ZXLB1600 datasheet(14/16 Pages) DIODES

14 / 16 page

Increased efficiency

If isolation of the coil from the supply is not needed, the

high side of this can be connected directly to VIN to

improve efficiency. This prevents power loss in the

internal PMOS switch and typical efficiency gains of 5%

can be achieved. (See efficiency vs. load curves). Some

applications may require the coil to be fed from a

separate supply with a different voltage to VIN. In this

case, the SW pin should be left floating.

Layout considerations

PCB tracks should be kept as short as possible to

minimize ground bounce and the ground pin of the

device should be soldered directly to the ground plane.

It is particularly important to mount the coil and the

input/output capacitors close to the device to minimize

parasitic resistance and inductance, which will

degrade efficiency and increase output ripple. The FB

and LBT pins are high impedance inputs, so PCB track

lengths to these should also be kept as short as

possible to reduce noise pickup. Output ripple is

typically only 50mV p-p, but a small feed-forward

capacitor (~100pF) connected from the FB pin to the

output may help to reduce this further. Capacitance

from the FB pin to ground should be avoided, but a

capacitor can be connected from the LBT pin to ground

to reduce noise pickup into the low battery comparator

if required.

Low battery detection circuit

The device contains an independent Low Battery

Detection Circuit that remains powered when the

device is shutdown. The detection threshold is set

internally to a default value of 1.98V, but can be

adjusted by means of external resistors as described

below.

Low battery threshold adjustment, LBT

The internal potential divider network R3/R4 sets the

detection threshold. This is accessible at the LBT pin

and can be shunted by means of external resistors to

set different nominal threshold voltages. The potential

divider defines threshold voltage according to the

relationship:

VLBT = (R3+R4)/R4 x 1.21V

When using external resistors, these should be chosen

with lower values than the internal resistors to

minimize errors caused by the

25% absolute value

variation of the internal resistors. The internal resistors

have high values in order to minimize these errors. It is

suggested to use values less than half those shown for

R3, R4 at the bottom of page 3.

Low battery flag output, LBF

This is an open drain output that switches low when the

battery voltage falls below the detection threshold. An

external pull-up resistor can be connected to this pin to

allow it to interface to any voltage up to a maximum of

29V. Current in the pull-up resistor should be limited to

a value below IBLOL.

ZXLB1600

ISSUE 3 - SEPTEMBER 2003

SEMICO NDUC TORS

ZXLB1600 Datasheet(PDF) 14 Page - Diodes Incorporated

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