tb62731fug TOSHIBA Semiconductor CORPORATION, tb62731fug Datasheet - Page 10

• Manufacturer Part Number: tb62731fug
• Description: Step-up Dc-dc Converter For White Led Driver
• Manufacturer: TOSHIBA Semiconductor CORPORATION

Output Derating Function
The TB62731FUG is designed to ensure safe and efficient driving of white LEDs used as backlight sources for color
LCDs. The IC incorporates a function that derates current according to the set temperature (the ambient
temperature when the IC is mounted), Ta.
when T
subtracting the self-generated temperature T
current change ratio and internally-detected temperature (IC temperature) T
The self-generated temperature T
Toshiba recommend derating the LED current depending on the increase in ambient temperature.
The IC features an output current that varies according to the internally-detected temperature T
The derating start temperature T
The derating characteristic is as shown in the graph below, Figure 5, which shows the relation between output
The parameters are described below:
DC resistance of inductor: RDC ( Ω )
LED forward current: If LED (A)
LED forward voltage: Vf LED (V)
Schottky diode forward voltage: Vf schottky (V)
Setting resistance: R_sens
Example of calculation: Where the measurement result for any lighting circuit shows the following values:
T
T
P loss: power loss
P parts: power loss of parts
θ
P loss (W) ∼ − P
P
η : power efficiency
P parts (W) ∼ − RDC × I
θ
max when IC mounted on PCB
P
V
I
P
P
V
I
RDC = 0.5 ( Ω ), P
(%)
o
IN
ja
ja
o
o
i
i
s
up
o
IN
: mean output current = set current
: input power
η (%) = 100 × P
: output power
: Vf LED output voltage
(W) = V
: package saturation thermal resistance ( Ω )
( ° C) = 45 ( ° C) − T
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(W) = V
: mean input current
( ° C/W) ≤ 260 ( ° C/W)
: input voltage
( ° C) = (P loss (W) − P parts (W)) × θ
= 45 ( ° C), output current is 100%; when T
IN
o
(V) × I
(V) × I
o
(W) ÷ η (%) − P
o
o
= 320 (mW), I
o
(W) ÷ P
IN
(A)
up
IN
(A)
( ° C) . . . formula 4
+ Vf schottky × If LED + R_sens × If LED × If LED
i
(W)
up
o
s
IN
(W)
( ° C) is calculated as follows:
( ° C) is determined based on Ta (Ta = Ts when the IC is not operating) by
= 0.1 (mA), I
ja
up
( ° C/W) ) . . . formula 5
( ° C) from T
js
o
= 20 (mA), R_sens = 1.8 ( Ω ), Vf schottky = 0.3 (V), and η = 70
= 100 ( ° C), output current is 0%.
10
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= 45 ( ° C).
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.
TB62731FUG
js
2006-06-14
as follows: