PRODUCT DATASHEET
AAT1218
SwitchReg
TM
Integrated Soft-Start
During start-up, the AAT1218's integrated soft-start cir-
cuitry gradually increases the peak inductor current until
it reaches normal operating value. The inrush current
during start-up can thus be minimized.
1A, 1.2MHz Synchronous Boost Converter
Application Information
Setting the Output Voltage
An external resistor divider is used to set the output volt-
age. The output voltage of the switching regulator (V
OUT
)
is determined by the following equation:
Current Mode PWM Control
The AAT1218 is based on a slope compensated current
mode control topology. It operates at a fixed frequency
of 1.2MHz. At the beginning of each clock cycle, the main
switch (NMOS) is turned on and the inductor current
starts to ramp up. After the maximum duty cycle or if the
sense current signal is equal to the error amplifier (EA)
output, the main switch is turned off and the synchro-
nous switch (PMOS) is turned on. This control scheme
has intrinsic cycle-by-cycle current limiting, which can
prevent the main switch from overstress and prevent
saturation of the external inductor.
R1
⎞
⎛
V
OUT
= 1.20V · 1 + R2
⎝
⎠
V
OUT
3.3V
5.0V
R1 (Ω)
1.02M
1.02M
R2 (Ω)
576k
324k
Table 1: Resistor Selection Example for Output
Voltage Setting.
Fixed output voltage devices are also available for 3.3V
and 5V outputs. These devices integrate the feedback
network into the die and can save two external resistors.
Do not connect any component to the FB pin and leave
it floating.
Power Saving Mode
At very light load, the AAT1218 automatically switches
into Power Saving Mode to improve efficiency. In this
mode, the PWM control will skip some pulses to maintain
regulation. If load increases and output voltage drops,
the device will automatically switch back to regular PWM
mode and maintain regulation.
Inductor Selection
The AAT1218's high switching frequency of 1.2MHz
allows for small surface mount inductors. For most
designs, the AAT1218 operates with inductors of 2.2μH
to 10μH depending on input/output voltage and load cur-
rent. First determine the worst case V
IN
, V
OUT
, and I
OUT
.
Then use the equation below to select the proper induc-
tor value; assume the converter is operating in continu-
ous current mode and is in steady state:
Anti-ringing Control
Anti-ringing circuitry is included to remove the high fre-
quency ringing that appears on the LX pin when the
inductor current decreases to zero. In this case, ringing
on the LX pin is caused by energy stored in parasitics of
the MOSFETs and the inductor. The anti-ringing circuitry
clamps the voltage to battery voltage internally and thus
dampens the ringing.
V
IN
D=1- V
OUT
ΔI
I
PEAK
= I
RMS
+ 2
V
IN
· D
ΔI
= L · f
SW
η
is the converter efficiency, I
PEAK
is the peak inductor
current,
ΔI
is the peak-peak inductor ripple current, I
RMS
is the RMS current of the inductor current. f
sw
is the
switching frequency, L is the inductance value and D is
the steady state duty cycle.
Device Enable
When EN is set logic high, the AAT1218 begins operation.
If EN is set logic low, the device is goes into shutdown
mode and consumes less than 1μA current. After start-
up, the internal circuitry is supplied by V
OUT
during normal
operation. However, if shutdown mode is enabled, the
internal circuitry will again be supplied by the battery.
1218.2008.10.1.1
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