SWITCHING REGULATOR CONTROL
M51995AP/FP
MITSUBISHI (Dig./Ana. INTERFACE)
( / 27 )
20
It is not necessary that the induced voltage is settled higher
than the operation start-up voltage Vcc(START)
,and the high gate
drive voltage causes high gate dissipation,on the other hand,too
low gate drive voltage does not make the MOS-FET fully on-
state or the saturation state.
It is recommend to use the third winding of "forward winding"
or "positive polarity" as shown in Fig.25,when the DC source
voltages at both the IC operation start and stop must be settled
at the specified values.
The input voltage(VIN(START)),at which the IC operation starts,is
decided by R1 and R2 utilizing the low start-up current
characteristics of type M51995AP/FP.
The input voltage(VIN(STOP)),at which the IC operation stops,is
decided by the ratio of third winding of transformer.
The VIN(START) and VIN(STOP) are given by following equations.
(2)The start-up circuit when it is not necessary to set the
start and stop input voltage
It is required that the VIN(START) must be higher than VIN(STOP).
When the third winding is the "fly back winding" or "reverse
polarity",the VIN(START) can be fixed,however,VIN(STOP) can not
be settled by this system,so the auxiliary circuit is required.
To avoid the abnormal IC operation,it is recommended to
design the Vcc is not vary abruptly and has few spike
voltage,which is induced from the stray capacity between the
winding of main transformer.
To reduce the spike voltage,the Cvcc,which is connected
between Vcc and ground,must have the good high frequency
characteristics.
To design the conductor-pattern on PC board,following cautions
must be considered as shown in Fig.26.
(a)To separate the emitter line of type M51995A from the GND
line of the IC
(b)The locate the CVCC as near as possible to type M51995A
and connect directly
(c)To separate the collector line of type M51995A from the Vcc
line of the IC
(d)To connect the ground terminals of peripheral parts of ICs to
GND of type M51995A as short as possible
(3)Notice to the Vcc,Vcc line and GND line
M51995A
THIRD WINDING OF
TRANSFORMER
PRIMARY WINDING
RECTIFIED DC
VOLTAGE FROM
Fig.25 Start-up circuit diagram when it is not
necessary to set the start and stop input voltage
VIN(START) R1 • ICCL + ( + 1) • Vcc(START)
R2
R1
~...............(9)
VIN(STOP) (Vcc(STOP)-VF) • ..........(10)
~NB
NP+2
1V'IN RIP(P-P)
where
ICCL is the operation start-up current of IC
Vcc(START) is the operation start-up voltage of IC
Vcc(STOP) is the operation stop voltage of IC
VF is the forward voltage of rectifier diode
V'IN(P-P) is the peak to peak ripple voltage of
Vcc terminal
~NP
NBV'IN RIP(P-P)
M51995A
Fig.26 How to design the conductor-pattern of type
M51995A on PC board(schematic example)
RCLM
MAIN
TRANSFORMER
THIRD
WINDING
(4)Power supply circuit for easy start-up
When IC start to operate,the voltage of the CVCC begins to
decrease till the CVCC becomes to be charged from the third
winding of main-transformer as the Icc of the IC increases
abruptly.In case shown in Fig.24 and 25,some "unstable start-
up" or "fall to start-up" may happen, as the charging interval of
CVCC is very short duration;that is the charging does occur only
the duration while the induced winding voltage is higher than
the CVCC voltage,if the induced winding voltage is nearly equal
to the "operation-stop voltage" of type M51995.
It is recommended to use the 10 to 47µF for CVCC1,and about 5
times capacity bigger than CVCC1 for CVCC2 in Fig.27.