FA7711V Datasheet, PDF (18/25 Pages) Fuji Electric – FUJI Power Supply Control IC

FA7711V
9. Design Advice
（1）Setting the oscillation frequency
As described at Section 8-(2), “Description of Each
Circuit,” a desired oscillation frequency can be
determined by the value of the resistor connected to
the RT pin. When designing an oscillation frequency,
you can set any frequency between 200kHz and
800kHz. You can obtain the oscillation frequency
from the characteristic curve “Oscillation frequency
(fosc) vs. timing resistor resistance (RT)” or the value
can be approximately calculated by the following
expression.
fosc = 4.1×10³ × RT ^−0.905
RT
=
⎜⎜⎝⎛
4.1×10³
fosc
⎟⎟⎠⎞^1.105
fosc: oscillation frequency [kHz]
RT ：timing resistor [kΩ]
This expression, however, can be used for rough
calculation, the obtained value is not guaranteed.
The operation frequency varies due to the conditions
such as tolerance of the characteristics of the ICs,
influence of noises, or external discrete components.
When determining the values, examine the
effectiveness of the values in an actual circuit. The
timing resistor RT should be wired to the GND pin as
shortly as possible because the RT pin is a high
impedance pin and is easy affected by noises.
（2）Determining soft start period
The period from the start of charging the capacitor
CCS to widening n% of output duty cycle can be
roughly calculated by the following expression: (see
Fig. 5 for symbols)
For CS1:
ts1[ms] = RCS1⋅CCS1⋅ ln_⎜⎛ ^VREF _⎟⎞
⎝^{VREF −VCS1n} ⎠
For CS2
ts2[ms] = RCS2 ⋅CCS2 ⋅ ln_⎜⎛ ^VREF _⎟⎞
⎝^VCS2n ⎠
For CS3
ts3[ms] = RCS3 ⋅CCS3 ⋅ ln_⎜⎛ ^VREF _⎟⎞
⎝^{VREF −VCS3n} ⎠
CCS1,CCS2,CCS3 :
Capacitance connected to CS* pin [μF]
RCS1,RCS2,RCS3 :
Resistance connected to CS* pin [kΩ]
VCS*n represents the voltage of CS Pin in the
output duty of n%, and it changes according to
the operation frequency. The value is obtained
simply from the chart of “CS Pin voltage vs.
output duty cycle” characteristic curves.
Charging of CS Pin begins after UVLO is
cancelled. Note that the time from power-on of
Power supply to start of charging Ccs* is t0,
which is not zero as described in Fig. 8. Be
careful.
Fuji Electric Systems Co., Ltd.
AN-060E Rev.1.0
Jun-2010
To reset the soft start function, the voltage of CS
pin is discharged with internal switch triggered by
lowering the voltage of Power supply below the
voltage of UVLO (3.3V, typ.). If Power supply
restarts before the voltage is sufficiently
discharged, the soft start function might not
properly operate. Accordingly, cautious care must
be taken about it.
Vcc
Threshold voltge
(3.3V typ.)
VCSn
CS pin voltage
t0
ts
Fig.9
（3）Setting the maximum output duty
If you need to control the maximum output duty
in the DC-DC converter circuit, you can control
pulse width by connecting VREF pin to CS pin
divided with resistors, as described in Fig. 10.
The output duty of the voltage of CS pin in this
case changes according to the operation
frequency, as described in the chart of “CS pin
voltage vs. Duty cycle” characteristic curves. Set
the output duty accordingly based on your
required operation frequency.
When the maximum duty cycle is limited, CS pin
voltage at start-up is described in Fig. 11, and the
approximate value of soft start period can be
obtained by the following expressions:
4 VREF
R1
11 CS1
R2
CCS1
R
R2
1+R
2
ï½¥V
RE
F
R
R6
5+R
6
ï½¥V
RE
F
VCS1n
VCS3n
R4
4 VREF
14 CS2
R3
CCS2
Fig.10
CS1,3 pin voltae
VCS2n
R
R3
3+R
4
ï½¥V
R
E
F
4 VREF
R5
16 CS3
R6
CCS3
CS2 pin voltge
ｔs
ｔs
Fig.11
For CS1
ts1[ms]
=
R
0
⋅
CCS1
⋅
ln_⎜⎛
⎝
VCS1
VCS1 −VCS1n
⎟⎞
⎠
R0 = ^R1⋅R2
R1+ R2
VCS1 = ^R2 ⋅VREF
R1+ R2
For CS2
ts
2[ms]
=
R
0
⋅
CCS
2
⋅
ln_⎜⎛
⎝
VREF −VCS2
VCS2n −VCS2
⎟⎞
⎠
R0 = ^R3⋅R4
R3+ R4
VCS2 = ^R3 ⋅VREF
R3+ R4
18
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