AS1324
Data Sheet - Application Information
Higher value, low cost ceramic capacitors are available in very small case sizes, and their high ripple current, high volt-
age rating, and low ESR make them ideal for switching regulator applications. Because the AS1324 control loop is not
dependant on the output capacitor ESR for stable operation, ceramic capacitors can be used to achieve very low out-
put ripple and accommodate small circuit size.
At light loads, the converter operates in powersave mode and the output voltage ripple is in direct relation to the output
capacitor and inductor value used. Larger output capacitor and inductor values minimize the voltage ripple in power-
save mode and tighten DC output accuracy in powersave mode.
Input Capacitor Selection
In continuous mode, the source current of thePMOS is a square wave of the duty cycle VOUT/VIN. To prevent large volt-
age transients while minimizing the interference with other circuits caused by high input voltage spikes, a low ESR
input capacitor sized for the maximum RMS current must be used. The maximum RMS capacitor current is given as:
(EQ 6)
VOUT × (VIN – VOUT
-----------------------------------------------------------
×
)
IRMS = IMAX
VIN
where the maximum average output current IMAX equals the peak current minus half the peak-to-peak ripple current,
IMAX = ILIM - ΔIL/2
This formula has a maximum at VIN = 2VOUT where IRMS = IOUT/2. This simple worst-case condition is commonly used
for design because even significant deviations only provide negligible affects.
The input capacitor can be increased without any limit for better input voltage filtering. Take care when using small
ceramic input capacitors. When a small ceramic capacitor is used at the input, and the power is being supplied through
long wires, such as from a wall adapter, a load step at the output, or VIN step on the input, can induce ringing at the VIN
pin. This ringing can then couple to the output and be mistaken as loop instability, or could even damage the part by
exceeding the maximum ratings.
Ceramic Input and Output Capacitors
When choosing ceramic capacitors for CIN and COUT, the X5R or X7R dielectric formulations are recommended.
These dielectrics have the best temperature and voltage characteristics for a given value and size. Y5V and Z5U
dielectric capacitors, aside from their wide variation in capacitance over temperature, become resistive at high frequen-
cies and therefore should not be used.
Table 6. Recommended Input and Output Capacitor
Part Number
C
TC Code Rated Voltage Dimensions (L/W/T)
Manufacturer
Taiyo Yuden
www.t-yuden.com
JMK212BJ226MG-T
22µF
X5R
6.3V
0805
0603
0805
Murata
www.murata.com
GRM188R60J106ME47
GRM21BR71A475KA73
10µF
X5R
X7R
6.3V
10V
4.7µF
Because ceramic capacitors lose a lot of their initial capacitance at their maximum rated voltage, it is recommended
that either a higher input capacity or a capacitance with a higher rated voltage is used.
Feedback Resistor Selection
In the AS1324-AD, the output voltage is set by an external resistor divider connected to VFB (see Figure 27). This cir-
cuitry allows for remote voltage sensing and adjustment.
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