AAT2749 Datasheet, PDF (10/17 Pages) Skyworks Solutions Inc. – 3.0MHz Step-Down Converter and Low-V in LDO

DATA SHEET
AAT2749
3.0MHz Step-Down Converter and Low-V_IN LDO
The output of the voltage error amplifier programs the
current mode loop for the necessary peak switch current
to force a constant output voltage for all load and line
conditions. Internal loop compensation terminates the
trans-conductance voltage error amplifier output.
Soft Start / Enable
Soft start limits the current surge seen at the input and
eliminates output voltage overshoot. When disabled, the
AAT2749 is in a low-power, non-switching state. The
total input current during shutdown is less than 1μA.
Current Limit
For overload conditions, the peak input current is limit-
ed. To minimize power dissipation and stresses under
current limit and short-circuit conditions, switching is
terminated after entering current limit for a series of
pulses. Switching is terminated for seven consecutive
clock cycles after a current limit has been sensed for a
series of four consecutive clock cycles.
LDO Regulator
The AAT2749 includes a low-dropout (LDO) linear regu-
lator. The LDO regulator operates from 1.62V to 5.5V;
however, V_{IN_LDO} cannot exceed the input voltage to the
step-down regulator (V_{IN_BUCK}). The linear regulator out-
put voltage is set by internal resistive voltage dividers.
The LDO regulator consumes about 50μA of quiescent
current.
The 300mA LDO regulator is stable with a 2.2μF ceram-
ic output capacitor. The LDO regulator has a current limit
to protect against short circuit conditions.
Over-Temperature Protection
Thermal protection completely disables the converters
when internal dissipation becomes excessive. The junc-
tion over-temperature threshold is 140°C with 15°C of
hysteresis. Once an over-temperature or over-current
fault conditions is removed, the output voltage auto-
matically recovers.
Chip Enable Pin (CE)
Logic High at CE pin enables step-down converter and
LDO.
Applications Information
Step-Down Converter Inductor Selection
The step-down converter uses peak current mode con-
trol with slope compensation to maintain stability for
duty cycles greater than 50%. The output inductor value
must be selected so the inductor current down slope
meets the internal slope compensation requirements.
The internal slope compensation for the adjustable and
low voltage fixed versions of channel 1 is 0.75A/μs. This
equates to a slope compensation that is 75% of the
inductor current down slope for a 1.8V output and 2.2μH
inductor.
m
=
0.75 ·
L
V_O
=
0.75 · 1.8V
1.8μH
=
0.75
A
μs
L=
0.75 ·
m
V_O
=
0.75
·
1.8V
A
=
1.8μH
0.75 _μs
The inductor should be set equal to the output voltage
numeric value in micro henries (μH). This guarantees
that there is sufficient internal slope compensation.
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.
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.
Step-Down Converter Input Capacitor
Select a 2.2μF to 4.7μF X7R or X5R ceramic capacitor for
the input. To estimate the required input capacitor size,
determine the acceptable input ripple level (V_PP) and
solve for C_IN. The calculated value varies with input volt-
age and is a maximum when V_IN is double the output
voltage.
C_IN =
V_O
V_IN
·
1-
V_O
V_IN
V_PP
I_O
- ESR
· F_S
V_{O · 1 -} V_O
V_IN
V_IN
=
1
4
for
V_IN
=
2
·
V_O
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [e-mail] • www.skyworksinc.com
10
202037A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • June 8, 2012