FA5626 FUJI [Fuji Electric], FA5626 Datasheet - Page 21

• Manufacturer Part Number: FA5626
• Description: Fuji Switching Power Supply Control IC
• Manufacturer: FUJI [Fuji Electric]

FA5626
(12)Soft-start
This IC has an adjustable soft start function by LAT pin capacitor.
Fig. 16 shows the soft-start timing chart at start up. (1)(2)(3) are soft start period in Fig.16.
When VCC voltage reaches uvlo on threshold voltage , LAT pin voltage rises gradually, and switching starts at LAT pin
voltage 2.1 V.
In period (1), a minimum ON width pulse are output 32 times after switching has started. The minimum ON width pulses
avoids Vds surge voltage of power MOSFET at start up.
In period (2), LAT pin voltage is discharged from 2.5 V to 2.0 V by constant current (70uA). In this period, the minimum ON
width pluses are output continuously.
Period (3) is effective soft-start period.
In period (3), the pulse width gradually widen from minimum ON width.
The soft-start time can be adjusted by capacitor connected to LAT pin.
Approximate effective soft start time (period(3)) can be calculated using the following expression.
Tss = 0.4×CLAT / Ilat
Tss :Soft-start time [sec]
CLAT: Capacitor connected to LAT pin [uF]
Ilat :LAT pin source current [uA] (70uA typ.)
(4) PWM operation start
Fig.16 Soft-start operation
Fuji Electric Co., Ltd.
AN-071E Rev.1.2
http://www.fujielectric.co.jp/products/semiconductor/
21
April-2011