MIC2172_05 Datasheet, PDF (11/17 Pages) Micrel Semiconductor – 100kHz 1.25A Switching Regulators

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
* MIC3172 only
Single point ground
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.
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
C1*
22µF
V_IN
D1
N/C SYNC
V_SW
MIC2172
1N5822
V_OUT
+12V, 0.14A
R1
10.7k
1%
R3
1k
COMP _GND FB
P1 P2 S
C3
1µF
C2
470µF
R2
1.24k
1%
* Locate near MIC2172 when supply leads > 2"
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.
(1)
I_OUT
≤


I_CL
2


V_IN δ
V_OUT
(1a) δ = ^{VOUT + VF ± VIN}
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