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AS1324

Datasheet - A p p l i c a t i o n I n f o r m a t i o n

9.4 Input Capacitor Selection

In continuous mode, the source current of the PMOS is a square wave of the duty cycle VOUT/VIN. To prevent large voltage 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:

where the maximum average output current IMAX equals the peak current minus half the peak-to-peak ripple current, IMAX = ILIM -

∆IL/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.

9.4.1

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 frequencies and therefore should not be used.

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.

9.5 Feedback Resistor Selection

In the AS1324-AD, the output voltage is set by an external resistor divider connected to VFB (see Figure 27). This circuitry allows for remote

voltage sensing and adjustment.

Figure 27. Setting the AS1324 Output Voltage

Resistor values for the circuit shown in Figure 27 can be calculated as:

The output voltage can be adjusted by selecting different values for R1 and R2. For R1 a value between 10k

Ω and 500kΩ is recommended. A

higher resistance of R1 and R2 will result in a lower leakage current at the output. It is recommended to keep VIN 500mV higher than VOUT.

Table 6. Recommended Input and Output Capacitor

Part Number

TC Code

Rated Voltage

Dimensions (L/W/T)

Manufacturer

JMK212BJ226MG-T

22µF

X5R

6.3V

0805

Taiyo Yuden

www.t-yuden.com

GRM188R60J106ME47

10µF

X5R

6.3V

0603

Murata

www.murata.com

GRM21BR71A475KA73

4.7µF

X7R

10V

0805

(EQ 6)

RMS

MAX

OUT

–

(

)

------------------------------------------------------------

0.6V

≤ VOUT ≤ 5.5V

AS1324

VFB

GND

R1<<R2

(EQ 7)

OUT

0,6

-------

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