1 - 4
1.2.3
Description of operation
(1) When the power is turned ON, C924 is charged by the starting resistors R903 and R934.
When this IC901 pin
voltage reaches 17.6V, the control circuit starts operation.
After the control circuit starts operation, power is attained by rectifying and smoothing the
coil voltage at the T901 auxiliary coil
-
.
(2) IC901 pin
OCP/FB terminal, oscillator and constant-voltage control circuit
The oscillator uses the HIC's internal CI charging/discharging. Pulse signals are generated
to determine the MOSFET OFF time. The constant-voltage control when the switching
power is configured fixes the MOSFET OFF time (50
µ
sec), and functions with the PRC
operation that changes the ON time and the pseudo-resonance operation.
The operation for no-constant voltage is shown in the figure below. When MOSFET is ON,
capacitor C1 in OSC is charged to the constant-voltage (5.6V). By passing the drain current I
P
to R906, a voltage drop (VR5) occurs at the pin
OCP/FB terminal. This voltage has the
same waveform as I
P
, and approximately the same voltage as VR5 is applied on the pin
.
When the pin
voltage reaches the threshold voltage 0.76V, Comp 1 operates, and MOSFET
turns OFF when the internal converter reverses. The C1 charge is released when MOSFET
turns OFF, and charging of C1 starts with the constant-current discharge circuit. The voltage
at both ends drops at the inclination determined by the capacitor in the OSC and the constant-
current discharge circuit. When the voltage at both ends of the capacitor in OSC drops to
approx. 1.2V, the oscillator output reverses again and MOSFET turns ON. At this time, the
capacitor in OSC is rapidly charged to 5.6V. MOSFET continues oscillation with a repetition of
these steps.
As explained above, the time determined by the VR5 (I
P
) inclination is the MOSFET ON time.
This fixed time is adjusted to 50
µ
sec by the constant-current discharge circuit.
To control the output, the F/B current passed to photocoupler IC903 is passed to R905 and
R935, and the voltage drop VR4 caused by this is applied on VR5. With this, the VR5 voltage
value (IP peak value) required to reverse Comp 1 is used as the control for the current mode
controlled by VR4 (FB current).
When MOSFET is OFF, the bias amount to just before MOSFET turns ON is lowered to approx.
half by the active lowpass filter's 0.8mA (typ.) constant-current bias circuit. As a result, the
surge noise that occurs when MOSFET turns ON is absorbed by C909.
Oscillation operation during constant-voltage control
5V
Oscillator voltage
0V
0.76V
Voltage at both ends of C5
0V
Oscillator output
0
MOSFET
Direct-current bias caused by photo-transistor
ON
ON
OFF
OSC
PWM Latch
S
Q
R
Q
S
R
INH Latch
Comp.1
+
-
+
-
Comp.2
Vth(1)
Vth(2)
Drive
Reg1
1
2
5
3
D
PC
R2
VR5
VR4
R4
R5
C5
OCP/FB/Tri
GND
S
Summary of Contents for NEC MultiSync LCD1550X LCD1550X LCD1550X
Page 88: ...SCHEMATIC DIAGRAM POWER LCD1550X Normal Power Save 1ch Pin 4 2ch Pin 1 3ch Pin 2 4ch Pin 5...
Page 89: ...SCHEMATIC DIAGRAM PWB MAIN POWER LCD1550X...
Page 90: ...SCHEMATIC DIAGRAM PWB MAIN INPUT LCD1550X...
Page 91: ...SCHEMATIC DIAGRAM PWB MAIN SYNC LCD1550X...
Page 92: ...SCHEMATIC DIAGRAM PWB MAIN TMDS LCD1550X...
Page 96: ...SCHEMATIC DIAGRAM PWB MAIN ASIC LCD1550X...
Page 97: ...SCHEMATIC DIAGRAM INVERTER LCD1550X 1ch Q701 B 2ch Q702 B...
Page 98: ...SCHEMATIC DIAGRAM PWB SW LCD1550X...
Page 100: ...SCHEMATIC DIAGRAM POWER LCD1550X...
Page 101: ...SCHEMATIC DIAGRAM PWB MAIN POWER LCD1550X...
Page 102: ...SCHEMATIC DIAGRAM PWB MAIN INPUT LCD1550X...
Page 103: ...SCHEMATIC DIAGRAM PWB MAIN SYNC LCD1550X...
Page 104: ...SCHEMATIC DIAGRAM PWB MAIN TMDS LCD1550X...
Page 105: ...SCHEMATIC DIAGRAM PWB MAIN MC LCD1550X...
Page 106: ...SCHEMATIC DIAGRAM PWB MAIN ASIC LCD1550X...
Page 107: ...SCHEMATIC DIAGRAM INVERTER LCD1550X...
Page 108: ...SCHEMATIC DIAGRAM PWB SW LCD1550X...
Page 127: ...16 Document No VSPF A028 Bave Average brightness of 1 to 9 10 50 90 90 10 50 5 1 2 3 6 9 8 7 4...
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