ZXLB1600 datasheet(13/16 Pages) DIODES

13 / 16 page

Inductor selection

The choice of inductor will depend on available board

space as well as required performance. Small value

inductors have the advantage of smaller physical size

and may offer lower series resistance and higher

saturation current compared to larger values. A

disadvantage of smaller inductors is that they result in

higher frequency switching, which in turn causes

reduced efficiency due to switch losses. Higher

inductor values can provide better performance at

lower supply voltages. However, if the inductance is

too high, the output power will be limited by the

internal oscillator, which will prevent the coil current

from reaching its peak value. This condition will arise

whenever the ramp time ILX(peak) x L/VIN exceeds the

preset 10

µs maximum ‘on’ time limit for the LX output.

The ZXLB1600 has been optimized for use with

inductor values in the range 10 Hto100 H. The typical

characteristics show how efficiency and available

output current vary with input voltage and inductance.

The inductor should be mounted as close to the device

as possible with low resistance connections to the LX

and SW pins.

Suitable coils for use with the ZXLB1600 are those in

the LP02506 and DO1608 series, made by Coilcraft.

Diode selection

The rectifier diode (D1) should be a fast low

capacitance switching type with low reverse leakage at

the working voltage. It should also have a peak current

rating above the peak coil current and a continuous

current rating higher than the maximum output load

current. Small schottky diodes such as the BAT54 are

suitable for use with the ZXLB1600 and this diode will

give good all round performance over the output

voltage and current range. At lower output voltages, a

larger schottky diode such as the ZHCS500 or MBR0540

will provide a smaller forward drop and higher

efficiency. At higher output voltages, where forward

drop is less important, a silicon switching diode such

as the 1N4148 can be used, however this will give lower

efficiency.

The BAT54S device specified in the application circuit

contains a second diode (D2) as one half of a series

connected pair. This second diode is used here to

clamp possible negative excursions (due to coil

ringing) from driving the drain of the output transistor

below -0.5V. This prevents internal coupling effects,

which might otherwise affect output regulation. The

table below gives some typical characteristics for

various diodes.

ZXLB1600

ISSUE 3 - SEPTEMBER 2003

SEMICO NDUC TORS

Diode

Forward voltage at 100mA (V)

Peak current

(mA)

Continuous current

(mA)

Reverse leakage (µA)

BAT54

530

300

200

ZHCS500

300

1000

500

MBR0540

390

1000

500

1N4148

950

450

200

0.025

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