aoz1281di Alpha & Omega Semiconductor, aoz1281di Datasheet - Page 7

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

Output Voltage Programming
Output voltage can be set by feeding back the output to
the FB pin with a resistor divider network. Refer to the
application circuit shown in Figure 1. The resistor divider
network includes R
by picking a fixed R
R
Some standard values of R
commonly used output voltage values are listed in
Table 1.
Table 1.
The combination of R
avoid drawing excessive current from the output, which
will cause power loss.
Protection Features
The AOZ1281 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 to 2 A.
When the load current reaches the current limit
threshold, the cycle-by-cycle current limit circuit
immediately turns off the high-side switch 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 decreases.
The AOZ1281 has internal short circuit protection to
protect itself from catastrophic failure under output short
circuit conditions. The FB pin voltage is proportional to
the output voltage. Whenever the FB pin voltage is below
0.2 V, the short circuit protection circuit is triggered.
As a result, the converter is shut down and hiccups.
V
1
Rev. 1.0 June 2011
O
with equation below.
=
V
0.8
O
1.8
2.5
3.3
5.0
(V)
×
⎛
⎜
⎝
1
+
R
------ -
R
1
2
and R
1
2
value and calculating the required
1
⎞
⎟
⎠
and R
R
1
80.6
49.9
49.9
49.9
2
1
(kΩ)
. Usually, a design is started
and R
2
should be large enough to
2
for the most
R
2
64.2
23.4
15.8
9.53
(kΩ)
www.aosmd.com
The converter will start up via a soft start once the short
circuit condition is resolved. In the short circuit protection
mode, the inductor average current is greatly reduced.
Under Voltage Lock Out (UVLO)
An UVLO circuit monitors the input voltage. When the
input voltage exceeds 2.9 V, the converter starts
operation. When input voltage falls below 2.3 V, the
converter will stop switching.
Thermal Protection
An internal temperature sensor monitors the junction
temperature. The sensor shuts down the internal control
circuit and high side NMOS if the junction temperature
exceeds 150 °C. The regulator will restart automatically
under the control of the soft-start circuit when the junction
temperature decreases to 100 °C.
Application Information
The basic AOZ1281 application circuit is shown in
Figure 1. Component selection is explained below.
Input Capacitor
The input capacitor must be connected to the VIN pin
and the GND pins of the AOZ1281 to maintain steady
input voltage and filter out the pulsing input current. The
voltage rating of the input capacitor must be greater than
maximum input voltage plus ripple voltage.
The input ripple voltage can be approximated by the
equation below:
Since the input current is discontinuous in a buck
converter, the current stress on the input capacitor is
another concern when selecting the capacitor. For a
buck circuit, the RMS value of input capacitor current can
be calculated by:
if we let m equal the conversion ratio:
ΔV
I
CIN_RMS
-------- -
V
V
IN
IN
O
=
=
---------------- -
f
×
m
=
I
O
C
I
IN
O
×
×
⎛
⎜
⎝
1
-------- - 1
V
V
–
IN
O
-------- -
V
V
⎛
⎜
⎝
IN
O
⎞
⎟
⎠
–
×
-------- -
V
V
-------- -
V
IN
V
O
IN
O
⎞
⎟
⎠
AOZ1281
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