FA5590 FUJI [Fuji Electric], FA5590 Datasheet - Page 17

• Manufacturer Part Number: FA5590
• Description: FUJI Power Supply Control IC
• Manufacturer: FUJI [Fuji Electric]

(Reference)
The above is a reference example, and it should be decided by
sufficiently verifying with actual application circuit.
Functions
(i) Set maximum on time
(ii) Set maximum oscillation frequency
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.
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.
(ii) Set maximum oscillation frequency
To improve the efficiency at light load, FA5590/91 limits swithching
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 8. 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.
Function
(i) Set zero current detection delay time
Application
(i) Set zero current detection delay time
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)
- Wiring
Connect R6 between RTZC terminal and GND as shown in Fig. 18.
For the resistance dependency of the delay time, see chapter 8.
Characteristic Curve.
Like the RT terminal as mentioned the previous section, the sourced
current is small, and it is recommended to connect a capacitor of about
0.01uF in parallel to the resistor, as shown in the figure.
Example of application circuit：C4=0.1uF
(3) Terminal No. 3 (RT terminal)
(4) Terminal No. 4 (RTZC terminal)
AN-016E Rev.1.2
April-2011
Fuji Electric Co., Ltd.
Ton
Ton
max

2

Vac

Vac
Lp
2
(min)
2




Po
Lp
2


Po

C5=0.15uF
R3=68kΩ

0
.
8
FA5590N,FA5591N
17
http://www.fujielectric.co.jp/products/semiconductor/
2.5V(typ)
2.5V(typ)
Fig.17 RT pin circuit
Fig.18 RTZC pin circuit
C7
C8
3
4
R5
R6
RAMP
_OSC
Delay