HT2800S

4/6

High Performance Primary Sensing Regulation Controller IC

www.hotchip.com.cn

Ver1.0

Function Description

HT2800S is specially designed for the charger/adapter of small power digital products. To use

PSR control technology to provide accurate (CV/CC) characteristics. The controller works in PFM

mode and the switching frequency can be automatically adjusted as per the load. Optimized

driving

circuit greatly promotes withstand voltage of the power BJT and minimizes the switching

loss, so as to make the circuit have excellent converting efficiency

Constant Voltage Operation (CV)

When the circuit is operates in CV area, FB will detect the output voltage when the power BJT

turn-off through the sensing voltage by auxiliary winding of the transformer, to make the output

voltage stabilized in the fixed value. In the turn-on period of the output rectifier diode D, it's given

the output winding voltage and auxiliary winding voltage as follows

(W here Vaux : auxiliary winding voltage； Nb：subsidiary

winding turns； NS:

output winding turns；

Vd：forward voltage drop of output rectifier diode)

FB will test the auxiliary winding voltage while the power BJT off-time to regulate the output

voltage. In order to avoid the peak influence happens in power BJT turn-off resulting from the

leakage inductance of the transformer, and considering the accuracy of sampling voltage, FB

captures the auxiliary

winding detection sampling points as below:

Constant Current Operation (CC)

HT2800S CC control circuit is requested the converter to operate in DCM. In CC operation

mode, control circuit will fix the proportion between the Tons and Toffs generated by rectifier

Diode. During Tons time, the primary energy of the transformer will be converted to the secondary

winding, via the rectifier diode then charge the output capacitor and supply load simultaneously.

During Toffs period, the primary coil stores the energy, the output filter capacitor makes load

discharge. In CC operation area, HT2800S switch frequency will follow the output load direct

ratio control detected by FB Voltage