MIC2171 Micrel Semiconductor, MIC2171 Datasheet - Page 6

• Manufacturer Part Number: MIC2171
• Description: 100kHz 2.5A Switching Regulator Preliminary Information
• Manufacturer: Micrel Semiconductor

MIC2171
Applications Information
Soft Start
A diode-coupled capacitor from COMP to circuit ground
slows the output voltage rise at turn on (Figure 3).
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
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.
Q1
V
IN
C1
R1
R2
V
IN
GND
D1
Figure 4. Current Limit
MIC2171
Figure 3. Soft Start
IN
COMP
D2
C1
SW
FB
R3
C2
MIC2171
Note: Input and output
IN
I
CL
COMP
returns not common.
0.6V/R2
R1
C2
IN
is removed.
V
OUT
4-8
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
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.
where:
As a practical example refer to Figure 1.
Then:
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).
calculated by modeling the switch as a resistor with the switch
duty cycle modifying the average power dissipation.
where:
From the Typical performance Characteristics:
Then:
= duty cycle
P
V
I
I
V
I
I
P
P
R
P
P
P
P
P
P
V
Q
CL
Q
CL
(bias+driver)
I
(bias
(bias+driver)
IN(min)
IN
IN(min)
(bias+driver)
SW
SW
SW)
(total)
(total)
= 66.2% (0.662)
SW
IN
= typical quiescent supply current
= 0.007A
= power switch current limit
= 2.21A
= 5.0V
= typical supply current increase
= (I
= (2.21)
= 0.37
= 1.2W
V
V
V
= 1.2 + 0.1
= 1.3W
SW
OUT
F
=
driver)
= supply voltage = V
SW
= 5 – (2.21
= D1 forward voltage drop at I
V
= I
OUT
)
= output voltage
2
= device operating losses
= 0.1W
= V I
2
CL
R
= (
V
SW
+ V – V
IN Q
(R
0.37
OUT
5
SW
F
IN
0.007) + (4.18
+ V
) and device supply current (I
)
0.662
F
V
IN(min)
0.37) = 4.18V
IN(min)
Power switch losses are
IN
– V
SW
I
SW
OUT
2.21
I
IN
Micrel
.009)
1997
Q
).