AAT2522 Datasheet, PDF (13/19 Pages) Skyworks Solutions Inc. – Dual High-Current, Low-Noise, Step-Down Regulator

DATA SHEET
AAT2522
Dual High-Current, Low-Noise, Step-Down Regulator
ing it more sensitive to external noise and interference.
Therefore, the recommended value range for R_FB-GND (R3
and R5 in Figure 2) is 100kΩ for good noise immunity or
221kΩ for reduced no load input current.
The external resistor R_OUT-FB (R2 and R4 in Figure 2), com-
bined with an external 100pF feed forward capacitor (C₅
and C₆ in Figure 2), delivers enhanced transient response
for extreme pulsed load applications and reduces ripple in
light load conditions. The addition of the feed forward
capacitor typically requires a larger output capacitor
(C_OUT) for stability. The external resistors set the output
voltage according to the following equation:
V_O = 0.6V ·
1 + ^ROUT-FB
R_FB-GND
or solving for R_OUT-FB
R_OUT-FB =
V_O
0.6V
-1
· R_FB-GND
V_OUT (V)
1.0
1.2
1.5
1.8
2.2
2.5
3.3
4.2
4.6
5
R3 = R5= 100kΩ
R2 = R4 (kΩ)
65.5
100
150
200
267
316
453
604
655
806
Table 1: Step-Down Converter Feedback Resistor
Selection for Different Output Voltages.
The typical circuit shown in the AAT2522 evaluation
schematic is intended to be general purpose and suitable
for most applications. In applications where transient
load steps are more severe and the restriction on output
voltage deviation is more stringent. To handle these
cases some simple adjustments can be made. The sche-
matic in Figure 2 shows the configuration for improved
transient response in an application where the output is
stepped down to 1.2V. The adjustments consist of adding
an additional 22μF output capacitor, increasing the value
of the feed forward capacitor C6 to 1nF, and adding the
bias RC filter networks R1, C3 and R6, C4 in Figure 2.
Applications Information
Thermal Calculations
There are three types of losses associated with the
AAT2522 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:
P_LOSS(RES) = I_O2 · R_DS(ON)H ·
V_O
V_IN
+ R_DS(ON)L ·
V_IN - V_O
V_IN
Switching losses are dominated by the gate charge of
the power output switching devices. At full load, assum-
ing continuous conduction mode (CCM), a simplified
form of the switching losses is given by:
P_LOSS(SW) = t_SW · f_SW · I_O · V_IN
The term t_SW is used to estimate the full load step-down
converter switching losses. Finally, the losses associated
with the controller bias requirements are based the
regulator’s quiescent current (I_Q):
P_LOSS(BIAS) = I_Q · V_IN
Summing the three power loss terms together provides
the total power loss that the AAT2522 package must dis-
sipate:
P_TOTAL = P_LOSS(RES) + P_LOSS(SW) + P_LOSS(BIAS)
For the condition where the step-down converter is in
dropout at 100% duty cycle, the total device dissipation
reduces to:
P_TOTAL = I_O2 · R_DS(ON)H =
V_O
V_IN
+ I_Q · V_IN
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.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [e-mail] • www.skyworksinc.com
202032A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • June 8, 2012
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