MIC4682(2003) Ver la hoja de datos (PDF) - Micrel
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MIC4682 Datasheet PDF : 15 Pages
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MIC4682
Block Diagram
Micrel
SHDN
ISET
200kHz
Oscillator
Internal
Regulator
Thermal
Shutdown
Current
Limit
Com-
parator
Reset
Driver
VIN
IN
VOUT
=
VFB
R1
R2
+ 1
VFB = 1.23V
R1
=
R2
VOUT
VFB
− 1
R2 = R1
VOUT
VFB
– 1
SW
2A
Switch
R1
FB
Error
Amp
1.23V
R2
Bandgap
Reference
MIC4682
GND
Figure 1. MIC4682 Block Diagram
VOUT
COUT
Functional Description
The MIC4862 is a constant frequency, voltage mode-switch-
ing 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 compara-
tor 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 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.
October 2003
9
M0334-102203
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