RT8249 datasheet(21/23 Pages) RICHTEK

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RT8249A/B/C

DS8249A/B/C-02

June 2014

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

Figure 6. RT8249B Timing

Output Inductor Selection

The switching frequency (on-time) and operating point

(% ripple or LIR) determine the inductor value as shown

below :

OUTx

LOAD(MAX)

t(V

)

LIR I







where LIR is the ratio of the peak-to-peak ripple current to

the average inductor current.

Find a low-loss inductor having the lowest possible DC

resistance that fits in the allotted dimensions. Ferrite cores

are often the best choice, although powdered iron is

inexpensive and can work well at 200kHz. The core must

be large enough not to saturate at the peak inductor

current, IPEAK :

IPEAK = ILOAD(MAX) + [ (LIR / 2) x ILOAD(MAX) ]

The calculation above shall serve as a general reference.

To further improve transient response, the output

inductance can be further reduced. Of course, besides

the inductor, the output capacitor should also be

considered when improving transient response.

Output Voltage Setting (FBx)

Connect a resistive voltage divider at the FBx pin between

VOUTx and GND to adjust the output voltage between 2V

and 5.5V (Figure 7). The recommended R2 is between

100k

Ω to 200kΩ, VOUT (vally) and solve for R1 using the

equation below :

OUT(Valley)

FBx

1 +









where VFBx is 2V (typ.).

Figure 7. Setting VOUTx with a resistive voltage divider

UGATEx

PHASEx

LGATEx

FBx

GND

VOUTx

VIN

LDO3

EN1

LDO5

5V VOUT

EN2

3.3V VOUT

PGOOD

VIN POR threshold

EN threshold

PGOOD

Delay

2.5V

Soft-Start Time

Start-Up Time

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