(

)

=

æ

ö

ç

÷

ç

÷

ç

÷ -

´

æ

ö

ç

÷

ç

÷

-

ç

÷

ç

÷

ç

÷

è

ø

è

ø

IND ripple

OUT

R1

R2

2.0

1

I

ESR

V

2

(

)

(

)

(

)

(

)

(

)

(

)

(

)

(

)

-

´

-

´

=

´

=

´

´

´

IN

OUT

OUT

IN

OUT

OUT

max

max

X

SW

IN

O

SW

IN

IND ripple

max

max

max

V

V

V

V

V

V

1

3

L

I

f

V

I

f

V

(

)

(

)

(

)

(

)

(

)

-

´

=

+

´

´

´

IN

OUT

OUT

max

TRIP

IND peak

X

SW

IN

DS on

max

V

V

V

V

1

I

8

R

L

f

V

(

)

(

)

(

)

æ

ö

æ

ö

-

ç

÷

´

-

´

´

ç

÷

ç

÷

ç

÷

´

´

è

ø

è

ø

=

IN

OUT

OUT

OCL

DS(on)

X

SW

IN

TRIP

TRIP

V

V

V

8

I

R

2

L

f

V

R

I

=

æ

ö

-

-

´

ç

÷

´

´

è

ø

EXT

IN

OUT

OUT

OCL

X

SW

IN

25 mV

R

V

V

V

I

2

L

f

V

´

=

X

X

EXT

ESL

C

R

R

TPS51219

www.ti.com

SLUSAG1B

– MARCH 2011 – REVISED OCTOBER 2011

External Components Selection

The external components selection is simple in D-CAP

™ mode.

1. DETERMINE THE VALUE OF R1 AND R2

The output voltage is determined by the value of the voltage-divider resistor, R1 and R2 as shown in Figure 25.

R1 is connected between VREF and REFIN pins, and R2 is connected between the REFIN pin and GSNS.

Setting R1 as 10-k

Ω is a good starting point. Determine R2 using Equation 10.

(10)

2. CHOOSE THE INDUCTOR

The inductance value should be determined to yield a ripple current of approximately

¼ to ½ of maximum output

current. Larger ripple current increases output ripple voltage and improves the signal-to-noise ratio and helps

stable operation.

(11)

The inductor needs a low direct current resistance (DCR) to achieve good efficiency, as well as enough room

above peak inductor current before saturation. The peak inductor current can be estimated in Equation 12.

(12)

3. CHOOSE THE OCL SETTING RESISTANCE

RTRIP for RDS(on) Sensing

Combining Equation 7 and Equation 8, RTRIP can be obtained using Equation 13.

(13)

REXT for Resistor Setting

Combining Equation 7 and Equation 9, REXT can be obtained using Equation 14.

(14)

For more accurate current sensing with an external resistor, the following technique is recommended. Adding an

RC filter to cancel the parasitic inductance (ESL) of resistor, this filter value is calculated using Equation 15.

(15)

The time-constant of CX and RX should match the one of ESL and REXT. Even when CX is not used, an RX of

100

Ω is recommended for noise suppression.

Copyright

© 2011, Texas Instruments Incorporated

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