FA7711V Datasheet, PDF (20/25 Pages) Fuji Electric – FUJI Power Supply Control IC

FA7711V
（8）Attention for driving bipolar transistor
If you use bipolar transistor as a switching device,
connect resistor R_B between Base of transistor and
OUT* pin, else there is a possibility of destroy the IC
by over current because OUT* pin driver does not
contain a current limit resistor. (Fig.13) Output current
of OUT* pins are below 40mA (continuous).
The connection of capacitor C_B is effective for speed
up the switching.
（9）ON/OFF control
ON/OFF control using CS pin is not available when
timer latch function is enable. In order to carry out
ON/OFF control with CS pin, CP pin should be
connected to GND so that timer latch function will be
disable as shown in Fig.14.
For ch.1 and ch.3, output pulses are disabled by
lowering CS pin voltage around zero. Ch.1 is
controlled by CS1 pin and ch.3 is controlled by CS3.
For ch.2, pulling up CS2 pin voltage to VREF voltage
disables output pulses.
Each channel will re-start operation with soft-start
function when CS pin is opened.
Fig.15 shows ON/OFF control method for Ch.1 and
Ch.3 when timer latched short protection is used for
all channels.
Please take care of that Ch.2 is not suitable for
ON/OFF control, because the error amplifier of Ch.2
sinks large current ( much larger than 2.7mA ) for
High Voltage of FB2 pin to stop switching of OUT2
pin.
The calculation example of ON/OFF circuit in Fig.15
is as follows.
VREF -> CS1 maximum current “ Ics1max ”
= VREF / R18 ( ex. 3.7V / 100k ohm = 37uA )
VREF -> CS3 maximum current “ Ics3max ”
= VREF / R38 ( ex. 3.7V / 100k ohm = 37uA )
Error Amp. Output Source Current “ ISOFB ”
= 185uA ( typ. ) for FB1 pin and FB3 pin
The base current ratio of QFB* and QCS* should be
same as the collector current ratio for each channel.
( above case : collector current ratio = 185 / 37 = 5
-> Base resistor ratio = R56 / R55 = R53 / R52 = 5 )
Base resistor voltage drop should be larger than
300mV in order to obtain good sharing of base
current for QFB* and QCS*.
Here, assuming hFE less than 100 of Q**,
each base current are over 1.85uA and over 0.37uA.
If R57 and R58 values to flow current over
( 1.85 uA + 0.37 uA ) are designed and
Fuji Electric Systems Co., Ltd.
AN-060E Rev.1.0
20
Jun-2010
R55 = R52 = 200k ohm and R56 = R53 = 1M ohm
are selected, base resistor voltage of R52, R53, R55
and R56 are over 370mV and these satisfy “ 300mV
condition ”. Therefore, this condition achieves normal
transistor operation.
Resistance of R57 and R58 should be smaller than
20% of above condition in order to obtain small hFE
of “ 20 ” which achieves low saturation voltage of Q**.
In this case, if Vcc = 17V (min.) and voltage drop
of R57 and R58 = Vcc - 2V,
R57 or R58
< 20% * (17V - 2V ) / (1.85 uA + 0.37 uA )
= 1.35 M ohm
VIN
VCC
18
PVCC
17
OUT RB
CB
9
GND
10
PGND
VIN
VCC
18
PVCC
17
CB
RB
OUT
9
GND
10
PGND
Fig.13
ON/OFF
CTRL
ch1
ch3
ch2
CCS1
4 VREF
11 CS1
CCS3
16 CS3
CCS2
14 CS2
CP 1
Fig.14
Timer latched Short Protection : not available
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