
Adjustable Output Resistor Selection
Thermal Calculations
For applications requiring an adjustable output voltage,
the 0.6V version can be programmed externally. Resistors
R1 through R4 of Table 2 program the output to regulate
at a voltage higher than 0.6V. To limit the bias current
required for the external feedback resistor string, the
minimum suggested value for R2 and R4 is 59kΩ.
Although a larger value will reduce the quiescent cur-
rent, it will also 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 voltages with R2 and R4 set to either
59kΩ for good noise immunity or 221kΩ for reduced no
load input current.
There are three types of losses associated with the
AAT2515 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 dominat-
ed by the gate charge of the power output switching
devices. At full load, assuming continuous conduction
mode (CCM), a simplified form of the dual converter
losses is given by:
IO12 · (RDSON(HS) · VO1 + RDSON(LS) · [VIN -VO1])
PTOTAL
=
+
VIN
IO22 · (RDSON(HS) · VO2 + RDSON(LS) · [VIN -VO2])
VIN
V
V
1.5V
0.6V
⎛
⎝
⎞
⎛
⎝
⎞
⎠
OUT
R1 =
-1 · R2 =
- 1 · 59kΩ = 88.5kΩ
⎠
REF
+ (tsw · F · [IO1 + IO2] + 2 · IQ) · VIN
The adjustable version of the AAT2515 in combination
with an external feedforward capacitor (C4 and C5 of
Figure 1) delivers enhanced transient response for
extreme pulsed load applications. The addition of the
feedforward capacitor typically requires a larger output
capacitor (C1 and C2) for stability.
IQ is the AAT2515 quiescent current for one channel and
tsw is used to estimate the full load switching losses.
For the condition where channel one is in dropout at
100% duty cycle, the total device dissipation reduces to:
R2, R4 = 59kΩ
R1, R3 (kΩ)
R2, R4 = 221kΩ
PTOTAL = IO12 · RDSON(HS)
VOUT (V)
R1, R3
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.8
1.85
2.0
2.5
3.3
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
124
137
187
267
75K
113K
150K
187K
221K
261K
301K
332K
442K
464K
523K
715K
1.00M
IO22 · (RDSON(HS) · VO2 + RDSON(LS) · [VIN -VO2])
+
VIN
+ (tsw · F · IO2 + 2 · 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.
Given the total losses, the maximum junction tempera-
ture can be derived from the θJA for the TDFN33-12 pack-
age which is 50°C/W.
Table 2: Adjustable Resistor Values
For Use With 0.6V Version.
TJ(MAX) = PTOTAL · ΘJA + TAMB
w w w . a n a l o g i c t e c h . c o m
12
2515.2007.12.1.0
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
