ZXGD3105 datasheet(8/13 Pages) DIODES

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ZXGD3105N8

Document Number DS35101 Rev. 1 – 2

8 of 13

www.diodes.com

November 2011

A Product Line of

Diodes Incorporated

ZXGD3105N8

Application Information

The purpose of the ZXGD3105 is to drive a MOSFET as a low-VF Schottky diode replacement in isolated AC/DC

converter. When combined with a low RDS(ON) MOSFET, the controller can yield significant power efficiency

improvement, whilst maintaining design simplicity and incurring minimal component count. Figure 1 shows the typical

configuration of ZXGD3105 for synchronous rectification in a low output voltage Flyback converter.

Figure 1 - Typical Flyback application schematic

Threshold voltage and resistor setting

Proper selection of external resistors RREF and RBIAS is important for optimum device operation. RREF and RBIAS supply

fixed current into the IREF and IBIAS pin of the controller. IREF and IBIAS combines to set the turn-off threshold voltage

level, VT. In order to set VT to -10mV, the recommended IREF and IBIAS are 1.02mA and 0.54mA respectively.

The values for RREF and RBIAS are selected based on the Vcc voltage. If the Vcc pin is connected to the power

converter’s output, the resistors should be selected based on the nominal converter’s output voltage. Table 1

provides the recommended resistor values for different Vcc voltages.

Supply, Vcc

Bias Resistor, RBIAS

Reference Resistor, RREF

5 V

9.6 k

Ω

4.3 k

Ω

10 V

18 k

Ω

9.1 k

Ω

12 V

24 k

Ω

11 k

Ω

15 V

30 k

Ω

15 k

Ω

Table 1 – Recommended resistor values for different Vcc voltages

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