MIC2172/3172
Micrel
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.
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 ) and device supply current (I ).
Soft Start
A diode-coupled capacitor from COMP to circuit ground
slows the output voltage rise at turn on (figure 8).
IN
Q
TheMIC2172/3172supplycurrentisalmostconstantregard-
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.
VIN
VIN
MIC2172/3172
0.004+δ
COMP
P
= V
(
I
+ V I
IN SW
)
(bias+driver)
IN Q
50
D1
D2
C1
R1
C2
where:
P
V
= device operating losses
(bias+driver)
= supply voltage
IN
Figure 8. Soft Start
I = quiescent supply current
Q
Theadditionaltimeittakesfortheerroramplifiertochargethe
capacitor corresponds to the time it takes the output to reach
regulation. Diode D1 discharges C1 when V is removed.
I
= power switch current
(see “ Design Hints: Switch Current
Calculations”)
SW
IN
Current Limit
δ = duty cycle
FordesignsdemandinglessoutputcurrentthantheMIC2172/
3172 is capable of delivering, P GND 1 can be left open
reducing the current capability of Q1 by one-half.
V
+ V – V
OUT
F
IN
δ =
V
+ V
OUT
F
V
= output voltage
OUT
VIN
V = D1 forward voltage drop
F
VIN
VSW
As a practical example refer to figure 1.
MIC2172/3172
V
= 5.0V
IN
VOUT
FB
P1 P2 S COMP
GND
I = 0.006A
Q
I
= 0.625A
SW
δ = 60% (0.6)
R1
ICL ≈ 0.6V/R2
R3
C2
Q1
Then:
C1
R2
Note: Input and output
returns not common.
0.004+0.6
50
P(bias+driver) = 5 × 0.006 + 5 0.625
(
)
Figure 9. Current Limit
P
= 0.068W
(bias+driver)
Alternatively,themaximumcurrentlimitoftheMIC2172/3172
can be reduced by adding a voltage clamp to the COMP
output (figure 9). This feature can be useful in applications
requiringeitheracompleteshutdownofQ1’sswitchingaction
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.
Power switch dissipation calculations are greatly simplified
bymakingtwoassumptionswhichareusuallyfairlyaccurate.
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
calculatedbymodelingtheswitchasaresistorwiththeswitch
duty cycle modifying the average power dissipation.
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.
2
P
= (I ) R
δ
SW
SW
SW
From the Typical performance Characteristics:
The junction temperature is determined by first calculating
the power dissipation of the device. For the MIC2172/3172,
R
= 1Ω
SW
4-22
1997
MICREL [ MICREL SEMICONDUCTOR ]
