AAT2612 Datasheet, PDF (12/18 Pages) Skyworks Solutions Inc. – Step-Down DC/DC Converter With Three High PSRR LDOs

DATA SHEET
AAT2612
Step-Down DC/DC Converter With Three High PSRR LDOs
Once the average inductor current increases to the DC
load level, the output voltage recovers. The above equa-
tion establishes a limit on the minimum value for the
output capacitor with respect to load transients.
Output Inductor
For most designs, the AAT2612 operates with inductor
values of 2.2μH to 4.7μH. Inductors with low inductance
values are physically smaller but generate higher induc-
tor current ripple leading to higher output voltage ripple.
The inductor value can be derived from the following
equation:
L = ^{VOUT · (VIN - VOUT)}
V_IN · ∆I_L · f_OSC
Where ΔI_L is inductor ripple current. Large value induc-
tors result in lower ripple current and small value induc-
tors result in high ripple current. Choose inductor ripple
current approximately 30% of the maximum load cur-
rent 0.6A, or
∆I_L = 180mA
Manufacturer’s specifications list both the inductor DC
current rating, which is a thermal limitation, and the
peak current rating, which is determined by the satura-
tion characteristics. The inductor should not show any
appreciable saturation under normal load conditions. The
DC current rating of the inductor should be at least equal
to the maximum load current plus half the inductor rip-
ple current to prevent core saturation (0.6A + 180mA).
Some inductors may meet the peak and average current
ratings yet result in excessive losses due to a high DCR.
Always consider the losses associated with the DCR and
its effect on the total converter efficiency when selecting
an inductor.
Feedback Resistor Selection
The Buck output voltage on the AAT2612 is adjustable
with external resistors R1 and R2 which program the
output to regulate at a voltage in the range of 0.9V to
0.85 ∙ V_INBK. To limit the bias current required for the
external feedback resistor string while maintaining good
noise immunity, the minimum suggested value for R2 is
59kΩ. Although a larger value will further reduce quies-
cent current, it will also increase the impedance of the
feedback node, making it more sensitive to external
noise and interference. The maximum value of R1 should
be below 1Mohm to keep reference voltage normal and
avoid noise coupling.
R1 =
V_{OUT - 1 · R2 =}
V_REF
V_{OUT - 1 · R2}
0.6V
Table 1 shows the standard 1% metal film resistor values
for different step-down output voltages
V_OUT (V)
1.2
2.8
3.3
3.6
4.2
R2 = 59kΩ,
R1 (kΩ)
59
216
265.5
259
354
Table 1: V_OUT Resistor Selection
Enable Function
The AAT2612 features one buck output enable/disable
function for buck converter. This pin (ENBK) is active
high and is compatible with CMOS logic. To assure the
buck output will switch on, the ENBK turn-on control
level must be greater than 1.5V. The buck converter will
go into the disable shutdown mode when the voltage on
the ENBK pin falls below 0.6V. If the enable function is
not needed in a specific application, it may be tied to V_IN
to keep the buck output in a continuously on state.
Low Dropout Regulator
Input Capacitor
Typically, a 2.2μF or larger capacitor is recommended for
C_IN in most applications. A C_IN capacitor is not required
for basic LDO regulator operation. However, if the LDO is
physically located any distance more than one or two
centimeters from the input power source, a C_IN capacitor
will be needed for stable operation. C_IN should be located
as closely to the device V_INL pin as practically possible.
C_IN values greater than 1μF will offer superior input line
transient response and will assist in maximizing the
power supply ripple rejection.
Ceramic, tantalum, or aluminum electrolytic capacitors
may be selected for C_IN as there is no specific capacitor
ESR requirement. For better performance, ceramic
capacitors are recommended for C_IN due to their inherent
capability over tantalum capacitors to withstand input
current surges from low impedance sources such as bat-
teries in portable devices.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [e-mail] • www.skyworksinc.com
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202407B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 20, 2013