AIC1550
ꢀAPPLICATION INFORMATION
increase efficiency. When control logic block turns
Q2 on, Q3 will turn off through anti-short-through
block. Similarly, when Q3 is on, Q2 will turn off.
Introduction
AIC1550 is a low-noise, pulse-width-modulated
(PWM), DC-DC step-down converter. It features
an internal synchronous rectifier, which eliminates
external Schottky diode. AIC1550 is suitable for
Li-lon battery applications, or can be used at 3V
or 5V fixed input voltage. It operates in one of
following four modes.
AIC1550 provides current limit function by using a
5Ω resistor. When Q1 turns on, current follows
through the 5Ω resistor. And current amplifier
senses the voltage, which crosses the resistor,
and amplifies it. When the sensed voltage gets
bigger than reference voltage, control logic shuts
the device off.
(1) Forced PWM mode operates at a
fixed frequency regardless of the
load.
PWM/PFM Function
When connecting SYNC/MODE pin to VIN, the
device is forced into PWM (Pulse-Width-
Modulated) mode with constant frequency.
Advantage of constant frequency is easily
reducing noise without complex post-filter. But its
disadvantage is low efficiency at light loading.
Therefore, AIC1550 provides a function to solve
this problem. When connecting SYNC/MODE pin
to GND, device is able to get into PWM/PFM
(Pulse-Frequency-Modulated) modes. Under a
light loading condition, the device turns to PFM
mode, which results in a higher efficiency. PWM
mode is on when heavy loading applies and the
noise is reduced.
(2) Synchronizable PWM mode allows
the synchronization by using an
external switching frequency with a
minimum harmonics.
(3) PWM/PFM Mode extends battery
life by switching to a PFM pulse-
skipping mode under light loads.
(4) Shutdown mode sets device to
standby, reducing supply current to
0.1µA or under.
Continuous output current of AIC1550 can be
upward to 800mA and output voltage can be
adjusted from 0.75V to VIN with an input range
from 2.5V to 6.5V by a voltage divider. AIC1550
also features high efficiency, low dropout voltage,
and a 0.75V reference with ±2% accuracy. It is
available in a space-saving 8-pin MSOP package.
Frequency Synchronization
Connecting an external clock signal to
SNYC/MODE pin can control switching frequency.
The acceptable range is from 500kHz to 1MHz.
This mode exhibits low output ripple as well as
low audio noise and reduces RF interference
while providing reasonable low current efficiency.
Operation
When power on, control logic block detects
SYNC/MODE pin connecting to VIN or GND to
determine operation function and gives a signal to
PWM/PFM control block to determine the proper
comparator (ref. Block Diagram). AIC1550 works
with an internal synchronous rectifier － Q3, to
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AIC [ ANALOG INTERGRATIONS CORPORATION ]
