AAT1275 Analogic Corporation, AAT1275 Datasheet - Page 10

• Manufacturer Part Number: AAT1275
• Manufacturer: Analogic Corporation

Step-Up Converter Application
Information
The AAT1275 step-up converter provides the ben-
efits of current mode control with a simple hys-
teretic feedback loop. The device maintains excep-
tional DC regulation, transient response, and cycle-
by-cycle current limit without additional compensa-
tion components. The AAT1275 modulates the
power MOSFET switching current in response to
changes in output voltage. The voltage loop pro-
grams 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 cur-
rent decays until the lower threshold, or zero induc-
tor current is reached. The lower current is equal to
the peak current minus a preset hystersis thresh-
old, 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 deter-
mines the average input current. Therefore, the
AAT1275 boost controller implements a pro-
grammed 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 components.
At light load, the inductor OFF interval current goes
to zero and the boost converter enters discontinu-
ous mode operation. Further reduction in the load
results in a corresponding reduction in the switch-
ing 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, increasing the output load will not signifi-
cantly change the operating frequency. A small
10
10
Boost Converter with USB Power Switch
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 resis-
tor divider network located from the OUT1 output
capacitor to the FB pin to ground.
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 overshoot across the
full input voltage range for all load conditions.
Current Limit and Over-Temperature
Protection
The switching of the N-channel MOSFET termi-
nates when current limit of 2.5A (typical) is exceed-
ed. This minimizes the power dissipation and com-
ponent stresses under overload and short-circuit
conditions. Switching resumes when the current
decays below the limit.
Thermal protection disables the AAT1275 boost
converter when the internal power dissipation
becomes excessive. The junction over-tempera-
ture threshold is 140°C with 15°C of temperature
hystersis. The output voltage automatically recov-
ers when the over-temperature or over-current fault
condition is removed.
Under-Voltage Lockout
Under-voltage lockout (UVLO) guarantees suffi-
cient V
circuitry prior to soft start. Internal bias of all circuits
is controlled via the VCC input, which is connected
to V
IN
.
IN
bias and proper operation of all internal
AAT1275
1275.2006.12.1.2
k