Micrel
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).
MIC2172/3172
Figure 8b. Without Soft Start
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.
Another soft start circuit is shown in figure 8A. The
circuit uses capacitor C1 to control the output risetime by
providing feedback from the output to the FB pin. The
output voltage starts to rise when the MIC3172 regulator
starts switching. This is the dv/dt of the output will force
a current through capacitor C1, which flows through the
lower feedback resistor, R2, increasing the voltage on
the FB pin. This increased voltage on the FB pin
reduces the duty cycle at the V
SW
pin, limiting the turn-on
time of the output. Increasing the value of C1 causes
the output voltage to rise more slowly. Diode D1 is
reverse biased in normal operation and prevents C1
from appearing in parallel with the upper voltage divider
resistor, which would affect stability and transient
response. Zener diode D2 clamps the voltage seen by
the feedback pin and provides a discharge path for C1
when the power supply is turned off.
Figure 8c. With Soft Start
Current Limit
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.
Figure 8a. Additional Soft Start Circuit
This circuit only limits the dv/dt of the output when the
boost converter is running. It will not decrease the dv/dt
or the initial inrush caused by applying the input voltage.
Figure 8B shows the turn-on without a soft start circuit
and Figure 8C shows how the soft start circuit reduces
inrush and prevents output voltage overshoot.
April 2006
12
Figure 9. Current Limit
Alternatively, the maximum current limit of the
MIC2172/3172 can be reduced by adding a voltage
M9999-041806
(408) 955-1690
MICREL [ MICREL SEMICONDUCTOR ]
