Icom BC-119N Service Manual page 5

SECTION 2 CIRCUIT DESCRIPTION
2-1 DC-DC CONVERTOR CIRCUIT
(STEP-UP/DOWN CONVERTOR)
The DC-DC convertor circuit regulates the charging voltage
from an AC adapter or a power supply to appropriate values
for various battery pack.
The applied voltage from an AC adapter or a power supply is
regulated at the step-up and down convertor circuit (Q5, D6,
D7, L2). The regulated voltage is passed through the con-
stant-current circuit.
• STEP-DOWN CONVERTOR CIRCUIT
The step-down convertor circuit activates when the charging
voltage is lower than the applied voltage from an AC adapter
or a power supply.
The applied voltage is chopped (turn ON and OFF) by the
step-down convertor circuit. The chopper voltage (square
wave) is passed through the smoothing circuit (C26) for con-
version to DC voltage.
The step-down convertor circuit is controlled by the CPU
(IC2) via the swtching regulator (IC5, Q8, Q9) and driver (Q3,
Q4, D4, D5) circuits.
• STEP-UP CONVERTOR CIRCUIT
The step-up convertor circuit activates when the charging
voltage is higher than the applied voltage from an AC adapter
or a power supply.
The voltage passed through the step-up convertor circuit is
chopped by the step-up convertor circuit.
Q5 is composed of two FETs (FET1 is pins 1–4, FET2 is pins
5–8). While FET2 is turned OFF, the same voltage as the
applied voltage is output from the step-up convertor circuit.
However, while FET2 is turned ON, stored energy at the coil
(L2) is driven into flywheel diode (D7). In other words, a back
electromotive force occurs when FET2 is turned ON. This
electromotive force is used for the step-up voltage.
• STEP-UP/DOWN CONVERTOR CIRCUIT
Power supply
or
AC adapter
From switching regulator
(IC5, pins 10, 11)
Q3, D5 Q4, D4
DRIVER
2 - 1
2-2 CONSTANT-CURRENT CIRCUIT
The constant-courrent circuit consists of DC-DC convertor
circuit (Q5, D6, D7, L2), IC4, IC5, R24, R44, and R45.
The current detector circuit (R24) detects a charging current
and generates a voltage in proportion to the current. The
generated voltage is applied to the amplifier section of IC4
(IC4a, pin 2).
The amplified voltage is applied to the CPU (IC2, pin 5) and
the swtiching regulator (IC5, pin 1). The CPU (IC2) is output
PWM signal from pin 2 to obtain stable output current. The
signal is applied to the IC5 via the smoothing circuit (Q8,
R32, C35) to convert to DC voltage. The DC voltage is
applied to the swtiching regulator circuit (IC5, pins 2, 3, 15).
The output current from switching regulator (IC5, pin 16)
passes through the R44 and R45 to limit the maximum volt-
age. The output voltage is applied to the driver (Q3, Q4, D4,
D5), and then applied to the DC-DC convertor circuit to
obtain constant current.
2-3 REGULATOR CIRCUIT
Part of voltage from an AC adapter or a power supply are
applied to the 1st regulator circuit (Q1,Q12) to convert to 12
V voltage signal. The 12 V signal is applied to the LED (DS1)
and buffer amplifier (IC4b, pin 8).
Part of 12 V signals are applied to the 2nd regulator circuit
(IC1, R25) to convert to the 5.15 V voltage signal. The 5.15
V signal is applied to the CPU (IC2, pin 25), voltage detector
(IC3, pin 2), etc.
R44
D7
L2
D6
to swiching regulator
(IC5, pin 16)
to adapter
to CPU (IC2, pin 7)