MIC2172/3172
Then:
Micrel
Applications and Design Hints
2
P
P
P
P
= (0.625) × 1 × 0.6
SW
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.
= 0.234W
(SW)
(total)
(total)
= 0.068 + 0.234
= 0.302W
Boost Conversion
Thejunctiontemperatureforanysemiconductoriscalculated
using the following:
Refer to figure 11 for a typical boost conversion application
where a +5V logic supply is available but +12V at 0.14A is
required.
T = T + P θ
(total) JA
J
A
Where:
+5V
(4.75V min.)
C1*
22µF
L1
27µH
T = junction temperature
J
VOUT
+12V, 0.14A
T = ambient temperature (maximum)
A
VIN
D1
VSW
N/C
SYNC
P
= total power dissipation
(total)
R1
10.7k
1%
1N5822
MIC2172
θ
= junction to ambient thermal resistance
JA
COMP
FB
For the practical example:
GND
R2
R3
1k
C2
470µF
P1 P2
S
1.24k
1%
C3
1µF
T = 70°C
A
θ
= 130°C/W (for plastic DIP)
JA
* Locate near MIC2172 when supply leads > 2"
Then:
T = 70 + 0.30 × 130
Figure 11. 5V to 12V Boost Converter
J
T = 109°C
J
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.
This junction temperature is below the rated maximum of
150°C.
4
Grounding
Refer to figure 10. Heavy lines indicate high current paths.
When L1 discharges its current completely during the
MIC2172/3172’s off-time, it is operating in discontinuous
mode.
VIN
VIN
EN*
VSW
MIC2172/3172
GND FB
L1 is operating in continuous mode if it does not discharge
completely before the MIC2172/3172 power switch is turned
on again.
P1 P2 S
VC
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.
Single point ground
* MIC3172 only
ICL
VIN
δ
Figure 10. Single Point Ground
2
IOUT
≤
(1)
VOUT
A single point ground is strongly recommended for proper
operation.
V
+ V – V
OUT
F
IN
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.
δ =
(1a)
V
+ V
OUT
F
Where:
I
= internal switch current limit
I
I
CL
= 1.25A when δ < 50%
= 0.833 (2 – δ) when δ ≥ 50%
CL
CL
(Refer to Electrical Characteristics.)
I
= maximum output current
OUT
V
= minimum input voltage
IN
δ = duty cycle
1997
4-23
MICREL [ MICREL SEMICONDUCTOR ]
