LSP3312
1A LED Driver with Internal Switch
Application Description
Setting nominal average output current with external resistor Rs
The nominal average output current in the LED(s) is determined by the value of the external
current sense resistor (R
S
) connected between VIN and ISENSE and is given by:
I
OUTnom
=
0.1
Rs
The table below gives values of nominal average output current for several preferred values of
current setting resistor (R
S
) in the typical application circuit shown on page1.
R
S
(Ω)
0.1
0.13
0.15
0.3
Nominal average output current (mA)
1000
760
667
333
The above values assume that he VSET pin is floating and at a nominal voltage of VREF (1.25V).
Note that Rs=0.1Ω is the minimum allowed value of sense resistor under these conditions to
maintain switch current below the specified maximum value. It is possible to use different values of
Rs if the VSET pin is driven from an external voltage.
Capacitor selection
A low ESR capacitor should be used for input decoupling, as the ESR of this capacitor appears in
series with the supply source impedance and lowers overall efficiency. This capacitor has to supply
the relatively high peak current to the coil and smooth the current ripple on the input supply. A
minimum value of 4.7uF is acceptable if the input source is close the device, but higher values will
improve performance at lower input voltages, especially when the source impedance is high. The
input capacitor should be placed as close as possible toe the IC.
For maximum stability over temperature and voltage, capacitors with X7R,X5R, or better dielectric
are recommended. Capacitors with Y5V dielectric are not suitable for decoupling in this application
and should NOT be used.
Inductor selection
Recommended inductor values for the LSP3312 are in the range 33uF to 100uF. Higher values of
inductance are recommended at higher supply voltages in order to minimize errors due to
switching delays, which result in increased ripple and lower efficiency. Higher values of inductance
also result in a smaller change in output current over the supply voltage range. The inductor should
be mounted as close the to device as possible with low resistance connections toe the LX and VIN
pins. The chosen coil should have a saturation current higher than the peak output current and a
continuous current rating above the required mean output current.
LSP3312 Rev. Preliminary 0.1
5 of 12
www.liteon-semi.com
2010/08/23
LITEON [ LITE-ON TECHNOLOGY CORPORATION ]
