aoz1025di Alpha & Omega Semiconductor, aoz1025di Datasheet - Page 8

• Manufacturer Part Number: aoz1025di
• Description: Ezbucktm 8a Synchronous Buck Regulator
• Manufacturer: Alpha & Omega Semiconductor

Protection Features
The AOZ1025D 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 AOZ1025D employs peak
current mode control, the COMP pin voltage is propor-
tional 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 V
catastrophic failure, a secondary current limit is designed
inside the AOZ1025D. The measured inductor current is
compared against a preset voltage which represents the
current limit. When the output current is more than
current limit, the high side switch will be turned off and
EN pin will be pulled down. The converter will initiate a
soft start once the over-current condition disappears.
Output Over Voltage Protection (OVP)
The AOZ1025D 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.
Under-voltage Lock-out (UVLO)
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.
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.
Power Good
The output of Power-Good is an open drain N-channel
MOSFET, which supplies an active high power good
stage. A pull-up resistor should connect this pin to a
DC power trail with maximum voltage no higher than 6V.
The AOZ1025D monitors the FB voltage: when FB pin
voltage is lower than 90% of the normal voltage,
N-channel MOSFET turns on and the Power-Good pin
is pulled low, which indicates the power is abnormal.
Rev. 1.3 September 2009
O
= 0V. To prevent
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Application Information
The basic AOZ1025D application circuit is show in
Figure 1. Component selection is explained below.
Input Capacitor
The input capacitor must be connected to the V
PGND pin of AOZ1025D 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 we 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 C
-------- -
V
V
I
I
ΔV
IN
CIN_RMS
CIN_RMS
IN
O
, C
I
IN
O
=
IN
(m)
Figure 2. I
=
m
is under the worst current stress. The worst
---------------- -
f
0.5
0.4
0.3
0.2
0.1
×
=
I
0
O
C
0
I
O
IN
CIN
×
×
IN
⎛
⎜
⎝
vs. Voltage Conversion Ratio
is 0.5 x I
-------- - 1
V
1
V
IN
–
O
-------- -
V
⎛
⎜
⎝
V
IN
O
–
⎞
⎟
⎠
O
-------- -
V
.
V
×
IN
0.5
O
m
-------- -
V
V
⎞
⎟
⎠
IN
O
AOZ1025D
Page 8 of 15
O
IN
is half of
pin and
1