MIC2171_07 Datasheet, PDF (7/12 Pages) Micrel Semiconductor – 100kHz 2.5A Switching Regulator

Micrel, Inc.
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 = 45°C/W (TO-220)
then:
T_J = 70 + (1.24 × 45)
T_J = 126°C
This junction temperature is below the rated maximum of
150°C.
Grounding
Refer to Figure 5. Heavy lines indicate high current
paths.
V_IN
IN
SW
MIC2171
GND
FB
VC
Single point ground
Figure 5. 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 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.
May 2007
MIC2171
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 whether the device can operate in
discontinuous mode without initiating the internal device
current limit.
(1)
I_OUT
≤
⎜⎜⎝⎛
I_CL
2
⎟⎟⎠⎞^VIN(min)δ
V_OUT
(1a)
δ = ^{VOUT + VF − VIN(min)}
V_OUT + V_F
where:
I_CL = internal switch current limit
I_CL = 2.5A when δ < 50%
I_CL = 1.67 (2 – δ) when δ ≥ 50%
(Refer to Electrical Characteristics.)
I_OUT = maximum output current
V_IN(min) = minimum input voltage = V_IN – V_SW
δ = duty cycle
V_OUT = required output voltage
V_F = D1 forward voltage drop
For the example in Figure 1.
I_OUT = 0.25A
I_CL = 1.67 (2–0.662) = 2.24A
V_IN(min) = 4.18V
δ = 0.662
then:
V_OUT = 12.0V
V_F = 0.36V (@ .26A, 70°C)
⎜⎛ ^2.235 ⎟⎞ × 4.178 × 0.662
I_OUT ≤ ^{⎝ 2 âŽ}  ₁₂
I_OUT ≤ 0.258A
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_OUT.
(2)
L1 ≥ ^{(VINδ )2}
2P_OUT f_SW
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