MIC2172_05 Datasheet, PDF (10/17 Pages) Micrel Semiconductor – 100kHz 1.25A Switching Regulators

MIC2172/3172
Micrel, Inc.
Figure 7 shows how one or more MIC2172s can be locked to
an external reference frequency. The slaves lock to the
negative (falling edge) of the external reference waveform.
Soft Start
A diode-coupled capacitor from COMP to circuit ground
slows the output voltage rise at turn on (figure 8).
V_IN
V_IN
MIC2172/3172
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 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 MIC2172/3172 supply current is almost constant regard-
less 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.
COMP
D1
D2
R1
( ) ^P_{(bias+driver)} ⁼
V_IN I_Q
+ V_IN
_ISW

0. 004
50
+
δ



where:
C1
C2
Figure 8. 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
P_{(bias+driver)} = device operating losses
V_IN = supply voltage
I_Q = quiescent supply current
I_SW = power switch current
(see “ Design Hints: Switch Current
Calculations”)
δ = duty cycle
For designs demanding less output current than the MIC2172/
3172 is capable of delivering, P GND 1 can be left open
reducing the current capability of Q1 by one-half.
δ = ^{VOUT + VF ± VIN}
V_OUT + V_F
V_IN
V_IN
V_SW
V_OUT = output voltage
V_F = D1 forward voltage drop
As a practical example refer to figure 1.
MIC2172/3172
GND
FB
P1 P2 S COMP
V_OUT
R1
Q1
C1
R2
R3
I_CL ≈ 0.6V/R2
C2
Note: Input and output
returns not common.
Figure 9. Current Limit
Alternatively, the maximum current limit of the MIC2172/3172
can be reduced by adding a voltage clamp to the COMP
output (figure 9). 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 MIC2172/3172 family contains thermal protec-
tion 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 MIC2172/3172,
V_IN = 5.0V
I_Q = 0.006A
I_SW = 0.625A
δ = 60% (0.6)
Then:
P_{(bias+driver)}
= (5 × 0.006) + 5
_{0. 625}

0.004 +
50
0.
6



P_{(bias+driver)} = 0.068W
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
From the Typical performance Characteristics:
R_SW = 1Ω
M9999-042205
10
April 2005