FA7703 Datasheet, PDF (18/21 Pages) Fuji Electric – FUJI Power Supply Control IC

FA7703/04
FA7703/7704
(7)Loss Calculation
Since it is difficult to measure IC loss directly,
the calculation to obtain the approximate loss of
the IC connected directly to a MOSFET is
described below.
When the supply voltage is VCC, the current
consumption of the IC is ICC1, the total input
gate charge of the driven MOSFET is Qg and the
switching frequency is fsw, the total loss Pd of
the IC can be calculated by:
Pd ≈ VCC ×(ICC1+Qg×fsw).
The value in this expression is influenced by
the effects of the dependency of supply voltage,
the characteristics of temperature, or the
tolerance of parameter. Therefore, evaluate the
appropriateness of IC loss sufficiently
considering the range of values of above
parameters under all conditions.
Example)
ICC1=1.8mA for VCC=6.0V in the case of a
typical IC from the characteristics curve.
Qg=6nC, fsw=190kHz, the IC loss ”Pd” is as
follows.
Pd ≈ 6.0× (1.6mA+6nC×190kHz) ≈17.6mW
if two MOSFETs are driven under the same
condition for 2 channels, Pd is as follows:
Pd ≈ 6.0×{1.8mA+2× (6nC×190kHz)}=24.5mW
(8)Performance of output stage
The performance of the output stage of
FA7703/04 is the maximum sink current of
150mA and the maximum source current of
400mA. The switching element externally
attached to FA7703/04 might affect switching
speed of the ICs. Cautious care must be taken
about it especially in high frequency operation. If
the performance of the ICs is not sufficient for
your design, consider adding a buffer circuit to
improve the performance.
OUT terminal within 50mA(continuous) in
operation. If you want to speed up the switching
speed, it is quite effective to install a capacitor
CB in addition.
power supply
9
Vcc
OUT1
CB
2
10 8
RB
GND
7
Fig.11
(10)ON/OFF control
FA7703/04 can be turned ON/OFF at CS
terminal by external signal. The way to conduct
ON/OFF control is also shown in Fig. 12. When
the voltage of CS terminal is below the threshold
voltage, the duty becomes 0% and the output
turns OFF. The current consumption in this case
is 1.3mA(typ.). To switch the ICs ON, just turning
CS terminal open, then the soft start function
restarts, and the output turns back ON. ON/OFF
control at CS terminal is used for both channel,
and the control of each channel is not allowed.
ON/OFF
Oscillation
output
DT terminal
voltage
12
Error Amplifier
output
2
CS terminal
voltage
DT terminal
voltage
6
Error Amplifier
output
+
-
+
+
+
-
+
+
7
output
output
PW M.Comp
Fig.12
(9)In the case of bipolar driving
If using a bipolar transistor as the switching
element, there is a possibility of the damage
from burnout due to excessive current flow
because the ICs doesn’t contain an internal
limiter resistance. Therefore, be sure to install a
base resistor. (Fig. 11) In the case of driving a
bipolar transistor, control the output current of
Fuji Electric Systems Co., Ltd.
AN-057E Rev.1.0
Jun-2010
18
http://www.fujielectric.co.jp/fdt/scd/