AS1335 データシートの表示（PDF） - austriamicrosystems AG

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AS1335

1.5A, 1.5MHz, Synchronous DC/DC Step-Down Converter

austriamicrosystems AG 

AS1335

Datasheet - Application Information

1 – V----O----U----T-

ΔVOUT

=

VOUT

×

------------V----I--N----

L×f

×

⎛

⎝

---------------1---------------

8 × COUT × f

+

E

SR⎠⎞

(EQ 5)

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 AS1335 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

lid 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.

a Input Capacitor Selection

v 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

ill input capacitor sized for the maximum RMS current must be used. The maximum RMS capacitor current is given as:

G st IRMS

=

IMAX

×

-----V----O---U----T----×-----(--V----I--N-----–----V----O----U----T---)

VIN

(EQ 6)

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

A t IMAX = ILIM - ΔIL/2

This formula has a maximum at VIN = 2VOUT where IRMS = IOUT/2. This simple worst-case condition is commonly used

s n for design because even significant deviations only provide negligible affects.

e The input capacitor can be increased without any limit for better input voltage filtering. Take care when using small

m t 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

a n 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.

o Ceramic Input and Output Capacitors

c 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

l dielectric capacitors, aside from their wide variation in capacitance over temperature, become resistive at high frequen-

cies and therefore should not be used.

a Table 4. Recommended External Components

ic Name

n COUT

CIN, COUT

ch L

Part Number

T520B107M006ATE040

GRM21BR60J226ME39

MOS6020-222ML

MOS6020-472ML

Value Rating

100µF 6.3V

22µF

2.2µH

4.7µH

6.3V

3.26A

1.82A

Type

Tantal

X5R

35mΩ

50mΩ

Size

Manufacturer

B

Kemet

(3.5x2.8x1.9mm) www.kemet.com

0805

Murata

www.murata.com

6.8x6.0x2.4mm Coilcraft

6.8x6.0x2.4mm www.coilcraft.com

e Because ceramic capacitors lose a lot of their initial capacitance at their maximum rated voltage, it is recommended

Tthat either a higher input capacity or a capacitance with a higher rated voltage is used.

www.austriamicrosystems.com/DC-DC_Step-Down/AS1335

Revision 1.03

13 - 18

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