MIC2172/3172
Then:
P
SW
=
(0.625)
2
×
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
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.
V
IN
V
IN
EN
*
V
SW
MIC2172/3172
GND
P1 P2 S
FB
V
C
* Locate near MIC2172 when supply leads > 2"
N/C
SYNC
MIC2172
R3
1k
COMP GND
FB
P1 P2 S
C3
1µF
V
IN
V
SW
1N5822
Micrel, Inc.
Applications and Design Hints
Access to both the collector and emitter(s) of the NPN power
switch makes the MIC2172/3172 extremely versatile and
suitable for use in most PWM power supply topologies.
Boost Conversion
Refer to figure 11 for a typical boost conversion application
where a +5V logic supply is available but +12V at 0.14A is
required.
+5V
(4.75V min.)
L1
27µH
D1
C1*
22µF
V
OUT
+12V, 0.14A
R1
10.7k
1%
R2
C2
1.24k
470µF 1%
Figure 11. 5V to 12V Boost Converter
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
mode is preferred because the feedback control of the
converter is simpler.
When L1 discharges its current completely during the
MIC2172/3172’s off-time, it is operating in discontinuous
mode.
L1 is operating in continuous mode if it does not discharge
completely before the MIC2172/3172 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.
I
CL
V
δ
2
IN
≤
V
OUT
Single point ground
* MIC3172 only
Figure 10. Single Point Ground
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
MIC2172/3172 or any switching regulator from functioning
properly.
(1)
I
OUT
(1a)
δ
=
V
OUT
+ V
F
± V
IN
V
OUT
+ V
F
Where:
I
CL
= internal switch current limit
I
CL
= 1.25A when
δ
< 50%
I
CL
= 0.833 (2 –
δ)
when
δ ≥
50%
(Refer to Electrical Characteristics.)
I
OUT
= maximum output current
V
IN
= minimum input voltage
δ
= duty cycle
April 2005
11
M9999-042205
MICREL [ MICREL SEMICONDUCTOR ]
