MAX1576 Datasheet, PDF (20/22 Pages) Maxim Integrated Products – 480mA White LED 1x/1.5x/2x Charge Pump for Backlighting and Camera Flash

THERMAL CONSIDERATIONS
The MC14489B is designed to operate with a chip–junction
temperature (TJ) ranging from – 40 to 130°C, as indicated in
the electrical characteristics tables. The ambient operating
temperature range (TA) is dependent on R_θJA, the internal
chip current, how many anode drivers are used, the number
of bank drivers used, the drive current, and how the package
is cooled. The maximum ratings table gives the thermal resis-
tance, junction–to–ambient, of the MC14489B mounted on a
pc board using natural convection to be 90°C per watt for the
plastic DIP. The SOG thermal resistance is 100°C per watt.
The following general equation (1) is used to determine the
power dissipated by the MC14489B.
PT = PD + PI
(1)
where
PT = Total power dissipation of the MC14489B
PD = Power dissipated in the driver circuitry (mW)
PI = Power dissipated by the internal chip
circuitry (mW)
The equations for the two terms of the general equation
are:
PD = (iOH) (N)(VDD – VLED)(B/5)
(2)
PI = (1.5 mA)(VDD) + IRx(VDD – IRxRx)
(3)
where
iOH = Peak anode driver current (mA)
IRx = iOH /10, with iOH = the peak anode driver current
(mA) when the dimmer bit is high
N = Number of anode drivers used
B = Number of bank drivers used
Rx = External resistor value (kΩ)
VDD = Maximum supply voltage, referenced to VSS
(volts)
VLED = Minimum anticipated voltage drop across the
LED
1.5 mA = Operating supply current of the MC14489B
The following two examples show how to calculate the
maximum allowable ambient temperature.
Worst–Case Analysis Example 1:
5–digit display with decimals (5 banks and 8 anode drivers)
DIP without heat sink on PC board
iOH = 20 mA max
VLED = 1.8 V min
VDD = 5.25 max
PD = (20)(8)(5.25 – 1.8)(5/5) = 552 mW
PI = (1.5)(5.25) + 2[5.25 – 2(2)] = 10 mW
Therefore, PT = 552 + 10 = 562 mW
and ∆Tchip = R_θJAPT = (90°C/W)(0.562) = 51°C
Finally, the maximum allowable
TA = TJmax – ∆Tchip = 130 – 51 = 79°C
Ref. (2)
Ref. (3)
Ref. (1)
That is, if TA = 79°C, the maximum junction temperature is
130°C. The chip’s average temperature for this example is
lower than 130°C because all segments are usually not illumi-
nated simultaneously for an indefinite period.
Worst–Case Analysis Example 2:
16 lamps (4 banks and 4 anode drivers)
SOG without heat sink on PC board
iOH = 30 mA max
VLED = 1.8 V min
VDD = 5.5 max
PD = (30)(4)(5.5 – 1.8)(4/5) = 355 mW
Ref. (2)
PI = (1.5)(5.5) + 3[5.5 – 3(1.0)] = 16 mW
Ref. (3)
Therefore, PT = 355 + 16 = 371 mW
Ref. (1)
and ∆Tchip = R_θJAPT = (100°C/W)(0.371) = 37°C
Finally, the maximum allowable
TA = TJmax – ∆Tchip = 130 – 37 = 93°C
To extend the allowable ambient temperature range or to
reduce TJ, which extends chip life, a heat sink such as shown
in Figure 20 can be used in high–current applications. Alter-
natively, heat–spreader techniques can be used on the PC
board, such as running a wide trace under the MC14489B and
using thermal paste. Wide, radial traces from the MC14489B
leads also act as heat spreaders.
AAVID #5804 or equivalent
(Tel. 603/524–4443, FAX 603/528–1478)
Motorola cannot recommend one supplier over another and
in no way suggests that this is the only heat sink supplier.
Figure 20. Heat Sink
Table 3. LED Lamp and Common–Cathode Display
Manufacturers
Supplier
QT Optoelectronics
Hewlett–Packard (HP), Components Group
Industrial Electronic Engineers (IEE), Component Products Div.
Purdy Electronics Corp., AND Product Line
NOTE: Motorola cannot recommend one supplier over another
and in no way suggests that this is a complete listing of
LED suppliers.
MC14489B
20
MOTOROLA