aoz1212 Alpha & Omega Semiconductor, aoz1212 Datasheet - Page 8

• Manufacturer Part Number: aoz1212
• Description: 3a Simple Buck Regulator
• Manufacturer: Alpha & Omega Semiconductor

The cycle-by-cycle current limit threshold is internally set.
When the load current reaches the current limit thresh-
old, the cycle-by-cycle current limit circuit turns off the
high side switch immediately to terminate the current
duty cycle. The inductor current stops 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 the peak inductor current. When
cycle-by-cycle current limit circuit is triggered, the output
voltage drops as the duty cycle decreases.
The AOZ1212 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
0.2V, the short circuit protection circuit is triggered. To
prevent current limit running away when the comp pin
voltage is higher than 2.1V, the short circuit protection is
also triggered. As a result, the converter is shut down
and hiccups at a frequency equals to 1/16 of normal
switching frequency. The converter will start up via a soft
start once the short circuit condition is resolved. In short
circuit protection mode, the inductor average current is
greatly reduced because of the low hiccup frequency.
Power-On Reset (POR)
A power-on reset circuit monitors the input voltage.
When the input voltage exceeds 4.3V, the converter
starts operation. When input voltage falls below 4.1V,
the converter will stop switching.
Thermal Protection
An internal temperature sensor monitors the junction
temperature. It shuts down the internal control circuit and
high side NMOS if the junction temperature exceeds
145°C. The regulator will restart automatically under the
control of soft-start circuit when the junction temperature
decreases to 100°C.
Application Information
The basic AOZ1212 application circuit is shown in
Figure 1. Component selection is explained below.
Input Capacitor
The input capacitor (C
the V
steady input voltage and lter out the pulsing input
current. The voltage rating of the input capacitor must be
greater than maximum input voltage + ripple voltage.
The input ripple voltage can be approximated by equation
below:
Rev. 1.1 October 2007
V
IN
IN
pin and GND pin of the AOZ1212 to maintain
=
------------------ -
f
I
O
C
IN
1
1
in Figure 1) must be connected to
–
--------- -
V
V
IN
O
--------- -
V
V
IN
O
www.aosmd.com
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 let m equal the conversion ratio:
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
V
current stress on CIN is 0.5 x I
For reliable operation and best performance, the input
capacitors must have a current rating higher than
I
capacitors are preferred for 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 also be
used. When selecting ceramic capacitors, X5R or X7R
type dielectric ceramic capacitors are preferred for their
better temperature and voltage characteristics. Note that
the ripple current rating from capacitor manufactures is
based on certain amount of life time. Further de-rating
may be necessary for practical design requirement.
Inductor
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,
I
--------- -
V
V
CIN_RMS
CIN _RMS
IN
I
IN
I
O
L
CIN_RMS
, C
=
I
=
IN
O
----------- -
f
Figure 2. I
m
(m)
V
is under the worst current stress. The worst
at the worst operating conditions. Ceramic
O
L
0.5
0.4
0.3
0.2
0.1
=
0
I
0
O
1
CIN
–
--------- -
V
vs. Voltage Conversion Ratio
V
--------- - 1
V
V
IN
O
IN
O
–
O
0.5
--------- -
V
.
m
V
IN
O
AOZ1212
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O
is half of
1