AIC1577 AIC [Analog Intergrations Corporation], AIC1577 Datasheet - Page 9

• Manufacturer Part Number: AIC1577
• Description: External NMOS Step-Down PWM Controller
• Manufacturer: AIC [Analog Intergrations Corporation]

in the voltage of V
remains increasing until the average inductor cur-
rent matches new load current. While the high-side
MOSFET turns off, the low-side MOSFET is turned
on to recharge bootstrap capacitor C3.
Main control loop is shutdown when I
below 0.8V. When I
by error amplifier, main control loop is enabled.
Low Current Operation
During heavy load current operation, AIC1577 op-
erates in PWM mode with a frequency of 500KHz.
Decreasing of the current will cause a drop in
I
mode operation for better efficiency. If the voltage
across R
comparator within a cycle, then the high-side and
internal MOSFETs will disable until I
over 1.33V.
Component Selection
AIC1577 can be used in many switching regulator
applications, such as step-down, step-up, SEPIC
and positive-to-negative converters. Among all,
step-down converter is the most common applica-
tion. External component selection, beginning with
selecting R
application. Once R
ductor, which is followed by selecting power
MOSFET and diode, can be easily chosen. Finally,
C
R
The choice of R
required output current. The threshold voltage of
current comparator decides peak inductor current,
which yields a maximum average output current
(I
peak-to-peak ripple current, ΔI
Allowing a margin for variation of AIC1577, external
component can be yielded as:
TH
MAX
IN
S
/RUN below 1.33V so that AIC1577 enters PFM
Selection
and C
). And the peak current is less than half of the
S
OUT
S
does not exceed the offset of current
,
depends on load requirement of the
can be determined.
S
FB
has substantial connection with
TH
S
pin. Thus the I
/RUN pulled up to 0.8V or up
is decided, the choice of in-
L
.
TH
TH
TH
/RUN voltage
/RUN goes
/RUN goes
Inductor Selection
With the operating frequency high to 500KHz,
smaller inductor value is favored. In general,
operating in high frequency will cause low efficiency
because of large MOSFET switching loss. Thus the
effect of inductor value on ripple current and low
current operation must be considered as well.
The inductor value has a direct influence on ripple
current ( Δ I
tance and increases with high V
V
Accepting a large value ofΔI
inductance, but yields high output ripple voltage
and large core loss. The inductor value also has an
effect on low current operation. Low inductor value
causes the PFM operation to begin at high load cur-
rent. The efficiency of the circuit decreases at the
beginning of low current operation. Generally
speaking, low inductance in PFM mode will cause
the efficiency to decrease.
Power MOSFET Selection
For an application of AIC1577, an external N-
channel power MOSFET, used as the high-side
switch, must be properly selected. To prevent
MOSFET from damage during high input voltage
operation, attention should be taken to the BV
specification for MOSFET.
Other important selection criteria for the power
MOSFET include the “ON” resistance R
put voltage and maximum output current.
R
∆I
D
S
L
is the drop voltage of the output Schottky diode.
=
=
100mV
V
I
MAX
IN
f
−
L
×
), which decreases with high induc-
V
L
OUT




V
OUT
V
IN
+
+
L
V
allows the use of low
V
D
IN
D
or V




AIC1577
OUT
DS(ON)
:
, in-
DSS
9