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TB62731FUG Fiches technique(PDF) 10 Page - Marktech Corporate

No de pièce TB62731FUG
Description  Step-up DC-DC Converter for White LED Driver
Download  17 Pages
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Fabricant  MARKTECH [Marktech Corporate]
Site Internet  http://www.marktechopto.com
Logo MARKTECH - Marktech Corporate

TB62731FUG Fiches technique(HTML) 10 Page - Marktech Corporate

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TB62731FUG
2004-09-14
10
Output Derating Function
Toshiba recommend derating the LED current depending on the increase in ambient temperature.
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.
The IC features an output current that varies according to the internally-detected temperature Tjs as follows:
when Tjs = 45 (°C), output current is 100%; when Tjs = 100 (°C), output current is 0%.
The derating start temperature Ts (°C) is determined based on Ta (Ta = Ts when the IC is not operating) by
subtracting the self-generated temperature Tup (°C) from Tjs = 45 (°C).
Ts (°C) = 45 (°C) − Tup (°C) . . . formula 4
The derating characteristic is as shown in the graph below, Figure 5, which shows the relation between output
current change ratio and internally-detected temperature (IC temperature) Tjs.
The self-generated temperature Tup (°C) is calculated as follows:
Tup (°C) = (P loss (W) − P parts (W)) × θja (°C/W) ) . . . formula 5
P loss: power loss
P parts: power loss of parts
θja: package saturation thermal resistance (Ω)
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
P loss (W) ∼− Po (W) ÷ η (%) − Po (W)
Po: output power
η: power efficiency
P parts (W) ∼− RDC × IIN + Vf schottky × If LED + R_sens × If LED × If LED
θja (°C/W) ≤ 260 (°C/W)
max when IC mounted on PCB
Po (W) = Vo (V) × Io (A)
Vo: Vf LED output voltage
Io: mean output current = set current
Pi (W) = VIN (V) × IIN (A)
Pi: input power
VIN: input voltage
IIN: mean input current
η (%) = 100 × Po (W) ÷ Pi (W)
Example of calculation: Where the measurement result for any lighting circuit shows the following values:
RDC = 0.5 (Ω), Po = 320 (mW), IIN = 0.1 (mA), Io = 20 (mA), R_sens = 1.8 (Ω), Vf schottky = 0.3 (V), and η = 70
(%)


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