SSC3S902 datasheet(13/32 Pages) SANKEN

13 / 32 page

SSC3S900 Series

SSC3S900 - DSJ Rev.2.4

SANKEN ELECTRIC CO.,LTD.

Apr. 01, 2016

http://www.sanken-ele.co.jp

8.2

Startup Operation

Figure 8-11 shows the VCC pin peripheral circuit.

When the following all conditions are fulfilled, the IC

starts the startup operation:

● The mains input voltage is provided, and the VSEN

pin voltage increases to the on-threshold voltage,

VSEN(ON) = 1.300 V, or more.

● The startup current, I

CC(ST), which is a constant current

of 6.0 mA, is provided from the IC to capacitor C2

connected to the VCC pin, C2 is charged, and the

VCC pin voltage increases to the operation start

voltage, VCC(ON) = 14.0 V, or more.

● The FB pin voltage increases to the oscillation start

threshold voltage, VFB(ON) = 0.30 V, or more.

After that, the startup circuit stops automatically, in

order to eliminate its own power consumption.

CSS

VCC

GND

VSPL

VSEN

Figure 8-11

VCC pin peripheral circuit

During the IC operation, the rectified voltage from the

auxiliary winding voltage, VD, of Figure 8-11 is a power

source to the VCC pin.

The winding turns of the winding D should be

adjusted so that the VCC pin voltage is applied to

equation (7) within the specification of the mains input

voltage range and output load range of the power supply.

The target voltage of the winding D is about 19 V.

⇒11.0 (V) < V

CC < 32.0 (V)

(7)

The startup time, tSTART, is determined by the value of

C2 and C6 connected to the CSS pin. Since the startup

time for C6 is much smaller than that for C2, the startup

time is approximately given as below:

(8)

where:

tSTART is the startup time in s,

VCC(INT) is the initial voltage of the VCC pin in V, and

ICC(ST) is the startup current, 6.0 mA

8.3

Undervoltage Lockout (UVLO)

Figure 8-12 shows the relationship of VCC and ICC.

After the IC starts operation, when the VCC pin

voltage decreases to VCC(OFF) = 9.8 V, the IC stops

switching operation by the Undervoltage Lockout

(UVLO) Function and reverts to the state before startup

again.

ICC

VCC（OFF）

VCC（ON）

VCC pin

voltage

Start

Stop

Figure 8-12

VCC versus ICC

8.4

Bias Assist Function

Figure 8-13 shows the VCC pin voltage behavior

during the startup period.

IC startup

VCC pin voltage

VCC(ON)

VCC(BIAS)

VCC(OFF)

Startup failure

Startup success

Target

operating

voltage

Time

Bias Assist period

Increasing by output

voltage rising

Figure 8-13

VCC pin voltage during startup period

When the conditions of Section 8.2 are fulfilled, the

IC starts operation. Thus, the circuit current, ICC,

increases, and 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 due to the increase of ICC and rising due to the

increase of the auxiliary winding voltage, VD. When the

VCC pin voltage decreases to VCC(OFF) = 9.8 V, the IC

stops switching operation and a startup failure occurs.

In order to prevent this, when the VCC pin voltage

decreases to the startup current threshold biasing voltage,

VCC(BIAS) = 11.0 V, the Bias Assist Function is activated.

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