aoz1915 Alpha & Omega Semiconductor, aoz1915 Datasheet - Page 7

• Manufacturer Part Number: aoz1915
• Description: 1.5a General Purpose Boost Regulator
• Manufacturer: Alpha & Omega Semiconductor

Detailed Description
The AOZ1915 is a current-mode step up regulator
(Boost Converter) with integrated NMOS switch. It
operates from a 2.7V to 5.5V input voltage range and
supplies up to 22V output voltage. The duty cycle can be
adjusted to obtain a wide range of output voltage up to
22V. Features include enable control, cycle by cycle
current limit, input under voltage lockout, adjustable
soft-start and thermal shut down.
The AOZ1915 is available in DFN 4x3 package
Enable and Soft Start
The AOZ1915 has the adjustable soft start feature to limit
in-rush current and ensure the output voltage ramps up
smoothly to regulation voltage. A soft start process
begins when the input voltage rises to 2.7V and voltage
on EN pin is HIGH. In soft start process, a 10µA internal
current source charges the external capacitor at SS.
As the SS capacitor is charged, the voltage at SS rises.
The SS voltage clamps the reference voltage of the error
amplifier, therefore output voltage rising time follows the
SS pin voltage. With the slow ramping up output voltage,
the inrush current can be prevented.
The EN pin of the AOZ1915 is active high. Connect the
EN pin to VIN if enable function is not used. Pull it to
ground will disable the AOZ1915. Do not leave it open.
The voltage on EN pin must be above 1.5 V to enable the
AOZ1915. When voltage on EN pin falls below 0.4V, the
AOZ1915 is disabled. If an application circuit requires the
AOZ1915 to be disabled, an open drain or open collector
circuit should be used to interface to EN pin.
Steady-State Operation
Under steady-state conditions, the converter operates in
fixed frequency.
The AOZ1915 integrates an internal N-MOSFET as the
control switch. Inductor current is sensed by amplifying
the voltage drop across the drain to source of the control
power MOSFET. 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, which is sum of inductor
current signal and ramp compensation signal, at PWM
comparator input. If the current signal is less than the
error voltage, the internal NMOS switch is on. The
inductor current ramps up. When the current signal
exceeds the error voltage, the switch is off. The inductor
current is freewheeling through the internal Schottky
diode to output.
Rev. 1.1 July 2009
www.aosmd.com
Switching Frequency
The AOZ1915 switching frequency is fixed and set by an
internal oscillator and FSEL. When the voltage of FSEL
is high (connected to Vin) The switching frequency is
1.2MHz; when the voltage of FSEL is low (connected to
GND), the switching frequency is 600 KHz.
Output Voltage Programming
Output voltage can be set by feeding back the output to
the FB pin with a resistor divider network. In the
application circuit shown in Figure 1. The resistor divider
network includes R
by picking a fixed R
R
Some standard value of R
output voltage values are listed in Table 1.
Table 1.
The combination of R
avoid drawing excessive current from the output, which
will cause power loss.
Protection Features
The AOZ1915 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 AOZ1915 employs peak
current mode control, the COMP pin voltage is
proportional to the peak inductor current. The peak
inductor current is automatically limited cycle by cycle.
When the current of control NMOS reaches the current
limit threshold, the cycle by cycle current limit circuit turns
off the NMOS 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
V
2
O
with equation below:
=
V
1.2
O
12
16
18
25
8
(V)
×
⎛
⎜
⎝
1
+
R
------ -
R
1
1
and R
2
1
value and calculating the required
1
⎞
⎟
⎠
and R
R
1
2
2
170
270
370
420
595
, R
. Usually, a design is started
(k
2
Ω
2
should be large enough to
for most commonly used
)
AOZ1915
R
Page 7 of 14
1
30
30
30
30
30
(k
Ω
)