aoz1092d Alpha & Omega Semiconductor, aoz1092d Datasheet - Page 8

• Manufacturer Part Number: aoz1092d
• Description: Ezbucktm 3a Simple Buck Regulator
• Manufacturer: Alpha & Omega Semiconductor

Since the switch duty cycle can be as high as 100%, the
maximum output voltage can be set as high as the input
voltage minus the voltage drop on upper PMOS and
inductor.
Protection Features
The AOZ1092D 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. Since AOZ1092D employs peak
current mode control, the COMP pin voltage is
proportional to the peak inductor current. The COMP pin
voltage is limited to be between 0.4V and 2.5V internally.
The peak inductor current is automatically limited cycle
by cycle.
The cycle by cycle current limit threshold is set between
4A and 5A. 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.
The AOZ1092D 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 FB pin voltage is below
0.2V, the short circuit protection circuit is triggered. As a
result, the converter is shut down and hiccups at a
frequency equals to 1/8 of normal switching frequency.
The converter will start up via a soft start once the short
circuit condition disappears. 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 4V, the converter starts
operation. When input voltage falls below 3.7V, the
converter will be shut down.
Output Over Voltage Protection (OVP)
The AOZ1092D monitors the feedback voltage: when the
feedback voltage is higher than 960mV, it immediate
turns-off the PMOS to protect the output voltage
overshoot at fault condition. When feedback voltage is
lower than 840mV, the PMOS is allowed to turn on in the
next cycle.
Rev. 1.3 February 2009
www.aosmd.com
Thermal Protection
An internal temperature sensor monitors the junction
temperature. It shuts down the internal control circuit and
high side PMOS if the junction temperature exceeds
150ºC.
Application Information
The basic AOZ1092D application circuit is shown in
Figure 1. Component selection is explained below.
Input Capacitor
The input capacitor must be connected to the V
PGND pin of the AOZ1092D to maintain 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
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 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 below. It can be seen that when V
half of V
worst current stress on C
-------- -
V
V
I
ΔV
CIN_RMS
IN
I
O
CIN_RMS
IN
I
=
O
(m)
Figure 2. I
=
m
IN
, C
---------------- -
f
0.5
0.4
0.3
0.2
0.1
×
=
0
IN
I
0
O
C
I
is under the worst current stress. The
O
IN
CIN
×
×
⎛
⎜
⎝
vs. Voltage Conversion Ratio
-------- - 1
V
1
V
IN
O
–
IN
-------- -
V
⎛
⎜
⎝
V
is 0.5 x I
IN
O
–
0.5
m
⎞
⎟
⎠
-------- -
V
V
×
IN
O
-------- -
V
V
⎞
⎟
⎠
IN
O
O
.
AOZ1092D
Page 8 of 16
1
IN
pin and
O
is