AIC2359 Datasheet, PDF (10/11 Pages) Analog Intergrations Corporation – 1A Dual Synchronous Step-down DC/DC Converter

AIC2359
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
12
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 (AIC2359 Adjustable
Version Only)
By connecting a resistive divider R₁₁ and R₁₂ (R₂₁ and R₂₂)_,
the output voltage of AIC2359 step-down converter can
be set. V_OUT1 and V_OUT2 can be calculated as:
V_{OUT 1}

0.6  _{}1 
R₁₁
R₁₂

V_{OUT 2}

0.6  _{}1 
R₂₁
R₂₂

The resistive divider should sit as close to VFB pin as
possible.
Layout Consideration
In order to ensure a proper operation of AIC2359, the fol-
lowing 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 synchro-
nous 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 resis-
tors directly and the route should be away from the
noise sources.
10