DN86

www.zetex.com

2

Issue 1 - October 2007

© Zetex Semiconductors plc 2007

Description

The system diagram of the MR16 lamp solution with ZXLD1350 and ZXSBMR16PT8 is shown in

Figure 2, and Table 1 provides the bill of materials.

Figure 2

System diagram of ZXLD1350 MR16 Lamp Solution

The ZXLD1350 is designed for LED current drive applications of up to 350mA. The monolithic

NMOSFET is sized appropriately to provide a cost-effective die size and is rated to 400mA, which

with the hysteretic mode of operation (the inductor current waveform will ramp +/-15% about the

nominal current set point) provides sufficient margin. The main features of the ZXLD1350 are:

•

Up to 380mA output current

•

Wide input voltage range: 7V to 30V

•

Internal 30V 400mA NDMOS switch

•

High efficiency (>90% possible)

•

Up to 1MHz switching frequency

The ZXSBMR16PT8 is a new space saving and thermally efficient device specifically designed for

the critical requirements of MR16 applications. The device encompasses a full bridge and a

freewheeling diode realized using extremely low leakage 1A, 40V Schottky diodes to allow a

nominal 12V AC input operations. The Schottky bridge together with the embedded freewheeling

diode enhance the system efficiency compared to the standard silicon diodes in a compact

format. The reference design has solder tag pins to bypass the bridge rectifier should the final

lamp design be used for purely DC operation.

As the ZXLD1350 has a hysteretic switching topology, the switching frequency is dependent on

several factors - input voltage, target current and number of LEDs. An Excel based calculator is

available for system initial evaluation and component choice.

See http://www.zetex.com/3.0/otherdocs/zxld1350calc.xls

System efficiency and LED current have been measured keeping the ADJ pin floating and the

current in the device at its rated value. The input impedance of the ADJ pin is high (200K) and is

susceptible to leakage currents from other sources. Anything that sinks current from this pin will

reduce the output current. In order to avoid any kind of electromagnetic coupling a guard track

around this pin is used.