AAT2612 Datasheet, PDF (13/18 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
Output Capacitor
For proper load voltage regulation and operational sta-
bility, a capacitor is required between pins VOUT and
GND. The C_OUT capacitor connection to the LDO regulator
ground pin should be made as direct as practically pos-
sible for maximum device performance. The AAT2612
LDOs have been specifically designed to function with
very low ESR ceramic capacitors. Although the device is
intended to operate with these low ESR capacitors, it is
stable over a very wide range of capacitor ESR, thus it
will also work with some higher ESR tantalum or alumi-
num electrolytic capacitors. For best performance,
ceramic capacitors are recommended.
The value of C_OUT typically ranges from 1μF to 10μF. 1μF
is sufficient for most operating conditions.
Enable Function
The AAT2612 features three LDO regulator enable/dis-
able function for LDO1/2/3 respectively. These pins
(ENL1, ENL2, and ENL3) are active high and are compat-
ible with CMOS logic. To assure the LDO regulator will
switch on, the EN turn-on control level must be greater
than 1.5V. The LDO regulator will go into the disable
shutdown mode when the voltage on the EN 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 LDO
regulator in a continuously on state.
Short-circuit and Thermal Protection
The AAT2612 LDOs are protected by both current-limiting
and over-temperature protection circuitry. The internal
short-circuit current limiting circuit is designed to activate
when the output load demand exceeds the maximum
rated output. If a short-circuit condition were to continu-
ally draw more than the current limit threshold, the LDO
regulator’s output voltage would drop to a level necessary
to supply the current demanded by the load. Under short-
circuit or other over-current operating conditions, the
output voltage would drop and the AAT2612’s die tem-
perature would rapidly increase. Once the regulator’s
power dissipation capacity has been exceeded and the
internal die temperature reaches approximately 140°C,
the system thermal protection circuit will become active.
The internal thermal protection circuit will actively turn off
the LDO regulator output pass device to prevent the pos-
sibility of over-temperature damage. The LDO regulator
output will remain in a shutdown state until the internal
die temperature falls back below the 140°C trip point.
The interaction between the short-circuit and thermal
protection systems allows the LDO regulator to with-
stand indefinite short-circuit conditions without sustain-
ing permanent damage.
No-Load Stability
The AAT2612 LDO is designed to maintain output voltage
regulation and stability under operational no-load condi-
tions. This is an important characteristic for applications
where the output current may drop to zero. An output
capacitor is required for stability under no-load operating
conditions. Refer to the Output Capacitor section of this
datasheet for recommended typical output capacitor val-
ues.
Internal Power Supply
The AAT2612 internal circuitry uses INL1 as the internal
power supply. The buck output will have no output when
INL1 is not connected to power.
Thermal Calculations
There are three types of losses associated with the
AAT2612 step-down converters: 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. Switching losses are
dominated by the gate charge of the power output
switching devices. At full load, with continuous conduction
mode (CCM), a simplified form of the losses is given by:
P_BUCK = I_O2 ·
R_DS(ON)P ·
V_O
V_IN
+ R_DS(ON)N ·
1-
V_O
V_IN
+ t_SW · f_S · I_O · V_IN + I_Q · V_IN
Where I_Q is the step-down converter quiescent current,
t_sw is the switching time, R_DS(ON)P and R_DS(ON)N are the high
side and low side switching MOSFETs’ on-resistance. V_IN,
V_O and I_O are the input voltage, the output voltage and
the load current.
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.
For all the LDOs,
P_D(MAX) = (V_IN – V_OUT) · I_OUT(MAX).
The total power losses of both step-down converter and
LDOs can be expressed as
P_TOTAL = P_BUCK + P_D(MAX).
Given the total losses, the maximum junction tempera-
ture can be derived from the θ_JA for the package.
T_J(MAX) = P_TOTAL · θ_JA + T_A
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [e-mail] • www.skyworksinc.com
13
202407B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 20, 2013