MIC3172YN Micrel Inc, MIC3172YN Datasheet - Page 12

• Manufacturer Part Number: MIC3172YN
• Description: IC REG SW 100KHZ 1.25A 8-DIP
• Manufacturer: Micrel Inc
• Type: Step-Down (Buck), Step-Up (Boost), Inverting, Cukr

Specifications of MIC3172YN

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Current - Output: 1.25A
• Frequency - Switching: 100kHz
• Voltage - Input: 3 ~ 40 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Through Hole
• Package / Case: 8-DIP (0.300", 7.62mm)
• Power - Output: 302mW
• Primary Input Voltage: 40V
• No. Of Outputs: 1
• Output Current: 3A
• No. Of Pins: 8
• Operating Temperature Range: -40°C To +85°C
• Supply Voltage Range: 3V To 40V
• Termination Type: Through Hole
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Output: -
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant, Lead free / RoHS Compliant
• Other names: 576-2255; MIC3172YN

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).
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
Another soft start circuit is shown in figure 8A.
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.
reduces the duty cycle at the V
time of the output. Increasing the value of C1 causes
the output voltage to rise more slowly.
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.
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.
Micrel
April 2006
IN
is removed.
Figure 8a. Additional Soft Start Circuit
This increased voltage on the FB pin
Figure 8. Soft Start
SW
pin, limiting the turn-on
Diode D1 is
The
12
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.
Alternatively,
MIC2172/3172 can be reduced by adding a voltage
Figure 8b. Without Soft Start
the
Figure 8c. With Soft Start
Figure 9. Current Limit
maximum
current
MIC2172/3172
[phone]
M9999-041806
limit
of
the