ZXLD1101
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 lower inductor values 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 (I
LX(peak)
x L/V
IN
) exceeds the nominal 5 s
maximum 'on' time limit for the LX output.
The graphs opposite show the ZXLD1101 performance
for given inductor values and different manufacturers.
Recommended inductor values for the ZXLD1101 are
in the range 6.8 H to 22 H. 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 ZXLD1101 are shown in
the table below:
Part No.
CMD4D11-100MC
DO1608-103
LQH31CN100
LB2012Y100MR
L
( H)
10
10
10
10
DCR
( )
0.457
0.16
1.3
0.5
I
SAT
(A)
0.5
1.1
0.23
0.1
Manufacturer
Sumida
www.sumida.com
Coilcraft
www.coilcraft.com
Murata
www.murata.com
Taiyo Yuden
www.t-yuden.co
Diode selection
The rectifier diode (D1) should be a fast low
capacitance schottky diode 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.
The table below gives some typical characteristics for
diodes that can be used with the ZXLD1101:
Layout considerations
PCB tracks should be kept as short as possible to
minimise 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 minimise
parasitic resistance and inductance, which will
degrade efficiency. The FB pin is a high impedance
input so PCB track lengths to this should also be kept as
short as possible to reduce noise pickup. Excess
capacitance from the FB pin to ground should be
avoided.
Ic (mA)
400
500
I
R
at 30V ( A)
15
15
Package
SOD323
SOT23
Diode
ZHCS400
ZHCS500
V
F
@ 100mA (mV)
300
300
I
FSM
(mA)
1000
1000
ISSUE 4 - JULY 2004
11
SEMICONDUCTORS
ZETEX [ ZETEX SEMICONDUCTORS ]
