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TK630xxB
GC3-J013
Page 25
12-2. Layout
Fig12-4: Layout example
1
GND
V
Cont
V
In
V
Out
GND
(Top View)
PCB Material : Glass epoxy
Size : 7mm
×8mm×0.8mm
Please do derating with 2.9mW/
°C at Pd=360mW and
25
°C or more. Thermal resistance (θ
ja) is=278°C/W.
Fig12-5: Derating curve
25
50
100
150°C
Pd(mW)
360
(85°C)
-2.9mW/°C
The package loss is limited at the temperature that the
internal
temperature
sensor
works
(about
150
°C).
Therefore, the package loss is assumed to be an internal
limitation. There is no heat radiation characteristic of the
package unit assumed because of its small size. Heat is
carried away from the device by being mounted on the
PCB. This value is directly effected by the material and
the copper pattern etc. of the PCB. The losses are
approximately 360mW. Enduring these losses becomes
possible in a lot of applications operating at 25
°C.
The overheating protection circuit operates when the
junction temperature reaches 150
°C (this happens when
the regulator is dissipating excessive power, outside
temperature is high, or heat radiation is bad). The output
current and the output voltage will drop when the
protection circuit operates. However, operation begins
again as soon as the output voltage drops and the
temperature of the chip decreases.
How to determine the thermal resistance when
mounted on PCB
The thermal resistance when mounted is expressed as
follows:
T
j=θja×Pd+Ta
T
j of IC is set around 150°C. Pd is the value when the
thermal sensor is activated.
If the ambient temperature is 25
°C, then:
150=
θ
ja×Pd+25
θ
ja=125/Pd (°C /mW)
Pd is easily calculated.
A simple way to determine Pd is to calculate V
In×IIn
when the output side is shorted. Input current gradually
falls as output voltage rises after working thermal
shutdown. You should use the value when thermal
equilibrium is reached. In many cases, heat radiation is
good, and Pd is 360mW or more.
Fig12-6: How to determine DPd
0
255075
100
150
Pd(mW)
Pd
D Pd
2
3
5
4
Ta (℃)
Procedure (When mounted on PCB.)
1. Find Pd (V
In×IIn
when the output side is short-
circuited).
2. Plot Pd against 25
°C.
3. Connect Pd to the point corresponding to the 150
°C
with a straight line.
4. In design, take a vertical line from the maximum
operating temperature (e.g., 75
°C) to the derating
curve.
5. Read off the value of Pd against the point at which the
vertical line intersects the derating curve. This is taken
as the maximum power dissipation DPd.
6. DPd
÷ (V
In,MAXVOut)=IOut (at 75°C)
The maximum output current at the highest operating
temperature will be I
Out DPd ÷ (VIn,MAXVOut).
Please use the device at low temperature with better
radiation. The lower temperature provides better quality.