AIC1577 Datasheet, PDF (8/14 Pages) Analog Intergrations Corporation – External NMOS Step-Down PWM Controller

AIC1577
PIN DESCRIPTIONS
PIN 1: CS
- Current sense comparator invert-
ing input, not to exceed V_IN volt-
age. Built in offsets between the
CS and V_IN pins in conjunction
with R_SENSE set the current trip
thresholds.
PIN 2:
I_TH/RUN -Combination of error amplifier
compensation point and run con-
trol inputs. The current compara-
tor threshold increases with this
control voltage. Forcing this pin
below 0.8V causes the device to
be shutdown.
PIN 3: FB
- Feedback error amplifier input, to
compare the feedback voltage
with the internal reference voltage.
Connecting a resistor R2 to con-
verter output node and a resistor
R1 to ground yields the output
voltage:
V_OUT=0.8 x (R1+R2)/ R1
PIN4: GND - Singal GND for IC. All voltage lev-
els are measured with respect to
this pin.
PIN 5: SW
- Switch node connection to induc-
tor. In buck converter applications
the voltage swing at this pin is
from a schottky diode voltage
drop below ground to V_IN
PIN 6: DRI
- External high-side N-MOSFET
gate drive pin. Connect DRI to
gate of the external high-side N-
MOSFET.
PIN 7: BOOST- Supply to high-side floating
driver. The bootstrap capacitor C3
is returned to this pin.
PIN 8: VIN
- The chip power supply pin. It also
provides the gate bias charge for
all the MOSFETs controlled by
the IC. Recommend supply volt-
age is 4.5V~24V.
APPLICATION INFORMATION
Introduction
AIC1577 is a current mode switching regulator con-
troller that drives external N-channel power
MOSFET with constant frequency architecture. It
uses external divider to adjust output voltage with
excellent line regulation and load regulation. A
maximum high duty cycle limit of 95% provides low
dropout operation, which extends operating time in
battery-operated system.
Wide input voltage ranges from 4.5V to 24V, and
switching frequency (500KHz) allows smaller sized
filter components. The operating current level is
user-programmable via an external current sense
resistor and it automatically enters PFM operation
at low output current to boost circuit efficiency.
A multifunction pin (I_TH/RUN) allows external
compensation plus shutdown. A built-in soft start
can properly provide sequence supplies. Available
packages are in SOP8 and MSOP8 for SMD.
Principle of Operation
AIC1577 uses a current mode with a constant fre-
quency architecture. Normally high-side MOSFET
turns on each cycle when oscillator sets RS latch
and it turns off when internal current comparator re-
sets RS latch. Voltage on I_TH/RUN pin, which is the
output voltage of voltage error amplifier, will control
peak inductor current. The output voltage feeds
back to V_FB pin so that the error amplifier receives
a voltage through external resistor divider. When
load current increases, it causes a slight decrease
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