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THEORY OF OPERATION/DESIGN PROCEDURE

Capacitor Selection

Power Efficiency

Power Dissipation

P

D max, package +

T

Jmax *

T

A

T

q

ja

TPS60230

SLVS516A – MAY 2004 – REVISED JUNE 2004

Ceramic capacitors such as X5R or X7R are recommended to be used with the TPS60230. For the two flying

capacitors C1 and C2, it is important to use low ESR capacitors to avoid unnecessary efficiency losses. Low

ESR capacitors on VOUT reduce the ripple voltage on the supply of the current sources. Table 2 lists capacitor

types that have been tested with the TPS60230.

Table 2. Capacitors

PART

VALUE

VOLTAGE

MANUFACTURER

SIZE

WEBSITE

C1608X5R1A105M

1

µF

10 V

0603

C1608X5R1A474M

0.47

µF

10 V

TDK

0603

www.componnent.tdk.com

C2012X7R1C105M

1

µ F

16 V

0805

LMK107BJ105MA

1

µF

10 V

0603

LMK107BJ474MA

0.47

µF

10 V

Taiyo Yuden

0603

www.t-yuden.com

LMK212BJ105MG

1

µ F

10 V

0805

The power conversion efficiency of the TPS60230 can be calculated by adding up the products of each LED

current and voltage and dividing it by the product of the input voltage and current. With a fully charged battery

where the input voltage is typically above the LED forward voltage, the charge pump operates in the 1x mode

and efficiency is very high. As the battery discharges, there is a point where the current sources no longer have

enough voltage overhead to maintain a constant current regulation. At that point, the charge pump switches into

the 1.5x mode. The conversion efficiency is lowest at the crossover. As the battery discharges further, the

efficiency again increases until at about 3.1 V where it reaches a second maximum. Below 3.1 V input voltage,

the maximum current per LED is less than 25 mA.

The maximum power dissipation inside the TPS60230 can be calculated based on the following equation:

The maximum power dissipation occurs when the input voltage is just low enough to operate in 1.5x mode, with

This needs to be lower than the maximum allowed power dissipation of the package, which can be calculated

using the following equation:

For example, the worst case power dissipation occurs at the input voltage level where the charge pump switches

from the 1x mode to the 1.5x mode. At this operating point, the supply voltage to the current sources is at its

maximum and the current sources must drop the most voltage in order to maintain a regulated output current.

The worst case power dissipation occurs when all 5 LED outputs are fully loaded with 25 mA of LED current.

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