Micrel
clamp to the COMP output (figure 9). This feature can be
useful in applications requiring either a complete
shutdown of Q1’s switching action or a form of current
fold-back limiting. This use of the COMP output does not
disable the oscillator, amplifiers or other circuitry,
therefore the supply current is never less than
approximately 5mA.
Thermal Management
Although the MIC2172/3172 family contains thermal
protection circuitry, for best reliability, avoid prolonged
operation with junction temperatures near the rated
maximum.
The junction temperature is determined by first
calculating the power dissipation of the device. For the
MIC2172/3172, the total power dissipation is the sum of
the device operating losses and power switch losses.
The device operating losses are the dc losses
associated with biasing all of the internal functions plus
the losses of the power switch driver circuitry. The dc
losses are calculated from the supply voltage (V
IN
) and
device supply current (I
Q
). The MIC2172/3172 supply
current is almost constant regardless of the supply
voltage (see “Electrical Characteristics”). The driver
section losses (not including the switch) are a function of
supply voltage, power switch current, and duty cycle.
MIC2172/3172
⎡
⎛
0.004
+
0.6
⎞⎤
P
(
bias
+
driver
)
=
(
5
×
0.006
)
+
5
⎢
0.625
⎜
⎟⎥
50
⎝
⎠⎦
⎣
P
(
bias
+
driver
)
=
0.068W
Power switch dissipation calculations are greatly
simplified by making two assumptions which are usually
fairly accurate. First, the majority of losses in the power
switch are due to on-losses. To find these losses, assign
a resistance value to the collector/emitter terminals of
the device using the saturation voltage versus collector
current
curves
(see
Typical
Performance
Characteristics). Power switch losses are calculated by
modeling the switch as a resistor with the switch duty
cycle modifying the average power dissipation.
P
SW
= (I
SW
) R
SW
δ
From the Typical performance Characteristics:
R
SW
= 1
Ω
Then:
2
P
SW
= (0.625) × 1 × 0.6
P
SW
= 0.234W
P
(total)
= 0.068 + 0.234
P
(total)
= 0.302W
The junction temperature for any semiconductor is
calculated using the following:
T
J
= T
A
+ P
(total)
θ
JA
Where:
T
J
= junction temperature
T
A
= ambient temperature (maximum)
P
(total)
= total power dissipation
2
⎡
⎛
0.004
+
δ
⎞⎤
P
(
bias
+
driver
)
=
(
V
IN
I
Q
)
+
V
IN
⎢
I
SW
⎜
⎟⎥
50
⎝
⎠⎦
⎣
where:
P
(bias+driver)
= device operating losses
V
IN
= supply voltage
I
Q
= quiescent supply current
I
SW
= power switch current
(see “Design Hints: Switch Current Calculations”)
δ
= duty cycle
V
+
V
F
±
V
IN
δ
=
OUT
V
OUT
+
V
F
V
OUT
= output voltage
V
F
= D1 forward voltage drop
As a practical example refer to figure 1.
V
IN
= 5.0V
I
Q
= 0.006A
I
SW
= 0.625A
δ
= 60% (0.6)
Then:
θ
JA
= junction to ambient thermal resistance
For the practical example:
T
A
= 70°C
θ
JA
= 130°C/W (for plastic DIP)
Then:
T
J
= 70 + 0.30
⋅
130
T
J
= 109°C
This junction temperature is below the rated maximum of
150°C.
Grounding
Refer to figure 10. Heavy lines indicate high current
paths.
April 2006
13
M9999-041806
(408) 955-1690
MICREL [ MICREL SEMICONDUCTOR ]
