FA5695 Datasheet, PDF (14/21 Pages) Fuji Electric – Fuji Switching Power Supply Control IC

FA5695
(4)FB short-circuit/open detection circuit
(Standby circuit)
In the PFC circuit of booster type, if feedback voltage is not properly
provided to the FB terminal due to short-circuit or open-circuit around R1,
R2, the error amplifier cannot control the constant voltage and the output
voltage abnormally rises. In such a case, the overvoltage protection
circuit also cannot operate because the detection of the output voltage is
abnormal.
To avoid such situation, this IC is equipped with FB short-circuit
detection circuit.
This circuit is composed of the reference voltage of 0.3V (typ.) and
comparator (SP), and when the input voltage of the FB terminal
becomes 0.3V or lower due to such trouble as short-circuit of R2 or
opening of R1, the output of the comparator (SP) inverts to stop the
output of the IC and the IC stops operation resulting in standby state.
Once the voltage of the FB terminal decreases to almost zero and the
output of the IC stops, and then when the voltage of the FB terminal
returns to 0.3V or higher, the IC resumes from the standby state and the
OUT pulse restarts.
Vout
R1
FB
1
R2
C3
VREF(2.5V)
COMP
ERRAMP
Short Ｃｏｍｐ
SP
Vthfb(0.3V)
Vsovp(1.08*VREF)
OVP
ＯＶＰ Ｃｏｍｐ
Vdovp(1.05*VREF)
RAMP OSC
Ｄｙｎａｍｉｃ ＯＶＰ
(5) Ramp oscillating circuit
The ramp oscillating circuit receives signal from the zero current
detection circuit or restart circuit, and outputs the set signal of F/F for
OUT output and saw tooth wave signal for deciding the duty of the PWM
comparator.
(5-1) Maximum frequency limiting
The switching frequency of PFC in the critical mode has
characteristic to rise at light load.
FA55901/91 has the maximum frequency limiting function to improve
the efficiency at light load and limits the switching frequency to Fmax
(Hz). (Fig. 10)
The maximum frequency Fmax depends on the resistance connected
between the RT terminal and GND.
When the switching frequency is lower than Fmax, the zero level of
the inductor current is detected and MOSFET is turned on after the zero
current detection delay Tzcd to adjust turning on take place at the
bottom of Vds wave, as shown in Fig. 11.
In case of light load where the switching frequency is limited to Fmax,
the zero level of the inductor current is detected and no turn-on occurs
after the zero current detection delay, but turn-on occurs at the cycle of
1/Fmax, as shown in Fig. 12.
Fig.8 FB pin circuit
L1
D1
C1
Q1
C2
Rs
R4
IS
5
1.4kΩ
C6
ZCD.comp
21.9kΩ ^20mV
C7
R5
3
RT
Delay
RAMP
OSC
46.7kΩ
1.5V
100mV
OCP.comp
OCP
Fig.9 Current detection circuit
OUT用
F/F
PWM
Comp
(6) Current detection circuit
The current detection circuit is composed of zero current detection
and overcurrent detection. (Fig. 9)
S_Sw_Wi_周tc_波hi_数ng frequency
(6-1) Zero current detection circuit
This IC performs the switching operation by self-oscillation in critical
mode instead of the oscillator with the fixed frequency. The zero current
detection circuit ZCD. Comp detects that the inductor current becomes
zero to perform the critical mode operation.
With the zero current detection, the voltage across the current
detection resistor Rs connected to the GND line is fed to the IS terminal,
and it is compared by the zero current detection comparator, and when it
becomes -4mV or more, the inductor current is regarded as zero level.
When the zero level is detected, the delay Tzcd is generated by the
zero cross delay detection circuit, and then set the F/F for OUT to make
MOSFET turn on.
Fmax
負_Lo荷_ad
Fig.10 maximum frequency limiting
Fuji Electric Co., Ltd.
AN-073E Rev.1.1
April-2011
http://www.fujielectric.co.jp/products/semiconductor/
14