Increased efficiency
Ifisolation of the coil fromthesupplyisnot needed, the
high side of this can be connected directly to VIN to
improve efficiency. This prevents power loss in the
internalPMOSswitchandtypicalefficiencygains of5%
canbeachieved.(See efficiency vs. load curves). Some
applications may require the coil to be fed from a
separate supply with a different voltage to V
IN
. 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
deviceshouldbesoldered 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:
V
LBT
= (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
variationof the internal resistors. The internalresistors
have high values in order to minimize these errors.It is
suggested to usevalues less than halfthose shown for
R3, R4 at the bottom of page 3.
Low battery flag output, LBF
Thisisanopendrain outputthatswitcheslowwhenthe
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 I
BLOL
.
ZXLB1600
ISSUE 3 - SEPTEMBER 2003
14
SEMICONDUCTORS
LAST TIME BUY
Closest Alternative Part AP5727WUG-7
