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AOZ1210AI

P1-P3P4-P6P7-P9P10-P12P13-P14
AOZ1210
Rev. 1.7 December 2009 www.aosmd.com Page 7 of 14
Detailed Description
The AOZ1210 is a current-mode step down regulator
with integrated high side NMOS switch. It operates from
a 4.5V to 27V input voltage range and supplies up to 2A
of load current. The duty cycle can be adjusted from 6%
to 85% allowing a wide range of output voltages. Fea-
tures include; enable control, Power-On Reset, input
under voltage lockout, fixed internal soft-start and ther-
mal shut down.
The AOZ1210 is available in SO-8 package.
Enable and Soft Start
The AOZ1210 has an internal soft start feature to limit
in-rush current and ensure the output voltage ramps up
smoothly to the regulation voltage. A soft start process
begins when the input voltage rises to 4.1V and voltage
on EN pin is HIGH. In the soft start process, the output
voltage is typically ramped to regulation voltage in 6.8ms.
The 6.8ms soft start time is set internally.
If the enable function is not used, connect the EN pin to
V
IN
. Pulling EN to ground will disable the AOZ1210. Do
not leave EN open. The voltage on the EN pin must be
above 2.5 V to enable the AOZ1210. When voltage on
EN pin falls below 0.6V, the AOZ1210 is disabled. If an
application circuit requires the AOZ1210 to be disabled,
an open drain or open collector circuit should be used to
interface with the EN pin.
Steady-State Operation
Under steady-state conditions, the converter operates in
fixed frequency and Continuous-Conduction Mode
(CCM).
The AOZ1210 integrates an internal N-MOSFET as the
high-side switch. Inductor current is sensed by amplifying
the voltage drop across the drain to source of the high
side power MOSFET. Since the N-MOSFET requires a
gate voltage higher than the input voltage, a boost
capacitor connected between the LX and BST pins drives
the gate. The boost capacitor is charged while LX is low.
An internal 10
Ω switch from LX to GND is used to ensure
that LX is pulled to GND even in the light load. Output
voltage is divided down by the external voltage divider at
the FB pin. The difference of the FB pin voltage and
reference is amplified by the internal transconductance
error amplifier. The error voltage, which shows on the
COMP pin, is compared against the current signal. The
current signal is the sum of inductor current signal and
ramp compensation signal, at the PWM comparator
input. If the current signal is less than the error voltage,
the internal high-side switch is on. The inductor current
flows from the input through the inductor to the output.
When the current signal exceeds the error voltage, the
high-side switch is off. The inductor current is freewheel-
ing through the Schottky diode to the output.
Switching Frequency
The AOZ1210 switching frequency is fixed and set by
an internal oscillator. The switching frequency is set to
370kHz.
Output Voltage Programming
Output voltage can be set by feeding back the output to
the FB pin with a resistor divider network. In the applica-
tion circuit shown in Figure 1. The resistor divider
network includes R
1
and R
2
. Typically, a design is started
by picking a fixed R
2
value and calculating the required
R
1
value with equation below.
Some standard values for R
1
and R
2
for the most
commonly used output voltages are listed in Table 1.
Table 1.
The combination of R
1
and R
2
should be large enough to
avoid drawing excessive current from the output, which
will cause power loss.
Protection Features
The AOZ1210 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 AOZ1210 employs peak
current mode control, the COMP pin voltage is propor-
tional to the peak inductor current. The COMP pin volt-
age is limited to be between 0.4V and 2.5V internally.
The peak inductor current is automatically limited cycle
by cycle.
V
O
(V) R
1
(kΩ) R
2
(kΩ)
0.8 1.0 Open
1.2 4.99 10
1.5 10 11.5
1.8 12.7 10.2
2.5 21.5 10
3.3 31.6 10
5.0 52.3 10
V
O
0.8 1
R
1
R
2
-------
+
⎝⎠
⎜⎟
⎛⎞
×=
AOZ1210
Rev. 1.7 December 2009 www.aosmd.com Page 8 of 14
The cycle-by-cycle current limit threshold is internally set.
When the load current reaches the current limit thresh-
old, the cycle-by-cycle current limit circuit turns off the
high side switch immediately to terminate the current
duty cycle. The inductor current stops 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 the peak inductor current. When
cycle-by-cycle current limit circuit is triggered, the output
voltage drops as the duty cycle decreases.
The AOZ1210 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 the FB pin voltage is below
0.2V, the short circuit protection circuit is triggered. To
prevent current limit running away when the comp pin
voltage is higher than 2.1V, the short circuit protection is
also triggered. As a result, the converter is shut down
and hiccups at a frequency equals to 1/16 of normal
switching frequency. The converter will start up via a soft
start once the short circuit condition is resolved. 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 4.3V, the converter
starts operation. When input voltage falls below 4.1V,
the converter will stop switching.
Thermal Protection
An internal temperature sensor monitors the junction
temperature. It shuts down the internal control circuit and
high side NMOS 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 AOZ1210 application circuit is shown in
Figure 1. Component selection is explained below.
Input Capacitor
The input capacitor (C
1
in Figure 1) must be connected to
the V
IN
pin and GND pin of the AOZ1210 to maintain
steady input voltage and filter out the pulsing input
current. The voltage rating of the input capacitor must be
greater than maximum input voltage + ripple voltage.
The input ripple voltage can be approximated by equa-
tion 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. It can be seen that when V
O
is half of
V
IN
, C
IN
is under the worst current stress. The worst
current stress on CIN is 0.5 x I
O
.
Figure 2. I
CIN
vs. Voltage Conversion Ratio
For reliable operation and best performance, the input
capacitors must have a current rating higher than
I
CIN_RMS
at the worst operating conditions. Ceramic
capacitors are preferred for input capacitors because of
their low ESR and high ripple current rating. Depending
on the application circuits, other low ESR tantalum
capacitor or aluminum electrolytic capacitor may also be
used. When selecting ceramic capacitors, X5R or X7R
type dielectric ceramic capacitors are preferred for their
better temperature and voltage characteristics. Note that
the ripple current rating from capacitor manufactures is
based on certain amount of life time. Further de-rating
may be necessary for practical design requirement.
Inductor
The inductor is used to supply constant current to output
when it is driven by a switching voltage. For given input
and output voltage, inductance and switching frequency
together decide the inductor ripple current, which is,
ΔV
IN
I
O
fC
IN
×
-----------------
1
V
O
V
IN
---------
–
⎝⎠
⎜⎟
⎛⎞
V
O
V
IN
---------
××=
I
CIN_RMS
I
O
V
O
V
IN
---------
1
V
O
V
IN
---------
–
⎝⎠
⎜⎟
⎛⎞
×=
V
O
V
IN
---------
m=
0
0.1
0.2
0.3
0.4
0.5
0 0.5 1
m
I
CIN_RMS
(m)
I
O
ΔI
L
V
O
fL×
-----------
1
V
O
V
IN
---------
–
⎝⎠
⎜⎟
⎛⎞
×=
AOZ1210
Rev. 1.7 December 2009 www.aosmd.com Page 9 of 14
The peak inductor current is:
High inductance gives low inductor ripple current but
requires larger size inductor to avoid saturation. Low
ripple current reduces inductor core losses. It also
reduces RMS current through inductor and switches,
which results in less conduction loss.
When selecting the inductor, make sure it is able to
handle the peak current without saturation even at the
highest operating temperature.
The inductor takes the highest current in a buck circuit.
The conduction loss on inductor needs to be checked for
thermal and efficiency requirements.
Surface mount inductors in different shape and styles are
available from Coilcraft, Elytone and Murata. Shielded
inductors are small and radiate less EMI noise. But they
cost more than unshielded inductors. The choice
depends on EMI requirement, price and size.
Output Capacitor
The output capacitor is selected based on the DC output
voltage rating, output ripple voltage specification and
ripple current rating.
The selected output capacitor must have a higher rated
voltage specification than the maximum desired output
voltage including ripple. De-rating needs to be consid-
ered for long term reliability.
Output ripple voltage specification is another important
factor for selecting the output capacitor. In a buck con-
verter circuit, output ripple voltage is determined by
inductor value, switching frequency, output capacitor
value and ESR. It can be calculated by the equation
below:
where;
C
O
is output capacitor value and
ESR
CO
is the Equivalent Series Resistor of output capacitor.
When low ESR ceramic capacitor is used as output
capacitor, the impedance of the capacitor at the switch-
ing frequency dominates. Output ripple is mainly caused
by capacitor value and inductor ripple current. The output
ripple voltage calculation can be simplified to:
If the impedance of ESR at switching frequency
dominates, the output ripple voltage is mainly decided by
capacitor ESR and inductor ripple current. The output
ripple voltage calculation can be further simplified to:
For lower output ripple voltage across the entire operat-
ing temperature range, X5R or X7R dielectric type of
ceramic, or other low ESR tantalum capacitor or
aluminum electrolytic capacitor may also be used as out-
put capacitors.
In a buck converter, output capacitor current is continu-
ous. The RMS current of output capacitor is decided by
the peak to peak inductor ripple current. It can be calcu-
lated by:
Usually, the ripple current rating of the output capacitor is
a smaller issue because of the low current stress. When
the buck inductor is selected to be very small and induc-
tor ripple current is high, output capacitor could be over-
stressed.
Schottky Diode Selection
The external freewheeling diode supplies the current to
the inductor when the high side NMOS switch is off. To
reduce the losses due to the forward voltage drop and
recovery of diode, a Schottky diode is recommended.
The maximum reverse voltage rating of the chosen
Schottky diode should be greater than the maximum
input voltage, and the current rating should be greater
than the maximum load current.
Loop Compensation
The AOZ1210 employs peak current mode control for
easy use and fast transient response. Peak current mode
control eliminates the double pole effect of the output
L&C filter. It greatly simplifies the compensation loop
design.
With peak current mode control, the buck power stage
can be simplified to be a one-pole and one-zero system
in frequency domain. The pole is the dominant pole and
can be calculated by:
I
Lpeak
I
O
ΔI
L
2
--------
+=
ΔV
O
ΔI
L
ESR
CO
1
8 fC
O
××
-------------------------
+
⎝⎠
⎛⎞
×=
ΔV
O
ΔI
L
1
8 fC
O
××
-------------------------
⎝⎠
⎛⎞
×=
ΔV
O
ΔI
L
ESR
CO
×=
I
CO_RMS
ΔI
L
12
----------
=
f
p1
1
2π C
O
R
L
××
-----------------------------------
=
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