AIC2302 Datasheet, PDF (10/11 Pages) Analog Intergrations Corporation – 700mA Synchronous PWM Step-Down

AIC2302
frequency decreases gradually to smoothly transit to
100% duty cycle operation.
If input voltage is very close to output voltage, the
switching mode goes from pure PWM mode to 100%
duty cycle operation. During this transient state
mentioned above, large output ripple voltage may
appear on output terminal.
Components Selection
Inductor
The inductor selection depends on the current ripple of
inductor, the input voltage and the output voltage.
L

V_OUT
f_OSC  I_L
^1
V_OUT
V_IN

Accepting a large current ripple of inductor allows the
use of a smaller inductance. However, higher current
ripple of inductor can cause higher output ripple
voltage and large core loss. By setting an acceptable
current ripple of inductor, a suitable inductance can be
obtained from above equation.
In addition, it is important to ensure the inductor
saturation current exceeds the peak value of inductor
current in application to prevent core saturation. The
peak value of inductor current can be calculated
according to the following equation.
I_PEAK
 I_{OUTmax } 
V_OUT
2  f_OSC
 L ^{1}
V_OUT
V_IN

Input Capacitor and Output Capacitor
To prevent the high input voltage ripple and noise
resulted from high frequency switching, the use of low
ESR ceramic capacitor for the maximum RMS current
is recommended. The approximated RMS current of
the input capacitor can be calculated according to the
following equation.
  _{ICINRMS }
I_{O2 UT(MAX)} 
V_OUT
V_IN  V_OUT
V_I2N
 ^IL2
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The selection of output capacitor depends on the
required output voltage ripple. The output voltage
ripple can be expressed as:
V_OUT

I_L
8  f_OSC  C_OUT
 ESR  I_L
For lower output voltage ripple, the use of low ESR
ceramic capacitor is recommended. The tantalum
capacitor can also be used well, but its ERS is larger
than that of ceramic capacitor.
When choosing the input and output ceramic
capacitors, X5R and X7R types are recommended
because they retain their capacitance over wider
ranges of voltage and temperature than other types.
Output Voltage Programming (AIC2302 Adjustable
Version Only)
By connecting a resistive divider R₁ and R_2, the output
voltage of AIC2302 step-down converter can be set.
V_OUT can be calculated as:
V_OUT

0.6  _{}1
R₁
R₂

The resistive divider should sit as close to VFB pin as
possible.
Layout Consideration
In order to ensure a proper operation of AIC2302, the
following points should be managed comprehensively.
1. The input capacitor and V_IN should be placed as
close as possible to each other to reduce the input
voltage ripple and noise.
2. The output loop, which is consisted of the inductor,
the internal main power switch, the internal syn-
chronous power switch and the 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 should
flow at the same direction.
5. The VFB pin should be connected to the feedback
resistors directly and the route should be away
from the noise sources.
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