AAT1276 Datasheet, PDF (10/18 Pages) Advanced Analogic Technologies – Boost Converter with USB Power Switch

SwitchReg^TM
Step-Up Converter
Application Information
The AAT1276 step-up converter provides the benefits of
current mode control with a simple hysteretic feedback
loop. The device maintains exceptional DC regulation,
transient response, and cycle-by-cycle current limit with-
out additional compensation components. The AAT1276
modulates the power MOSFET switching current in
response to changes in output voltage. The voltage loop
programs the required inductor current in response to
changes in the output load and input voltage.
The switching cycle initiates when the N-channel MOSFET
is turned ON and the inductor current ramps up. The ON
interval is terminated when the inductor current reaches
the programmed peak current level. During the OFF
interval, the input current decays until the lower thresh-
old, or zero inductor current is reached. The lower cur-
rent is equal to the peak current minus a preset hyster-
esis threshold, which determines the inductor ripple
current. The peak current is adjusted by the controller
until the output current requirement is met.
The magnitude of the feedback error signal determines
the average input current. Therefore, the AAT1276 boost
controller implements a programmed current source
connected to the output capacitor and load resistor.
There is no right-half plane zero, and loop stability is
achieved with no additional external compensation com-
ponents.
At light load, the inductor OFF interval current goes to
zero and the boost converter enters discontinuous mode
operation. Further reduction in the load results in a cor-
responding reduction in the switching frequency, which
reduces switching losses and maintains high efficiency at
light loads.
The operating frequency varies with changes in the input
voltage, output voltage, and inductor size. Once the
boost converter has reached continuous mode, increas-
ing the output load will not significantly change the oper-
ating frequency. A small 2.2μH (± 20%) inductor is
selected to maintain high frequency operation for the 5V
USB output voltage.
Output Voltage Programming
The output voltage is programmed through a resistor
divider network located from the OUT1 output capacitor
to the FB pin to ground.
PRODUCT DATASHEET
AAT1276
Boost Converter with USB Power Switch
Soft Start / Enable
The input disconnect switch is activated when a valid
input voltage is present and the EN pin is pulled high.
The slew rate control on the P-channel MOSFET ensures
minimal inrush current as the output voltage is charged
to the input voltage prior to switching of the N-channel
power MOSFET. The soft-start circuitry guarantees
monotonic turn-on and eliminates output voltage over-
shoot across the full input voltage range for all load
conditions.
Input Current Limit and
Over-Temperature Protection
The switching of the N-channel MOSFET terminates
when input current limit of 2.5A (typical) is exceeded.
This minimizes the power dissipation and component
stresses under overload and short-circuit conditions.
Switching resumes when the current decays below the
limit.
Thermal protection disables the AAT1276 boost con-
verter when the internal power dissipation becomes
excessive. The junction over-temperature threshold is
140°C with 15°C of temperature hysteresis. The output
voltage automatically recovers when the over-tempera-
ture or over-current fault condition is removed.
Shutdown and Output Disconnect
A typical synchronous step-up (boost) converter has a
conduction path from the input to the output via the
body diode of the P-channel MOSFET. The AAT1276
design disconnects this body diode from the output and
eliminates this conduction path. This enables the AAT1276
to provide true load disconnect during shutdown and
inrush current limit at turn-on.
Short-Circuit Protection
The P-channel synchronous MOSFET body diode discon-
nect feature also gives the AAT1276 the ability to pro-
vide output short-circuit current limit protection.
Under-Voltage Lockout
Under-voltage lockout (UVLO) guarantees sufficient V_IN
bias and proper operation of all internal circuitry prior to
soft start. Internal bias of all circuits is controlled via the
VCC input, which is connected to V_IN.
10
www.analogictech.com
1276.2007.11.1.1