MIC2171 Micrel Semiconductor, MIC2171 Datasheet - Page 7

• Manufacturer Part Number: MIC2171
• Description: 100kHz 2.5A Switching Regulator Preliminary Information
• Manufacturer: Micrel Semiconductor

MIC2171
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 prop-
erly.
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. Discontinuous
1997
V
IN
T
T
T
P
T
T
T
JA
JA
J
J
A
A
J
J
(total)
= T
= junction temperature
= 70 + (1.24
= 126 C
= ambient temperature (maximum)
= 70 C
= junction to ambient thermal resistance
= 45 C/W (TO-220)
A
= total power dissipation
Figure 5. Single Point Ground
+ P
(total) JA
GND
MIC2171
IN
45)
COMP
SW
FB
Single point ground
4-9
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 whether the device can operate in 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 require-
ment, so we can proceed to find the minimum inductance
value of L1 for discontinuous operation at P
(2)
Where:
I
I
V
V
V
I
I
V
V
V
I
P
f
I
I
L1
CL
OUT
OUT
CL
OUT
SW
OUT
OUT
IN(min)
OUT
F
IN(min)
OUT
F
OUT
= duty cycle
= 0.662
=
= D1 forward voltage drop
= 0.36V (@ .26A, 70 C)
= internal switch current limit
= 1.67 (2–0.662) = 2.24A
V
= 1 10
= maximum output current
= 0.25A
OUT
I
I
(Refer to Electrical Characteristics.)
= required output voltage
= 12.0V
= 12
CL
CL
0.258A
2 P
= minimum input voltage = V
= 4.18V
= 2.5A when < 50%
= 1.67 (2 – ) when
V
V
+ V – V
OUT SW
I
2.235
OUT
CL
IN
5
2
Hz (100kHz)
2
0.25 = 3W
F
V
f
+ V
2
OUT
V
IN(min)
F
IN(min)
4
12
.178
0.662
50%
IN
OUT
– V
.
SW
Micrel
4