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6
Functional description and component design
6.1
Startup time
Startup time is counted from applying input voltage to IC turn on. ICE3BRXX65J(Z)(G) has a startup cell
which is connected to input bulk capacitor. When there is input voltage, the startup cell will act as a constant
current source to charge up the Vcc capacitor and supply energy to the IC. When the Vcc capacitor reaches
the Vcc_on threshold 18V, the IC turns on. Then the startup cell is turned off and the Vcc is supplied by the
auxiliary winding. Start up time is independent from the AC line input voltage and it can be calculated by the
equation (1). Figure 6 shows the start up time of 85Vac line input.
⋅
V
C
=
VCCon
Vcc
t
STARTUP
I
VCCch
arg e
3
where, I
: constant current to charge Vcc capacitor ( 0.7mA ),
VCCcharge3
V
: IC turns on threshold ( 18V ), C
VCCon
Pls refer to the datasheet for the symbol used in the equation.
V
CS
V
CC
V
FB
V
BA
Figure 6
The startup delay time at AC line input voltage of 85Vac
Precaution : For a typical application, start up should be VCC ramps up first, other pin (such as FB pin)
voltage will follow VCC voltage to ramp up. It is recommended not to have any voltage on other
pins (such as FB; BA and CS) before VCC ramps up.
6.1.1
Vcc capacitor
The minimum value of the Vcc capacitor is determined by voltage drop during the soft start time. The formula
is expressed in equation (2).
⋅
I
t
2
VCC
sup
2
ss
=
⋅
C
VCC
V
3
CChys
where, I
: IC consumption current ( 4.2mA ), t
VCCsup2
V
: Vcc turn-on/off hysteresis voltage ( 7.5V )
CChys
Therefore, the minimum Vcc capacitance can be 7.4µF. In order to give more margins, 22uF is taken for the
design. The startup time t
filtering capacitor is always needed to add as near as possible to the Vcc pin to filter the high frequency
noise. The filter capacitor C6 and the auxiliary series resistor R2 form a R-C filter which can effectively filter
the transformer switching noise from auxiliary winding going into the IC.
Application Note
VCC
0.54s
is then 0.528s. The measured start up time is 0.54s (Figure 6). A 0.1uF
STARTUP
: Vcc capacitor
Channel 1; C1 : CS Voltage (V
Channel 2; C2 : Supply Voltage (V
Channel 3; C3 : FB voltage (V
Channel 4; C4 : BA Voltage (V
Startup time = 0.54s
Startup delay time @ Vin=85Vac & 12W
load
: soft start time ( 20ms ),
ss
9
ICE3BRXX65J(Z)(G)
(1)
)
CS
)
CC
)
FB
)
BA
(2)
2010-06-20
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