PN8370

Chipown

13/F, Building C, Wangzhuang Technology Innovation Center, Longshan Road,New District, Wuxi

Tel: +86(510)85217718 http://www.chipown.com.cn

Rev.A.1507 6 / 10

Functional Description

The PN8370 is a high performance CC/CV primary-side controller. PN8370 operates in primary-side sensing and

regulation, so opto-coupler and TL431 could be eliminated. Proprietary built-in CV and CC control can achieve high

precision CC/CV control meeting most charger and adaptor application requirements. Internal HV Start-up circuit and

the chip’s low consumption help the system to meet strict standby power standard.

1. HV Start up Control

At start up, the internal high-voltage start-up circuit provides the internal bias and charges the external VDD capacitor, so

that PN8370 starts up quickly. When VDD reaches VDDon, the device starts switching and the internal high-voltage current

source stops charging the capacitor. The device keeps in normal operation provided as long as VDD keeps above VDDoff.

After startup, the bias is supplied from the auxiliary transformer winding, the current of HV start-up circuit is designed to

be very low so that the power consumption is very low.

2. CC Operation Mode

In CC operation mode, the PN8370 captures the auxiliary flyback signal at FB pin through a resistor dividing-network.

The pulse width of the auxiliary flyback signal determines the PN8370 oscillator frequency. The higher the output voltage

is, the shorter the pulse width is, and the higher the chip oscillator frequency is, thus the constant output current can be

achieved.

The current waveform of DCM mode is shown in Figure 1. During MOSFET turn-on time, the current in the primary

winding (Ipri) ramps up. When MOSFET turns off, the energy stored in the primary winding is transferred to the

secondary side, so the peak current in the secondary winding is

ps

pk

pri

pk

N

I

I

×

=

_

sec_

（

1）

The output current is

P

demag

sense

ipk

PS

P

demag

pk

O

T

T

R

V

N

T

T

I

I

2

1

2

sec_

=

×

=

（

2）

Because Vipk is constant and Tp is equal to tow times Tdemag, the output current Io is constant.

Figure 1 The current waveform of DCM mode

3. CV Operation Mode

In CV mode，PN8370 uses a pulse to sample VFB and it is hold until the next sampling. The sampled voltage is compared

with VREF_CV and the error is amplified. The error amplified output reflects the load condition and controls the switching

off time to regulate the output voltage, thus constant output voltage can be achieved.

The relationship between the output voltage and VREF_CV is