AIC2822 Datasheet, PDF (7/9 Pages) Analog Intergrations Corporation – 2A 23V Synchronous PWM Step-Down Converter

AIC2822
 ^{APPLICATION INFORMATIONS}
The AIC2822 is a synchronous high voltage buck
converter that can support the input voltage range from
4.75V to 23V and the output current can be up to 2A.
Setting the Output Voltage
The output voltage is set using a resistive voltage
divider connected from the output voltage to FB. The
voltage divider divides the output voltage down to the
feedback voltage by the ratio:
R2
V_FB = V_{OUT R1 + R2}
Thus the output voltage is:
R1 + R2
V_OUT = 0.925 × _R2
For example, for a 3.3V output voltage, R2 is 10kΩ,
and R1 is 26.1kΩ.
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

Soft-Start
The AIC2822 provides the soft-start function. Initially,
the voltage at SS pin is 0V. Then an internal current
source of 6A (typ.) charges an external soft-start ca-
pacitor. During the soft-start period, the voltage at SS
pin will limit the feedback threshold voltage at FB pin.
When the voltage at SS pin is higher than 0.925V, the
feedback threshold voltage at FB pin reaches the de-
sired value. The soft-start time can be calculated in
accordance with the following equation.
0.925V
t_SS = C5 × _6μA
The soft-start capacitor is discharged to GND when the
EN pin is connected to GND.
Optional Schottky Diode
A Schottky diode with low forward drop voltage and
fast reverse recovery is the ideal choice for better
efficiency. The forward drop voltage of a Schottky
diode will result in the conduction losses in the diode,
and the diode capacitance (C_T or C_D) will cause the
switching losses. Therefore, it is necessary to consider
both forward voltage drop and diode capacitance for
diode selection. In addition, the rating of selected
Schottky diode should be able to handle the input
voltage and the maximum peak diode current.
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)}  ^VOUT
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:
7