MIC2171 Datasheet, PDF (6/10 Pages) Micrel Semiconductor – 100kHz 2.5A Switching Regulator Preliminary Information

MIC2171
Applications 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
Micrel
The device operating losses are the dc losses associated
with biasing all of the internal functions plus 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) ^{× I}_SW ^{× ∆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
IN
SW
MIC2171
GND
FB
COMP
V_OUT
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 com-
plete 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.
V_IN(min) = 5 – (2.21 × 0.37) = 4.18V
P_{(bias + driver)} = (5 × 0.007) + (4.18 × 2.21 × .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 satura-
tion voltage versus collector current curves (see Typical
Performance Characteristics). 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:
R_SW = 0.37Ω
Then:
P_SW = (2.21)² × 0.37 × 0.662
P_SW) = 1.2W
P_(total) = 1.2 + 0.1
P_(total) = 1.3W
4-8
1997