mic4682 Micrel Semiconductor, mic4682 Datasheet - Page 9

• Manufacturer Part Number: mic4682
• Description: Mic4682 Precision Current Limit So-8 Superswitcher? Buck Regulator
• Manufacturer: Micrel Semiconductor

Functional Diagram
Functional Description
The MIC4862 is a constant frequency, voltage mode-
switching regulator. Referring to the block diagram,
regulation is achieved when the feedback voltage is
equal to the band gap reference. The FB pin senses the
output voltage and feeds into the input of the Error Amp.
The output of the Error Amp produces a positive voltage
to compare with the 200kHz saw-tooth waveform. These
two signals are fed into the comparator to generate the
Pulse Width Modulation (PWM) signal to turn on and off
the internal switch. The duty cycle is defined as the time
the switch turns on divided by the period of the saw-
tooth oscillator. Initially, when power is applied to the IN
pin, the duty cycle is high because the feedback is close
to ground. As the output and feedback voltage start to
rise, the duty cycle decreases.
During the on time, current flows through the switch and
into the inductor until it reaches the peak current limit or
Micrel, Inc.
June 2007
SHDN
ISET
MIC4682
Oscillator
200kHz
parator
Com-
Error
Amp
Shutdown
Regulator
Thermal
Figure 1. MIC4682 Block Diagram
Internal
Reset
GND
1.23V
Bandgap
Reference
Current
Driver
Limit
9
the maximum duty cycle which turns the switch off. The
external resistor at the ISET pin sets the peak current
limit. The maximum duty cycle is controlled by the Reset
circuitry. At this time, energy is stored in the inductor.
The current charges the output capacitor and supplies to
the load. The Schottky diode is reversed bias.
When the internal switch is off, the stored energy in the
inductor starts to collapse. The voltage across the
inductor reverses polarity and the inductor current starts
to decrease. The Schottky diode clamps the switch
voltage from going too negative and provides the path
for the inductor current. During the off time, the inductor
and the output capacitor provide current to the load.
An internal regulator provides power to the control
circuitry and the thermal protection circuitry turns off the
internal switch when the junction temperature exceeds
about 160°C.
Switch
IN
2A
V
IN
SW
FB
R1 R2
R2
V
V
FB
OUT
=
=
=
⎛
⎜
⎝
=
1.23V
V
V
V
⎛
⎜
⎝
OUT
FB
FB
V
R1
V
OUT
FB
⎛
⎜
⎝
– 1
R2
R1
−
⎞
⎟
⎠
R1
R2
+
1
⎞
⎟
⎠
1
⎞
⎟
⎠
V
C
OUT
OUT
M9999-061507
MIC4682