AOZ1280
The AOZ1280 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. 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.
The relationship between the input capacitor RMS
current and voltage conversion ratio is calculated and
shown in Figure 2. It can be seen that when V is half of
O
V , C is under the worst current stress. The worst
IN
IN
current stress on C is 0.5 x I .
IN
O
0.5
0.4
0.3
0.2
0.1
0
Under Voltage Lock Out (UVLO)
ICIN_RMS(m)
IO
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
0
0.5
m
1
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 soft-start circuit when the junction
temperature decreases to 100 °C.
Figure 2. ICIN vs. Voltage Conversion Ratio
For reliable operation and best performance, the input
capacitors must have a current rating higher than
I
at the worst operating conditions. Ceramic
CIN_RMS
capacitors are preferred for use as input capacitors
because of their low ESR and high ripple current rating.
Depending on the application circuits, other low ESR
tantalum capacitor or aluminum electrolytic capacitor
may be used. When selecting ceramic capacitors,
X5R or X7R type dielectric ceramic capacitors are
preferred for their better temperature and voltage
characteristics.
Application Information
The basic AOZ1280 application circuit is shown in
Figure 1. Component selection is explained below.
Input Capacitor
The input capacitor must be connected to the V pin and
IN
the GND pin of the AOZ1280 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.
Note that the ripple current rating from capacitor
manufactures are based on a fixed life time. Further de-
rating may be necessary for practical design
requirement.
The input ripple voltage can be approximated by
equation below:
Inductor
I
V
V
O
O
O
-----------------
--------
--------
V
=
1 –
The inductor is used to supply constant current to output
when it is driven by a switching voltage. For given input
and output voltage, inductance and switching frequency
together decide the inductor ripple current, which is:
IN
f C
V
V
IN
IN
IN
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:
V
V
O
O
----------
--------
I =
1 –
L
f L
V
IN
The peak inductor current is:
V
V
O
O
--------
--------
I
= I
1 –
I
L
CIN_RMS
O
V
V
--------
= I +
O
I
IN
IN
Lpeak
2
if we let m equal the conversion ratio:
High inductance provides a low inductor ripple current
but requires larger size inductor to avoid saturation.
Low ripple current reduces inductor core losses and also
V
O
--------
= m
V
IN
Rev. 1.1 August 2011
www.aosmd.com
Page 8 of 13
AOS [ ALPHA & OMEGA SEMICONDUCTORS ]
