MIC2171_07 Datasheet, PDF (6/12 Pages) Micrel Semiconductor – 100kHz 2.5A Switching Regulator

Micrel, Inc.
Application Information
Soft Start
A diode-coupled capacitor from COMP to circuit ground
slows the output voltage rise at turn on (Figure 3).
V_IN
IN
MIC2171
COMP
D1
D2
R1
C1
C2
Figure 3. Soft Start
The additional time it takes for the error amplifier to
charge the capacitor corresponds to the time it takes the
output to reach regulation. Diode D1 discharges C1
when V_IN is removed.
Current Limit
V_IN
IN
SW
MIC2171
GND
FB
COMP
V_{O U T}
R1
Q1
C1
R2
R3
I_CL 0.6V/R2
C2
Note: Input and output
returns not common
Figure 4. Current Limit
The maximum current limit of the MIC2171 can be
reduced by adding a voltage clamp to the COMP output
(Figure 4). This feature can be useful in applications
requiring either a complete shutdown of Q1’s switching
action or a form of current fold-back limiting. This use of
the COMP output does not disable the oscillator,
amplifiers or other circuitry, therefore, the supply current
is never less than approximately 5mA.
Thermal Management
Although the MIC2171 family contains thermal protection
circuitry, for best reliability, avoid prolonged operation
with junction temperatures near the rated maximum.
The junction temperature is determined by first
calculating the power dissipation of the device. For the
MIC2171, the total power dissipation is the sum of the
device operating losses and power switch losses.
The device operating losses are the dc losses
associated with biasing all of the internal functions plus
May 2007
MIC2171
the losses of the power switch driver circuitry. The dc
losses are calculated from the supply voltage (V_IN) and
device supply current (I_Q).The MIC2171 supply current is
almost constant regardless of the supply voltage (see
“Electrical Characteristics”). The driver section losses
(not including the switch) are a function of supply
voltage, power switch current, and duty cycle.
P_{(bias+driver)} = (V_IN I_Q) + (V_IN(min) x I_SW x ∆I_IN)
where:
P_{(bias+driver)} = device operating losses
V_IN(min) = supply voltage = V_IN – V_SW
I_Q = typical quiescent supply current
I_CL = power switch current limit
∆I_IN = typical supply current increase
As a practical example refer to Figure 1.
V_IN = 5.0V
I_Q = 0.007A
I_CL = 2.21A
δ = 66.2% (0.662)
then:
V_IN(min) = 5.0V – (2.21 x 0.37) = 4.18V
P_{(bias+driver)} = (5 x 0.007) + (4.18 x 2.21 x 0.009)
P_{(bias+driver)} = 0.1W
Power switch dissipation calculations are greatly
simplified by making two assumptions which are usually
fairly accurate. First, the majority of losses in the power
switch are due to on-losses. To find these losses, assign
a resistance value to the collector/emitter terminals of
the device using the saturation voltage versus collector
current curves (see Typical Performance Character-
istics). Power switch losses are calculated by modeling
the switch as a resistor with the switch duty cycle
modifying the average power dissipation.
P_SW = (I_SW)² R_SW δ
where:
δ = duty cycle
δ = ^{VOUT + VF − VIN(min)}
V_OUT + V_F
V_SW = I_CL (R_SW)
V_OUT = output voltage
V_F = D1 forward voltage drop at I_OUT
From the Typical performance Characteristics:
then:
R_SW = 0.37Ω
P_SW = (2.21)² × 0.37 × 0.662
P_SW = 1.2W
P_(total) = 1.2 + 0.1
P_(total) = 1.3W
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