aoz1242 Alpha & Omega Semiconductor, aoz1242 Datasheet - Page 8

• Manufacturer Part Number: aoz1242
• Description: Ezbucktm 3a Simple Buck Regulator
• Manufacturer: Alpha & Omega Semiconductor

When cycle by cycle current limit circuit is triggered, the
output voltage drops as the duty cycle decreasing.
The AOZ1242 has internal short circuit protection to pro-
tect itself from catastrophic failure under output short cir-
cuit conditions. The FB pin voltage is proportional to the
output voltage. Whenever FB pin voltage is below 0.2V,
the short circuit protection circuit is triggered. To prevent
current limit running away, when comp pin voltage is
higher than 2.1 V, the short circuit protection is also trig-
gered. As a result, the converter is shut down and hic-
cups 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 disappears. In short circuit pro-
tection 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 AOZ1242 application circuit is shown in
Figure 1. Component selection is explained below.
Input Capacitor
The input capacitor (C
to the V
steady input voltage and filter out the pulsing input
current. The voltage rating of input capacitor must be
greater than maximum input voltage plus ripple voltage.
The input ripple voltage can be approximated by equa-
tion 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:
I
ΔV
CIN_RMS
Rev. 1.4 November 2010
IN
IN
=
pin and GND pin of the AOZ1242 to maintain
---------------- -
f C
×
=
I
O
I
IN
O
×
×
⎛
⎜
⎝
1
1
-------- - 1
V
V
in Figure 1) must be connected
IN
–
O
-------- -
V
V
⎛
⎜
⎝
IN
O
–
⎞
⎟
⎠
-------- -
V
V
×
IN
O
-------- -
V
V
⎞
⎟
⎠
IN
O
www.aosmd.com
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 current rating higher than I
at worst operating conditions. Ceramic 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,
The peak inductor current is:
High inductance gives low inductor ripple current but
requires larger size inductor to avoid saturation. Low
ripple current reduces inductor core losses. It also
-------- -
V
V
ΔI
I
IN
Lpeak
IN
I
O
CIN_RMS
, C
L
=
=
I
IN
O
Figure 2. I
---------- -
f
m
(m)
is under the worst current stress. The worst
=
V
×
O
I
L
0.5
0.4
0.3
0.2
0.1
O
0
×
+
0
⎛
⎜
⎝
ΔI
------- -
1
CIN
2
–
L
-------- -
V
vs. Voltage Conversion Ratio
V
IN
O
⎞
⎟
⎠
O
0.5
.
m
AOZ1242
Page 8 of 18
O
is half of
CIN_RMS
1