UVLO Protection

Thermal Shutdown

External Parts Selection

1. Determine output voltage

(

)

OUT

V

2.0

R1

R2

2.0

-

=

´

(5)

2. Choose the Inductor

(

)

(

)

(

)

(

)

(

)

(

)

(

)

(

)

f

f

OUT

OUT

OUT

OUT

IN max

IN max

IND ripple

IN max

OUT max

IN max

1

3

I

I

V

V

V

V

V

V

L

V

V

´

´

´

-

´

-

´

=

=

´

(6)

(

)

( )

(

)

(

)

(

)

f

OUT

OUT

IN max

TRIP

IND peak

DS on

IN max

V

1

R

L

V

V

V

I

V

´

-

´

=

+

´

(7)

3. Choose the Output Capacitor(s)

(

) (

)

( )

(

)

( )

f

OUT

RIPPLE

V

20 mV

1 D

20 mV

L

ESR

2 V

I

2 V

´

´

-

´ ´

=

-

´

(8)

TPS51123

TPS51123 has VREG5 under voltage lock out protection (UVLO). When the VREG5 voltage is lower than UVLO

threshold voltage both switch mode power supplies are shut off. This is non-latch protection. When the VREG3

voltage is lower than (VO2 - 1 V), both switch mode power supplies are also shut off

TPS51123 monitors the temperature of itself. If the temperature exceeds the threshold value (typically 150°C),

TPS51123 is shut off including LDOs. This is non-latch protection.

The external components selection is much simple in D-CAP™ Mode.

The output voltage is programmed by the voltage-divider resistor, R1 and R2, as shown in Figure 36. R1 is

connected between VFBx pin and the output, and R2 is connected betwen the VFBx pin and GND.

Recommended R2 value is from 10 k

Ω to 20 kΩ. Determine R1 using equation as below.

The inductance value should be determined to give the ripple current of approximately 1/4 to 1/2 of maximum

output current. Larger ripple current increases output ripple voltage and improves S/N ratio and helps stable

operation.

The inductor also needs to have low DCR to achieve good efficiency, as well as enough room above peak

inductor current before saturation. The peak inductor current can be estimated as follows.

Organic semiconductor capacitor(s) or specialty polymer capacitor(s) are recommended. Determine ESR to meet

required ripple voltage. A quick approximation is as shown in Equation 8. This equation is based on that required

where

• D is the duty cycle

Copyright © 2008, Texas Instruments Incorporated

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