aoz1024d Alpha & Omega Semiconductor, aoz1024d Datasheet - Page 8

• Manufacturer Part Number: aoz1024d
• Description: 4a Synchronous Buck Regulator
• Manufacturer: Alpha & Omega Semiconductor

Table 1.
The combination of R
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 AOZ1024D has multiple protection features to pre-
vent system circuit damage under abnormal conditions.
Over Current Protection (OCP)
The sensed inductor current signal is also used for over
current protection. Since the AOZ1024D 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 slowly during
a switching cycle because of V
strophic failure, a secondary current limit is designed
inside the AOZ1024D. The measured inductor current is
compared against a preset voltage which represents the
current limit, between 5.0A and 6.0A. 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 will be shut down.
Rev. 1.1 November 2007
V
O
0.8
1.2
1.5
1.8
2.5
3.3
5.0
(V)
1
and R
R
1
4.99
12.7
21.5
31.6
52.3
1.0
10
(k )
2
should be large enough to
O
= 0V. To prevent cata-
R
Open
2
11.5
10.2
10
10
10
10
(k )
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
150°C. The regulator will restart automatically under the
control of soft-start circuit when the junction temperature
decreases to 100ºC.
Application Information
The basic AOZ1024 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 AOZ1024D 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 relation between the input capacitor RMS current
and voltage conversion ratio is calculated and shown in
Figure 2 on the next page. It can be seen that when V
half of V
worst current stress on C
--------- -
V
I
V
CIN _RMS
V
IN
O
IN
=
=
IN
m
, C
------------------ -
f
=
IN
I
O
C
I
is under the worst current stress. The
O
IN
--------- - 1
V
1
V
IN
–
O
IN
--------- -
V
V
is 0.5 x I
IN
O
–
--------- -
V
V
IN
O
--------- -
V
V
O
IN
O
.
AOZ1024D
Page 8 of 16
IN
pin and
O
is