AOZ1282CI
Detailed Description
The AOZ1282CI is a current-mode step down regulator
with integrated high side NMOS switch. It operates from
a 4.5V to 36V input voltage range and supplies up to
1.2A 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.
Switching Frequency
The AOZ1282CI switching frequency is fixed and set by
an internal oscillator. The switching frequency is set
internally 450kHz.
Output Voltage Programming
Output voltage can be set by feeding back the output to
the FB pin with a resistor divider network. In the
application circuit shown in Figure 1. The resistor divider
network includes R1 and R2. Usually, a design is started
by picking a fixed R2 value and calculating the required
R1 with equation below.
The AOZ1282CI is available in SOT23-6L package.
Enable and Soft Start
The AOZ1282CI has 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 the voltage higher
than UVLO and voltage on EN pin is HIGH. In soft start
process, the output voltage is ramped to regulation
voltage in typically 400µs. The 400µs soft start time is set
internally.
R
1
------
V
= 0.8 1 +
O
R
2
Some standard values of R1 and R2 for the most
commonly used output voltage values are listed in
Table 1.
The EN pin of the AOZ1282CI is active high. Connect the
EN pin to VIN if enable function is not used. Pull it to
ground will disable the AOZ1282CI. Do not leave it open.
The voltage on EN pin must be above 1.2 V to enable the
AOZ1282CI. When voltage on EN pin falls below 0.4V,
the AOZ1282CI is disabled.
Vo (V)
R1 (kΩ)
R2 (kΩ)
1.8
2.5
3.3
5.0
80.6
49.9
49.9
49.9
64.2
23.4
15.8
9.53
Steady-State Operation
Under steady-state conditions, the converter operates in
fixed frequency and Continuous-Conduction Mode
(CCM).
Table 1.
The combination of R1 and R2 should be large enough to
avoid drawing excessive current from the output, which
will cause power loss.
The AOZ1282CI integrates an internal NMOS as the
high-side switch. Inductor current is sensed by amplifying
the voltage drop across the drain to 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 is amplified by the
internal transconductance error amplifier. The error
voltage is compared against the current signal, which is
sum of inductor current signal and ramp compensation
signal, at 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.
Protection Features
The AOZ1282CI has multiple protection features to
prevent system circuit damage under abnormal
conditions.
Over Current Protection (OCP)
The sensed inductor current signal is also used for over
current protection.
The cycle by cycle current limit threshold is set normal
value of 1.9A. When the load current reaches the current
limit threshold, the cycle by cycle current limit circuit turns
off the high side switch immediately to terminate the
current duty cycle. The inductor current stop rising. The
cycle by cycle current limit protection directly limits
inductor peak current. The average inductor current is
also limited due to the limitation on peak inductor current.
When cycle by cycle current limit circuit is triggered, the
output voltage drops as the duty cycle decreasing.
Rev. 0.5 September 2012
www.aosmd.com
Page 7 of 13
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