TOSHIBA

T B 6 2 7 3 2 F U

27, Feb 2002 (Ver.04)

9

ABOUT THE CURRENT DERATING FUNCTION

As for the LED current, generally the current derating recommended against the rise in the ambient temperature.

The TB62732FU aims at turning on white LED of the back light illuminant of the color LCD safely and efficiently,

and builds in the current derating function which set temperature was presumed.

This IC has the character which makes the current 100(%) in the case of Tjs=45 (degree) and which makes

the current 0(%) again in the case of Tjs=100 (degree) corresponding to internal detection temperature Tjs.

When it is seen from set temperature Ta (degree), the temperature that self-fever temperature Tup (degree)

in the operation was reduced from Tjs = 45 (degree) becomes a starting temperature Ts (degree).

Starting temperature Ts (degree) = 45 (degree) - self-fever temperature Tup (degree)

--- Form 4

Therefore, the derating character functions as the figure 6, and shows internal detection temperature Tjs and

the rate of change of the outout current.

Self-fever temperature Tup (degree) in the operation is calculated with the Form 5.

Self-fever temperature Tup (degree)

---- ceremony 5

= (P loss (W) - P parts(W))*Rth(j-a) (degree/W)

It is described in the following about each clause.

DC resistance of the inductance : RDC(ohm), Forward current of LED : IF LED(A),

Total forward voltage of LED : VF LED(V), Forward voltage : VF schottky(V),

Setup resistance : R_sens(ohm), Power loss of lighting circuit : P loss(W)=(Po/efficiency)-Po(W),

Power loss of parts : P parts(W)=RDC*Iin+VF schottky*IF LED+R_sens*IF LED*IF LED,

Saturation thermal resistance of package :

Rth(j-a) (degree/W) =< 260 - - - when on board, muximum

Output power :

Po(W) = VF LED*Io(A)

Input power :

Pi(W) = Vin (V)*Iin(A)

Efficiency :

Efficiency(%)=100*(Po/Pi)

Example : When the measurement of the lighting circuit tightened each following value.

RDC=0.5(ohm), Po=320(mW), Iin=0.1(A), Io=20(mA), R_sens=1.8(ohm), VF schottky=0.3(V),

Efficiency=70(%)

In this case, self-fever temperature Tup

Tup (degree)= ((0.32-(0.32*0.7))-(0.5*0.1+0.3*0.0196+2.7*0.0196*0.0196))*260 =10.16 (degree)

Start temperatue Ts(degree)=45(degree)-10.16(degree)=33.4(degree)

Io is controlled as the Fig 6 as a result within the recommendation current area of LED.

The saturation thermal resistance Rth(j-a) = 260 (degree /W) is maximum value. They sometimes become

the Rth(j-a) = 210 - 260 (degree /W) by mounting.

And, the individual difference is formed in the starting temperature with the character of the IC and the influence

of the difference in the environment of the use.

Fig 6 : the derating function of the setup current

0

25

50

75

100

Ts/Tjs (degree)

0

20

40

60

80

100

120

The change of

Ts=34.4(degree)

(20mA=100%)

The

recommendation

current area of LED

(25mA conversion)

The change by Tjs