MIC2171 Datasheet, PDF (7/10 Pages) Micrel Semiconductor – 100kHz 2.5A Switching Regulator Preliminary Information

MIC2171
Micrel
The junction temperature for any semiconductor is calculated mode is preferred because the feedback control of the
using the following:
converter is simpler.
T_J = T_A + P_(total) θ_JA
Where:
When L1 discharges its current completely during the MIC2171
off-time, it is operating in discontinuous mode.
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
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


V_IN(min) δ
V_OUT
(1a) δ = ^{VOUT + VF – VIN(min)}
V_OUT + V_F
Refer to Figure 5. Heavy lines indicate high current paths. ^Where:
V_IN
IN
SW
MIC2171
FB
GND COMP
I_CL = internal switch current limit
I_CL = 2.5A when δ < 50%
I_CL = 1.67 (2 – δ) when δ ≥ 50%
4
(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.
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 prop-
erly.
Boost Conversion
I_OUT = 0.25A
I_CL = 1.67 (2–0.662) = 2.24A
V_IN(min) = 4.18V
δ = 0.662
V_OUT = 12.0V
V_F = 0.36V (@ .26A, 70°C)
Then:
I_OUT
≤
 2.2235
× 4.178 × 0.662
12
I_OUT ≤ 0.258A
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_OUT.
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
( ) _{(2) L1 ≥}
V_IN δ ²
2 P_OUT f_SW
Where:
P_OUT = 12 × 0.25 = 3W
f_SW = 1×10⁵Hz (100kHz)
1997
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