AOZ1021(2007) Ver la hoja de datos (PDF) - Alpha and Omega Semiconductor
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AOZ1021 Datasheet PDF : 15 Pages
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AOZ1021
The 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 AOZ1021 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 the AOZ1021 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.
When the output is shorted to ground under fault
conditions, the inductor current decays very slow during
a switching cycle because of VO = 0V. To prevent cata-
strophic failure, a secondary current limit is designed
inside the AOZ1021. The measured inductor current is
compared against a preset voltage which represents the
current limit, between 3.5A and 5.0A. When the output
current is more than current limit, the high side switch will
be turned off. The converter will initiate a soft start once
the over-current condition is resolved.
Power-On Reset (POR)
A power-on reset circuit monitors the input voltage.
When the input voltage exceeds 4.1V, the converter
starts operation. When input voltage falls below 3.7V,
the converter shuts 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
150°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 AOZ1021 application circuit is show in
Figure 1. Component selection is explained below.
Input Capacitor
The input capacitor must be connected to the VIN pin and
PGND pin of AOZ1021 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:
∆V IN
=
-------I--O---------
×
1
–
-V-----O----
×
-V-----O----
f × CIN V IN V 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:
I CIN _RMS = I O ×
-V-----O----
1
–
-V-----O----
V IN V IN
if we 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
ICIN_RMS(m) 0.3
IO
0.2
0.1
0
0
0.5
1
m
Figure 2. ICIN vs. Voltage Conversion Ratio
Rev. 1.1 November 2007
www.aosmd.com
Page 8 of 15
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