AAT1217
600mA, 1.2MHz, Micropower
Synchronous Step-Up Converter
Current Mode Operation
Application Information
The AAT1217 is based on a slope compensated
current mode PWM 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. After the
maximum duty cycle or the sense current signal
equals the error amplifier (EA) output, the main
switch is turned off and the synchronous switch
(PMOS) is turned on. This control topology fea-
tures cycle-by-cycle current limiting which can pre-
vent the main switch from overstress and the exter-
nal inductor from saturating.
Adjustable Output Voltage
An external resistor divider is used to set the output
voltage. The output voltage of the switching regula-
tor (VOUT) is determined by the following equation:
R1
V
OUT
= 1.23V · 1 +
R2
Table 1 lists the recommended resistor values for
particular output voltage settings.
Ω
Ω
VOUT
3.3V
5.0V
R1( )
R2( )
Pulse Skipping Mode Operation
1.02M
1.02M
604k
332k
At very light load, the AAT1217 automatically
switches into pulse skipping mode operation to
improve efficiency. During this mode, the PWM
control will skip some pulses to maintain regulation.
If the load increases and the output voltage drops,
the device will automatically switch back to normal
PWM mode and maintain regulation.
Table 1: Resistor Selection for Output Voltage
Setting.
Fixed Output Voltage
AAT1217 has two fixed output voltage options:
3.3V and 5V. An internal resistor divider is con-
nected to the FB pin inside the package which elim-
inates the need for external feedback resistors.
When designing with the fixed output voltage
option, remember to leave the FB pin open; other-
wise the output voltage will be affected. However, a
feed-forward capacitor can still be added between
the FB and VOUT pins to enhance the control loop
performance.
Anti-Ringing Control
An anti-ringing circuitry is included to remove the
high frequency ringing that appears on the SW pin
when the inductor current goes to zero. In this
case, a ringing on the SW pin is induced due to
remaining energy stored in parasitic components of
switch and inductor. The anti-ringing circuitry
clamps the voltage internally to the battery voltage
and therefore dampens this ringing.
Inductor Selection
Device Shutdown
The high switching frequency of 1.2MHz allows for
small surface mount inductors. For most applica-
tions, the AAT1217 operates with inductors from
2.2µH to 10µH. Use the following equations to
select the proper inductor value for a particular
application condition:
When SHDN is set logic high, the AAT1217 is put
into active mode operation. If SHDN is set logic
low, the device is put into shutdown mode and con-
sumes less than 1µA of current. After start-up, the
internal circuitry is supplied by VOUT, however, if
shutdown mode is enabled, the internal circuitry
will be supplied by the input source again.
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1217.2007.07.1.0
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
