PL3378 datasheet(7/8 Pages) PMICRO

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Datasheet

PL3378

Fig.2. Voltage of Auxiliary-winding Waveform

Via a resistor divider connected between the

auxiliary winding and FB, the auxiliary voltage

is sampled based on the internal timing control.

Through the built-in error amplifier the sampled

voltage could be regulated at a preset level,

thus constant output voltage can be achieved.

During the CC mode of operation PL3378 will

regulate the output current at a constant level

regardless of the output voltage, while avoiding

continuous conduction mode.

9.4 Adjustable CC Point and Output

Power

In application of AC/DC power supply

application, we could change the Rs to get

different CC points and the maximum output

power only using one model .The larger Rs, the

smaller CC point is, and the smaller output

power becomes.

9.5 Switching Frequency and Lm

Tolerance Compensation

The system load condition and the operation

mode decide the switching frequency of

PL3378.The maximum switching frequency is

set by system design. In DCM assuming the

efficiency to 100%, the output power is given by

smooth it away, the switching frequency is

locked by an internal loop such that the

switching frequency is

fsw = 4/(7*Tdemag)

(4)

Where Tdemag is the discharge time of

inductance.

Since

Tdemag

inversely

proportional to the inductance, the error of

current in CC mode and max output power

caused by the variation of inductance will be

eliminated.

9.6 Adjustable Cable Drop

Compensation

Built-in cable drop compensation is used to get

better load regulation.

Without compensation of cable drop, the

voltage at the end of the cable between no load

and full load will not be the same caused by

cable drop. To get better load regulation, an

offset voltage is generated at FB pin by an

internal current flowing into the resister divider.

The current is proportional to the switching off

time, as a result, it is inversely proportional to

the output load current, thus the drop due to the

cable loss can be compensated.

In different applications using different cables,

we could adjust the resistance of the divider

connected to the FB pin. The larger impedance

of the cable, the larger divider will be used.

Po  1 LmfswI 2

Ppk



Vo IO

(3)

9.7 Protection Control

PL3378 has built-in rich protection features

Where Lm indicates the inductance of the

primary winding and IPpk is the peak current of

the primary winding.

Refer to the equation 3, the change of Lm

results in the change of Po and the constant

output current in the CC mode. The tolerance of

Lm will make CC worse in mass production. To

including Cycle-by-Cycle Current Limiting, VDD

over voltage, Under Voltage Lockout on VDD,

Output OVP and OTP Protection.

PL3378 is shut down when VDD drops below

the UVLO (ON) limit ,and the power converter

enters the power on start-up sequence

thereafter.

10 Packaging Information

PL3378 데이터시트(PDF) 7 Page - Suzhou Juyuan Microelectronics Co., Ltd

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