SSC1S311A Ver la hoja de datos (PDF) - Sanken Electric co.,ltd.
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SSC1S311A Datasheet PDF : 28 Pages
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SSC1S311A
8. Operational Description
All the characteristic values given in this section are
typical values, unless they are specified as minimum or
maximum. Current polarities are defined as follows:
current going into the IC (sinking) is positive current
(+); current coming out of the IC (sourcing) is negative
current (−).
8.1. Startup Operation
8.1.1. Startup Period
Figure 8-1 shows VCC pin peripheral circuit.
The built-in startup circuit is connected to the ST pin,
and it generates a constant current, ICC(STARTUP) = –
3.1mA, to charge capacitor C2 connected to the VCC pin.
During this process, when the VCC pin voltage reaches
VCC(ON) = 15.1V, the control circuit starts operation.
After that, the startup circuit stops automatically, in
order to eliminate its own power consumption.
The approximate startup time, tSTART, is calculated as
follows:
t START
= C2 ×
VCC(ON)－ VCC(INT)
ICC(STARTUP)
,
(1)
where:
tSTART is the startup time in s, and
VCC(INT) is the initial voltage of the VCC pin in V.
VAC
D1
RST
T1
C1 P
4
ST
VCC 7
U1
GND 8
D2 R2
C2 VD
D
Figure 8-1. VCC Pin Periheral Circuit
8.1.2. Undervoltage Lockout (UVLO)
Circuit
Figure 8-2 shows the relationship of VCC and ICC.
When the VCC pin voltage increases to VCC(ON) = 15.1 V,
the control circuit starts operation and the circuit current,
ICC, increases. In operation, when the VCC pin voltage
decreases to VCC(OFF) = 9.4 V, the control circuit stops
operation, by the UVLO (Undervoltage Lockout) circuit,
and reverts to the state before startup.
The voltage rectified the auxiliary winding voltage,
VD, in Figure 8-1 becomes a power source to the control
circuit after the operation start.
The VCC pin voltage should become as follows within
the specification of input voltage range and the output
load range of power supply, taking account of the
winding turns of the D winging. The target voltage of
the VCC pin voltage is about 20 V.
12.5(V)(V
)
CC ( BIAS) MAX
<
V
CC
<
28.5(V)(V
CC ( OVP
) MIN
)
,
(2)
Circuit current, ICC
ICC（ON）=3.7mA
（MAX）
9.4V
VCC（OFF）
15.1V VCC pin voltage
VCC（ON）
Figure 8-2. VCC vs. ICC
8.1.3. Bias Assist Function
Figure 8-3 shows the VCC pin voltage behavior during
the startup period. When the VCC pin voltage reaches
VCC(ON) = 15.1 V, the control circuit starts operation, the
circuit current, ICC, increases, and thus the VCC pin
voltage begins dropping. At the same time, the auxiliary
winding voltage, VD, increases in proportion to the
output voltage rise. Thus, the VCC pin voltage is set by
the balance between dropping by the increase of ICC and
rising by the increase of the auxiliary winding voltage,
VD.
Just at the turning-off of the power MOSFET, a surge
voltage occurs at the output winding. If the feedback
control is activated by the surge voltage under light load
condition at startup, and the VCC pin voltage decreases to
VCC(OFF) = 9.4V, a startup failure can occur, because the
output power is restricted and the output voltage
decreases. In order to prevent this, during a state of
operating feedback control (that is, the FB/OLP pin
SSC1S311A-DSE Rev.1.3
SANKEN ELECTRIC CO., LTD.
9
Dec. 22, 2017
http://www.sanken-ele.co.jp/en
© SANKEN ELECTRIC CO., LTD. 2012
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