AAT1121 Datasheet, PDF (10/19 Pages) Advanced Analogic Technologies – 1.5MHz, 250mA Step-Down Converter

Control Loop
The AAT1121 is a 250mA current mode step-down con-
verter. The current through the P-channel MOSFET (high
side) is sensed for current loop control, as well as short-
circuit and overload protection. A fixed slope compensa-
tion signal is added to the sensed current to maintain
stability for duty cycles greater than 50%. The peak cur-
rent mode loop appears as a voltage-programmed cur-
rent source in parallel with the output capacitor.
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
transconductance voltage error amplifier output. The
error amplifier reference is fixed at 0.6V.
Soft Start / Enable
Soft start increases the inductor current limit point in
discrete steps when the input voltage or enable input is
applied. It limits the current surge seen at the input and
eliminates output voltage overshoot. When pulled low,
the enable input forces the AAT1121 into a low-power,
non-switching state. The total input current during shut-
down is less than 1μA.
Current Limit and
Over-Temperature Protection
For overload conditions, the peak input current is limit-
ed. As load impedance decreases and the output voltage
falls closer to zero, more power is dissipated internally,
raising the device temperature. Thermal protection com-
pletely disables switching when internal dissipation
becomes excessive, protecting the device from damage.
The junction over-temperature threshold is 140°C with
15°C of hysteresis.
Under-Voltage Lockout
Internal bias of all circuits is controlled via the V_IN power.
Under-voltage lockout (UVLO) guarantees sufficient V_IN
bias and proper operation of all internal circuits prior to
activation.
DATA SHEET
AAT1121
1.5MHz, 250mA Step-Down Converter
Applications Information
Inductor Selection
The step-down converter uses peak current mode control
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 the AAT1121 is 0.45A/μs. This equates
to a slope compensation that is 75% of the inductor cur-
rent down slope for a 1.8V output and 3.0μH inductor.
m
=
0.75 ·
L
V_O
=
0.75 · 1.8V
3.0µH
=
0.45
A
µs
This is the internal slope compensation for the AAT1121.
When externally programming to 3.0V, the calculated
inductance is 5.0μH.
L=
0.75 · V_O
m
=
0.75 · V_O
A
»
1.67
µs
A
·
V_O
0.45A _µs
=
1.67
µs
A
· 3.0V
=
5.0µH
In this case, a standard 4.7μH value is selected.
For most designs, the AAT1121 operates with an inductor
value of 1μH to 4.7μH. Table 1 displays inductor values
for the AAT1121 with different output voltage options.
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.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [e-mail] • www.skyworksinc.com
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201974B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 15, 2013