aoz1281di Alpha & Omega Semiconductor, aoz1281di Datasheet - Page 6

• Manufacturer Part Number: aoz1281di
• Description: Ezbucktm 1.8 A Simple Buck Regulator
• Manufacturer: Alpha & Omega Semiconductor

Efficiency
Detailed Description
The AOZ1281 is a current-mode step down regulator
with integrated high side NMOS switch. It operates from
a 3 V to 26 V input voltage range and supplies up to 1.8 A
of load current. Features include: enable control, under
voltage lock-out, internal soft-start, output over-voltage
protection, over-current protection, and thermal shut
down. The AOZ1281 is available in a DFN 2x2, 8L
package.
Enable and Soft Start
The AOZ1281 has an internal soft start feature to limit
in-rush current and ensure the output voltage ramps up
smoothly to regulation voltage. A soft start process
begins when the input voltage rises to a voltage higher
than UVLO and the voltage level on the EN pin is HIGH.
In the soft start process, the output voltage is typically
ramped to regulation voltage in 400 μs. The 400 μs
soft start time is set internally.
The EN pin of the AOZ1281 is active high. Connect the
EN pin to VIN if the enable function is not used. Pulling
EN to ground will disable the AOZ1281. Do not leave EN
open. The voltage on the EN pin must be above 1.2 V to
enable the AOZ1281. When voltage on the EN pin falls
below 0.4 V, the AOZ1281 is disabled.
Rev. 1.0 June 2011
95
90
85
80
75
70
65
60
0
0.2
0.4
Efficiency (V
0.6
www.aosmd.com
Load Current (A)
IN
0.8
= 12V) vs. Load Current
Steady-State Operation
Under steady-state conditions, the converter operates
in fixed frequency and Continuous-Conduction Mode
(CCM).
The AOZ1281 integrates an internal NMOS as the
high-side switch. Inductor current is sensed by amplifying
the voltage drop across the drain to the source of the
high-side power MOSFET. Output voltage is divided
down by the external voltage divider at the FB pin.
The difference of the FB pin voltage and reference
voltage is amplified by the internal transconductance
error amplifier. The error voltage is compared against
the current signal, which is sum of inductor current signal
plus ramp compensation signal, at the PWM comparator
input. If the current signal is less than the error voltage,
the internal high-side switch is on. The inductor current
flows from the input through the inductor to the output.
When the current signal exceeds the error voltage, the
high-side switch is off. The inductor current is
freewheeling through the external Schottky diode to
output.
Switching Frequency
The AOZ1281 switching frequency is fixed and set by an
internal oscillator. The switching frequency is set
internally 1.5 MHz.
1.0
1.2
1.4
5.0V OUTPUT
3.3V OUTPUT
1.8V OUTPUT
1.2V OUTPUT
1.6
1.8
AOZ1281
Page 6 of 12