MIC2171WU Micrel Inc, MIC2171WU Datasheet - Page 7

• 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

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 5. Heavy lines indicate high current
paths.
A single point ground is strongly recommended for
proper operation.
The signal ground, compensation network ground, and
feed-back network connections are sensitive to minor
voltage variations. The input and output capacitor
grounds and power ground conductors will exhibit
voltage drop when carrying large currents. Keep the
sensitive circuit ground traces separate from the power
ground traces. Small voltage variations applied to the
sensitive circuits can prevent the MIC2171 or any
switching regulator from functioning properly.
Boost Conversion
Refer to Figure 1 for a typical boost conversion
application where a +5V logic supply is available but
+12V at 0.25A is required.
The first step in designing a boost converter is
determining whether inductor L1 will cause the converter
to operate in either continuous or discontinuous mode.
Micrel, Inc.
May 2007
V
IN
T
T
T
P
θ
T
θ
T
T
JA
JA
J
J
A
A
J
J
(total)
= T
= junction temperature
= 70 + (1.24 × 45)
= 126°C
= ambient temperature (maximum)
= 70°C
= junction to ambient thermal resistance
= 45°C/W (TO-220)
Figure 5. Single Point Ground
= total power dissipation
A
+ P
(total)
GND
MIC2171
θ
IN
JA
V C
S W
F B
Single point ground
7
Discontinuous mode is preferred because the feedback
control of the converter is simpler.
When L1 discharges its current completely during the
MIC2171 off-time, it is operating in discontinuous mode.
L1 is operating in continuous mode if it does not
discharge completely before the MIC2171 power switch
is turned on again.
Discontinuous Mode Design
Given the maximum output current, solve equation (1) to
determine
discontinuous mode without initiating the internal device
current limit.
(1)
(1a)
where:
For the example in Figure 1.
then:
This value is greater than the 0.25A output current
requirement, so we can proceed to find the minimum
inductance value of L1 for discontinuous operation at
P
(2)
OUT
.
I
I
V
δ = duty cycle
V
V
I
I
V
δ = 0.662
V
V
I
I
δ
I
L1
CL
OUT
OUT
CL
OUT
OUT
OUT
IN(min)
OUT
F
IN(min)
OUT
F
=
= D1 forward voltage drop
= 0.36V (@ .26A, 70°C)
= internal switch current limit
= 1.67 (2–0.662) = 2.24A
≥
= maximum output current
= 0.25A
≤ 0.258A
whether
V
= required output voltage
= 12.0V
≤
≤
2P
I
I
(Refer to Electrical Characteristics.)
OUT
(V
CL
CL
= minimum input voltage = V
= 4.18V
⎛
⎜ ⎜
⎝
⎛
⎜
⎝
OUT
I
IN
2.235
= 2.5A when δ < 50%
= 1.67 (2 – δ) when δ ≥ 50%
CL
V
2
δ
+
OUT
2
f
V
)
⎞
⎟ ⎟
⎠
V
SW
2
V
OUT
F
IN(min)
⎞
⎟
⎠
+
the
−
×
V
V
4.178
12
F
IN(min)
δ
device
×
0.662
can
M9999-051107
IN
operate
– V
MIC2171
SW
in