m51996ap Renesas Electronics Corporation., m51996ap Datasheet - Page 15

• Manufacturer Part Number: m51996ap
• Description: Switching Regulator Control
• Manufacturer: Renesas Electronics Corporation.

M51996AP/AFP
2. Oscillator operation when the SOFT (soft start) circuit is operating.
REJ03D0836-0201 Rev.2.01 Nov 14, 2007
Page 15 of 35
Figure 3 Schematic Diagram of Charging and Discharging Control Circuit for OSC Capacitor C
Output transistor is protected from rush current by CLM function at the start time of power on. SOFT terminal is
used to improve the rising response of the output voltage of power supply (prevention of overshooting).
The ON duration of output is kept constant, and the OFF duration is extended as the SOFT terminal voltage
becomes lower by the soft start circuit of this IC.
The maximum value of extension is set internally at approximately sixteen times of the maximum ON duration.
The features of this method are as follows :
(1) It is ideal for primary control as IC driving current is supplied from the third winding of the main transformer at
(2) It is possible to get a wide dynamic range for ON/OFF ratio by pulse-by-pulse current limit circuit.
(3) The response characteristics at power-on is not affected by input voltage as the pulse-by-pulse limit current
Figure 4 shows the circuit diagram of the soft start. If SOFT terminal voltage is low, T-OFF terminal voltage
becomes low and V
the start-up because constant ON duration is obtained from start-up.
value is not affected by the input voltage.
Figure 4 Circuit Diagram of SOFT Terminal Section and T-OFF Terminal Section
R
R
OFF
C
ON
F
T-OFF
terminal
SOFT
R
C
in equations (3) and (4) become low.
Terminal
SOFT
SOFT
To REG
M51996A
T-ON
T-OFF
CF
Note: Active when operation stops.
To REG
Terminal
Discharging transistor
IC's internal circuit
From VF
signal
Vz = 4.2 V
Q3
1/16
Q4
Note
Vz ≈ 4.2 V
Q2
Q1
SW2
Charging
SW1
Discharging
T-OFF
terminal
5.8 V
Switched by
charging and
discharging signal
GND
terminal
F