LC5525F datasheet(21/40 Pages) SANKEN

21 / 40 page

SANKEN ELECTRIC CO., LTD.

LC5500-AN, Rev.1.2

Operation Modes at Startup

Figure 24 shows the operation modes during the startup phase of

the LC551xD, and figure 25 shows those for the LC552xD and

LC552xF. Note that OCP pin voltage, which determines the tim-

ing of quasi-resonant operation, is in positive voltage on the OCP

pin, in reference to the S/GND pin.

During two periods below at startup, IC operation is set to PWM,

with fOSC = 14 kHz:

• While the COMP pin voltage (for LC551xD) and FB pin volt-

age (for LC552xD and LC552xF), in reference to S/GND, are

0 to 0.9 V (the control voltage lower limit for the COMP pin,

VCOMP(MIN), and FB pin, VFB(MIN)): During this period, on-width

is fixed at the Leading Edge Blanking Time, tBW = 500 ns.

• Until the quasi-resonant signal (OCP pin voltage) reaches the

Quasi-Resonant Operation Threshold Voltage-1, VBD(TH1)=

0.24 V: During this period, the output voltage is low; therefore,

the auxiliary winding voltage, VD , is low. Thus the quasi-reso-

nant signal is low.

After those startup operations the output voltage starts increasing,

when the OCP pin voltage reaches VBD(TH1) = 0.24 V, the IC is

switched to quasi-resonant operation (figure 26).

Soft-Start Function

The soft-start function reduces power stress on the incorporated

MOSFET and secondary rectifier during the startup phase.

LC551xD series (non-isolated designs)

The soft-start operation

begins when the COMP pin voltage reaches VCOMP(MIN) = 0.9 V

and lasts until the output current becomes constant. During that

period, the output power gradually increases.

During this period, check the items below:

• VCC pin voltage does not drop to the Operation Stop Voltage,

VCC(OFF)

• Output current reaches the target value before the overload pro-

tection (OLP) is activated by the COMP pin voltage reaching

VCOMP(OLP)2 = 4.5 V

CC Pin

Voltage

COMP Pin

Voltage

Output (LED)

Current, I

OUT

Drain

Current, I

S/GND

IC turn on

S/GND

GND(I

OUT)

Duration

PWM

(QR)

VCOMP(MIN)= 0.90 V

VCC(BIAS 1

) = 11.0 V

Soft-Start Period

Target

Current

GND(I

D )

Constant current operation

ON= tBW(500 ns)

VCC Pin

Voltage

FB Pin

Voltage

Output (LED)

Current, I

OUT

Drain

Current, I

S/GND

IC turn on

S/GND

GND(I

OUT)

Duration

PWM

(QR)

VFB (MIN)= 0.90 V

VCC(BIAS 1

) = 11.0 V

Soft-Start Period

Target

Current

GND(I

D )

Constant current operation

ON= tBW(500 ns)

Figure 24. Soft-start operation waveforms at startup (LC551xD)

Figure 25. Soft-start operation waveforms at startup (LC552xD/ LC552xF)

GND(ID)

S/GND

VBD(TH1)

PWM operation Quasi-resonant operation (QR)

OCP Pin

Voltage

Drain

Current, ID

Figure 26. OCP Pin Voltage (with time scale expanded)

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