MIC3172YN Micrel Inc, MIC3172YN Datasheet - Page 13

• Manufacturer Part Number: MIC3172YN
• Description: IC REG SW 100KHZ 1.25A 8-DIP
• Manufacturer: Micrel Inc
• Type: Step-Down (Buck), Step-Up (Boost), Inverting, Cukr

Specifications of MIC3172YN

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Current - Output: 1.25A
• Frequency - Switching: 100kHz
• Voltage - Input: 3 ~ 40 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Through Hole
• Package / Case: 8-DIP (0.300", 7.62mm)
• Power - Output: 302mW
• Primary Input Voltage: 40V
• No. Of Outputs: 1
• Output Current: 3A
• No. Of Pins: 8
• Operating Temperature Range: -40°C To +85°C
• Supply Voltage Range: 3V To 40V
• Termination Type: Through Hole
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Output: -
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant, Lead free / RoHS Compliant
• Other names: 576-2255; MIC3172YN

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
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
device supply current (I
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.
where:
As a practical example refer to figure 1.
Then:
P
Micrel
April 2006
(
bias
P
V
I
I
(see “Design Hints: Switch Current Calculations”)
δ = duty cycle
V
I
I
δ = 60% (0.6)
Q
SW
Q
SW
(bias+driver)
IN
IN
+
= quiescent supply current
= 0.006A
driver
= supply voltage
= 5.0V
= power switch current
= 0.625A
)
=
= device operating losses
(
V
IN
I
Q
)
+
V
Q
IN
). The MIC2172/3172 supply
V
⎡
⎢
⎣
I
F
SW
= D1 forward voltage drop
⎛
⎜
⎝
0.004
determined by first
δ
V
OUT
50
=
V
+
= output voltage
OUT
δ
V
⎞
⎟
⎠
OUT
⎤
⎥
⎦
+
V
+
F
IN
V
±
) and
F
V
IN
13
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
Characteristics). Power switch losses are calculated by
modeling the switch as a resistor with the switch duty
cycle modifying the average power dissipation.
From the Typical performance Characteristics:
Then:
The junction temperature for any semiconductor is
calculated using the following:
Where:
For the practical example:
Then:
This junction temperature is below the rated maximum of
150°C.
Grounding
Refer to figure 10. Heavy lines indicate high current
paths.
P
R
P
P
P
P
T
T
T
P
θ
T
θ
T
T
P
P
JA
JA
J
J
A
A
J
J
SW
SW
SW
(total)
(total)
(total)
(
(
SW
bias
bias
= T
= junction temperature
= 70 + 0.30 ⋅ 130
= 109°C
= ambient temperature (maximum)
= 70°C
= junction to ambient thermal resistance
= 130°C/W (for plastic DIP)
= (0.625)
= 0.234W
= (I
= 1 Ω
+
+
= 0.068 + 0.234
= 0.302W
= total power dissipation
A
driver
driver
+ P
curves
SW
)
(total)
2
)
)
R
=
=
2
SW
× 1 × 0.6
(
0.068W
θ
5
JA
×
δ
0.006
(see
)
+
5
Typical
⎡
⎢
⎣
0.625
⎛
⎜
⎝
0.004
MIC2172/3172
[phone]
M9999-041806
Performance
50
+
0.6
⎞
⎟
⎠
⎤
⎥
⎦