AAT1121
1.5MHz, 250mA Step-Down Converter
Control Loop
Applications Information
The AAT1121 is a 250mA current mode step-down
converter. The current through the P-channel
MOSFET (high side) is sensed for current loop
control, as well as short-circuit and overload pro-
tection. A fixed slope compensation signal is added
to the sensed current to maintain stability for duty
cycles greater than 50%. The peak current mode
loop appears as a voltage-programmed current
source in parallel with the output capacitor.
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 induc-
tor 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/µsec. This equates to a slope
compensation that is 75% of the inductor current
down slope for a 1.8V output and 3.0µH inductor.
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 compen-
sation terminates the transconductance voltage
error amplifier output. The error amplifier reference
is fixed at 0.6V.
0.75 ⋅ VO 0.75 ⋅ 1.8V
= 0.45
A
µsec
m =
=
L
3.0µH
This is the internal slope compensation for the
AAT1121. When externally programming to 3.0V,
the calculated inductance is 5.0µH.
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 shutdown is less than 1µA.
0.75 ⋅ VO
0.75
⋅
VO
A
µsec
⋅ VO
A
L =
=
≈
1.67
m
0.45A
µsec
µsec
A
= 1.67
⋅ 3.0V = 5.0µH
Current Limit and
In this case, a standard 4.7µH value is selected.
Over-Temperature Protection
For overload conditions, the peak input current is lim-
ited. As load impedance decreases and the output
voltage falls closer to zero, more power is dissipated
internally, raising the device temperature. Thermal
protection completely disables switching when inter-
nal dissipation becomes excessive, protecting the
device from damage. The junction over-temperature
threshold is 140°C with 15°C of hysteresis.
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
saturation 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 loss-
es 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.
Under-Voltage Lockout
Internal bias of all circuits is controlled via the VIN
power. Under-voltage lockout (UVLO) guarantees
sufficient VIN bias and proper operation of all inter-
nal circuits prior to activation.
10
1121.2007.03.1.2
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
