N

out

swpeak

V

D

I

I

=

+

2

s

L

1

D

´

´ ¦ ´

-

I

(

)

IN

V

D

Iout =

1

D

Isw

2

s

L

æ

ö

´

-

-

ç

÷

´ ¦ ´

è

ø

NEG

IN

NEG

V

D =

V

η + V

´

IN

out

swpeak

V

D

I

I

=

+

2

s

L

1

D

´

´ ¦ ´

-

(

)

IN

V

D

Iout =

1

D

Isw

2

s

L

æ

ö

´

-

-

ç

÷

´ ¦ ´

è

ø

TPS65137

www.ti.com

SLVS929A – MAY 2010 – REVISED OCTOBER 2012

(2)

The maximum output current is given at the highest switching frequency of typically 1.4MHz and minimum switch

current limit of 0.9A. Equation 3 is used to calculate the switch peak current.

(3)

The inductor needs to be rated for this switch peak current to avoid inductor saturation.

The boost converter output capacitor is connected to pin CB and a 4.7µF capacitor is sufficient. A 2.2µF

Table 3. Output Capacitor Selection

CAPACITOR

COMPONENT SUPPLIER

SIZE

4.7

μF/10 V

Taiyo Yuden LMK107BJ475

0603

2.2

μF/10 V

Taiyo Yuden LMK107BJ225

0603

The negative output is generated with a buck boost converter. The component values and output current are

calculated at maximum load current in continuous conduction operation. The typical switching frequency during

this operation mode is 1.4MHz.

The buck boost converter duty cycle is:

(4)

To calculate the duty cycle a good estimation for the efficiency,

η, is 75% or it can be taken out of the typical

curve in Figure 1. In order to calculate the maximum output current of the buck boost converter for a certain input

voltage, the following formula is used:

(5)

The maximum output current is given at the highest switching frequency of typically 1.4MHz and minimum switch

current limit of 1.1A. Equation 6 is used to calculate the switch peak current.

(6)

The inductor needs to be rated for this switch peak current to avoid inductor saturation. Refer to Table 4 for

possible inductors for this application. A 4.7

values can be used to minimize the output voltage ripple. Refer to Table 3 for output capacitor selection.

INDUCTOR SELECTION

The device is optimized to operate with 4.7uH inductors. Different inductor values will change the converter

efficiency and output voltage ripple. A 2.2uH inductor is also a possible solution. Any other inductor values will

degrade device performance and stability which is not recommended for this device.

Table 4. Inductor Selection

INDUCTOR VALUE

COMPONENT

DIMENSIONS in mm

Isat/DCR

SUPPLIER

4.7

μH

TDK VLF4012

3.7 × 3.5 × 1.2

1.1A/140m

Ω

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