AOZ1018 查看數據表（PDF） - Alpha and Omega Semiconductor
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AOZ1018 Datasheet PDF : 14 Pages
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AOZ1018
Combination of R1 and R2 should be large enough to
avoid drawing excessive current from the output, which
will cause power loss.
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 AOZ1018 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 AOZ1018 employs peak current
mode control, the COMP pin voltage is proportional to
the peak inductor current. The COMP pin voltage is lim-
ited 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
2.54A and 3.65A. When the load current reaches the cur-
rent 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 AOZ1018 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 opera-
tion. When input voltage falls below 3.7V, the converter
will be shut down.
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
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 AOZ1018 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
PGND pin of the AOZ1018 to maintain steady input volt-
age 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:
∆V IN
=
-------I--O---------
f × CIN
×
1
–
V-V-----IO-N---
×
-V-----O----
V IN
Since the input current is discontinuous in a buck con-
verter, 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 calcu-
lated by:
ICINRMS = IO ×
-V-----O----
1
–
-V-----O----
V IN V IN
if let m equal the conversion ratio:
-V-----O---- = m
V IN
The relation between the input capacitor RMS current
and voltage conversion ratio is calculated and shown in
Figure 2 below. It can be seen that when VO is half of VIN,
CIN is under the worst current stress. The worst current
stress on CIN is 0.5 x IO.
0.5
0.4
ICINRMS(m) 0.3
IO
0.2
0.1
0
0
0.5
1
m
Figure 2. ICIN vs. Voltage Conversion Ratio
Rev. 1.0 November 2006
www.aosmd.com
Page 7 of 14
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