AIC1555 데이터 시트보기 (PDF) - Analog Intergrations
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AIC1555 Datasheet PDF : 15 Pages
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AIC1555
Since the diode conducts during the off time, the
peak current and voltage of converter is not
allowed to exceed the diode ratings. The ratings of
diode can be calculated by the following formulas:
VD,MAX(OFF) = VIN
(5)
ID,MAX(ON)
=
IOUT,MAX
+
∆IL
2
(6)
ID,AVG(ON) = IOUT − IIN = IOUT − D × IOUT
= (1− D) × IOUT
(7)
Adjustable Output Voltage
AIC1555 appears a 0.75V reference voltage at FB
pin. Output voltage, ranging from 0.75V to VIN,
can be set by connecting two external resistors,
R1 and R2. VOUT can be calculated as:
VOUT
=
0.75V × (1+
R1 )
R2
(8)
Applying a 12pF capacitor parallel with R1 can
prevent stray pickup. They should sit as close to
AIC1555 as possible. But load transient response
is degraded by this capacitor.
Over Current Protection
The current limiter circuit monitors the current
flowing through the P-channel MOS connected to
the Lx pin, and features a constant-current type
current limit mode. If the inductor current does not
exceed the current limit, the high-side MOSFET
turns on normally. When the driver current is
greater than a specific level, the constant-current
type current limit function operates to turn off the
pulses from the Lx pin at any switching cycle. This
features a constant-current type over current
protection
Layout Consideration
To ensure a proper operation of AIC1555, the
following points should be given attention to:
1. Input capacitor and Vin should be placed as
close as possible to each other to reduce the
input ripple voltage.
2. The output loop, which is consisted of
inductor, Schottky diode and output capacitor,
should be kept as small as possible.
3. The routes with large current should be kept
short and wide.
4. Logically the large current on the converter,
when AIC1555 is on or off, should flow at the
same direction.
5. The FB pin should connect to feedback
resistors directly. And the route should be
away from the noise source, such as inductor
of LX line.
6. Grounding all components at the same point
may effectively reduce the occurrence of loop.
A stability ground plane is very important for
gaining higher efficiency. When a ground
plane is cut apart, it may cause disturbed
signal and noise. If possible, two or three
through-holes can ensure the stability of
grounding.
Example
Here are two examples to prove the components
selector guide above.
1. Tantalum capacitors application:
Assume AIC1555 is used for mobile phone
application, which uses 1-cell Li-ion battery with
2.7V to 4.2V input voltage for power source. The
required load current is 700mA, and the output
voltage is 1.8V. Substituting VOUT=1.8V, VIN=4.2V,
∆I=250mA, and f=500KHz to equation (1)
L=
1.8V
1 − 1.8V = 8.23µH
500KHz × 250mA 4.2V
Therefore, 10µH is proper for the inductor. And the
inductor of series number SLF6025-100M1R0
from TDK with 57.3mΩ series resistor is
recommended for the best efficiency.
12
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