FA5695 Datasheet, PDF (17/21 Pages) Fuji Electric – Fuji Switching Power Supply Control IC

FA5695
(3) Terminal No. 3 (RT terminal)
Functions
(i) Set maximum on time
(ii) Set maximum oscillation frequency
(iii) Set delay time for zero current detection
Application
(i) Set maximum on time
On time Ton in each switching cycle with input and output conditions is
theoretically expressed by the following formula.
2  Lp  Po
Ton 
Vac²  
Input Voltage (Vrms): Vac
Inductor (H): Lp
Maximum Output Power (W): Po
Efficiency: η
The maximum on time Tonmax must be set equal to or more than the on
time at minimum input voltage Vac (min) at which the on time is
maximum. In soft start, the maximum on time is limited to 80%, and
therefore, the maximum on time should be set as shown by the following
formula.
2  Lp  Po
Ton max 
Vac(min)²    0.8
(ii) Set maximum oscillation frequency
To improve the efficiency at light load, FA5695 limits switching frequency
at light load to Fmax (Hz). The maximum frequency Fmax depends on
the resistance connected between RT terminal and GND.
- Wiring
Connect R5 between RT and GND as shown in Fig. 17.
For the resistance dependency of the maximum on time and maximum
oscillation frequency, see Chapter 7. Characteristic Curve.
The current sourced from the RT terminal changes depending on the
resistance connected. When R5 is relatively large, for example, 82kΩ,
the current is about 10uA. When the current is relatively small, it is
recommended to connect a capacitor of about 0.01uF in parallel to the
resistor to stabilize the RT voltage, as shown in the figure.
(iii) Set delay time for zero current detection
Select a resistance value so as to set such delay time that MOSFET will
turn on at the bottom of the vds waveform. (Vds is almost 0V)
However, very smaller resistance makes maximum ON width Tonmax
narrower and maximum output power fewer. Avoid choosing the
resistance which gives narrower ON width than the result of above
equation.
If the resistance gives much longer delay time, we recommend an
adjustment of the delay time by the resonant capacitor.
(4) Terminal No. 4 (OVP terminal)
Function
(i) Set detection level of OVP
It sets a voltage which detects an over voltage of output and which stop
switching operation.
For avoiding malfunction by noise, the recommended resistance of the
detection circuit is 10Mohm or smaller.
2.5V(typ)
RAMP Delay
_OSC
3
C7
R5
Fig.17 RT pin circuit
Vout
R1
OVP
4
R2
C3
Fig.18 OVP pin circuit
Fuji Electric Co., Ltd.
AN-073E Rev.1.1
April-2011
http://www.fujielectric.co.jp/products/semiconductor/
17