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
disadvantageoflowerinductorvaluesisthattheyresult
in higher frequency switching, which in turn causes
reducedefficiencydueto switchlosses.Higherinductor
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
peakvalue.Thisconditionwillarise whenever the ramp
time (I
LX(peak)
x L/V
IN
) exceeds the nominal 5s
maximum ‘on’ time limit for the LX output.
ThegraphsoppositeshowtheZXLD1100performance
forgiveninductorvaluesanddifferentmanufacturers.
Recommended inductor values for the ZXLD1100 are
in the range 6.8Hto22H. The inductor should be
mounted as close to the device as possible with low
resistance connections to the LX and VIN pins.
Suitable coils for use with the ZXLD1100 are shown in
the table below:
Part No.
L
(
H)
DCR
( )
I
SAT
(A)
Manufacturer
CMD4D11-100MC 10 0.457 0.5 Sumida
www.sumida.com
DO1608-103 10 0.16 1.1 Coilcraft
www.coilcraft.com
LQH31CN100 10 1.3 0.23 Murata
www.murata.com
LB2012Y100MR 10 0.5 0.1 Taiyo Yuden
www.t-yuden.com
Diode selection
The rectifier diode (D1) should be a fast low
capacitance schottky diodewith low reverseleakage 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.
The table below gives some typical characteristics for
diodes that can be used with the ZXLD1100:
Diode V
F
@ 100mA (mV) I
FSM
(mA) Ic (mA) I
R
at 30V ( A) Package
ZHCS400 300 1000 400 15 SOD323
ZHCS500 300 1000 500 15 SOT23
Layout considerations
PCB tracks should be kept as short as possible to
minimize ground bounce, and the ground pin of the
deviceshould be soldered directly to theground 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. The FB pin is a high impedance
inputsoPCBtracklengthstothisshouldalsobekept as
short as possible to reduce noise pickup. Excess
capacitance from the FB pin to ground should be
avoided.
ZXLD1100
SEMICONDUCTORS
ISSUE 4 - JULY 2004
10
LAST TIME BUY
Closest Alternative is AP5724FDCG-7
