MIC2171WU Micrel Inc, MIC2171WU Datasheet - Page 6

• Manufacturer Part Number: MIC2171WU
• Description: IC REG SW 100KHZ 2.5A TO263-5
• Manufacturer: Micrel Inc
• Type: Step-Down (Buck), Step-Up (Boost), Inverting, Cukr

Specifications of MIC2171WU

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Current - Output: 2.5A
• Frequency - Switching: 100kHz
• Voltage - Input: 3 ~ 40 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: D²Pak, TO-263 (5 leads + tab)
• Power - Output: 1.3W
• Number Of Pwm Outputs: 1
• On/off Pin: No
• Adjustable Output: No
• Switching Freq: 112KHz
• Operating Supply Voltage (max): 40V
• Output Current: 2.5A
• Output Voltage: 65V
• Synchronous Pin: No
• Operating Temperature Classification: Industrial
• Mounting: Surface Mount
• Pin Count: 5 +Tab
• Package Type: TO-263
• Duty Cycle: 95%
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Output: -
• Lead Free Status / RoHS Status: Compliant, Lead free / RoHS Compliant
• Other names: 576-1063

Application Information
Soft Start
A diode-coupled capacitor from COMP to circuit ground
slows the output voltage rise at turn on (Figure 3).
The additional time it takes for the error amplifier to
charge the capacitor corresponds to the time it takes the
output to reach regulation. Diode D1 discharges C1
when V
Current Limit
The maximum current limit of the MIC2171 can be
reduced by adding a voltage clamp to the COMP output
(Figure 4). 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 MIC2171 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
MIC2171, 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
Micrel, Inc.
May 2007
Q1
IN
V
is removed.
IN
C1
R 1
R 2
V
IN
GND
Figure 4. Current Limit
D1
Figure 3. Soft Start
MIC2171
IN
COMP
C1
S W
D2
F B
R 3
C2
MIC2171
Note: Input and output
IN
I
C L
COMP
returns not common
0.6V/R2
R 1
C2
V
OU T
6
the losses of the power switch driver circuitry. The dc
losses are calculated from the supply voltage (V
device supply current (I
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:
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 Character-
istics). Power switch losses are calculated by modeling
the switch as a resistor with the switch duty cycle
modifying the average power dissipation.
where:
From the Typical performance Characteristics:
then:
P
P
V
I
I
∆I
V
I
I
δ = 66.2% (0.662)
V
P
P
P
δ = duty cycle
V
V
V
R
P
P
P
P
δ
Q
CL
Q
CL
(bias+driver)
(bias+driver)
IN(min)
IN
IN(min)
(bias+driver)
(bias+driver)
SW
SW
OUT
F
SW
SW
(total)
(total)
SW
IN
= typical quiescent supply current
= 0.007A
=
= D1 forward voltage drop at I
= power switch current limit
= 2.21A
= 5.0V
= typical supply current increase
= (I
= I
= (2.21)
= 1.2W
V
= 0.37Ω
= output voltage
= 1.2 + 0.1
= 1.3W
OUT
= supply voltage = V
= 5.0V – (2.21 x 0.37) = 4.18V
CL
SW
V
(R
)
= (V
= device operating losses
= (5 x 0.007) + (4.18 x 2.21 x 0.009)
= 0.1W
+
2
OUT
SW
R
V
2
F
× 0.37 × 0.662
SW
)
IN
+
Q
−
).The MIC2171 supply current is
I
δ
V
Q
V
) + (V
F
IN(min)
IN(min)
IN
x I
– V
SW
OUT
SW
M9999-051107
x ∆I
MIC2171
IN
)
IN
) and