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PROTECTION CIRCUIT FOR EXCESSIVE
VOLTAGE
The norminal voltage of Zener diode D2 is 22V
and the reverse voltage by leakage current is about
14.5V.
If the input voltage is under 22V, the Q1 stays off
and the gate of FET Q2 becomes low level and
turns on main path. If the voltage is higher than
22V or excessive impulsive voltage is supplied, D2
becomes breakdown and turns on Q1 and turns off
Q2 by raising the gate voltage to high level.
MICROPOWER VOLTAGE REGULATOR
When power is on and off, pin 5 of U1 (LP2951)
detects the output voltage and keeps high/low
level and the reset pin 18 of MPU becomes active
by the voltage level of pin 5.
The error flag becomes high impedance state at the
input voltage of under 1.3V and changes the state
to low level when the voltage reaches to 5V and
then resets MPU. Also the error flag becomes low
state at the output voltage of under 4.75V and
resets MPU.
By this way MPU can keep the input voltage of
CMOS or TTL device stable and prevents
malfunctions.
Samsung Electronics
STEP-DOWN REGULATOR
The step-down buck converter is composed of U2
(LM2576T-ADJ), L1, D4 and C6. And U2 choppers
the output by switching operation in 52 kHz
internally fixed frequency. Pin 5 is connected to
pin 3 of MPU and becomes on/off and controls the
output voltage by adding/subtracting the
feedback voltage of pin 4. D4 is a diode to
flywheel the energy of L1 when the switching TR
of Buck circuit is off.
CONSTANT CURRENT CIRCUIT
As the constant current circuit which is adapted
with PCS/RC, is just one directional, it can be
used as power source of OP-amp.
By applying and fixing input bias current to N.1
terminal of U3, the sink current is controlled by
shunt resistor R7, R8, R12 and RR of the battery
pack.
Circuit Description
4-9
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