LATCHED LED DRIVER
The load current per bit (IO) is set by the external resistor
(REXT) as shown in the figure below.
Package Power Dissipation (PD). The maximum al-
lowable package power dissipation is determined as
PD(max) = (150 - TA)/RθJA.
The actual package power dissipation is
PD(act) = DC • (VCE • IO • 16) + (VDD • IDD) ,
where DC is the duty cycle.
When the load supply voltage is greater than 3 V to 5 V,
considering the package power dissipating limits of these
devices, or if PD(act) > PD(max), an external voltage re-
ducer (VDROP) should be used.
Load Supply Voltage (VLED). These devices are de-
signed to operate with driver voltage drops (VCE) of
0.4 V to 0.7 V with LED forward voltages (VF) of 1.2 V to
4.0 V. If higher voltages are dropped across the driver,
package power dissipation will be increased significantly.
To minimize package power dissipation, it is recom-
mended to use the lowest possible load supply voltage or
to set any series dropping voltage (VDROP) as
VDROP = VLED - VF - VCE
with VDROP = Io • RDROP for a single driver, or a Zener
diode (VZ), or a series string of diodes (approximately
0.7 V per diode) for a group of drivers. If the available
voltage source will cause unacceptable dissipation and
series resistors or diode(s) are undesirable, a regulator
such as the Sanken Series SAI or Series SI can be used to
provide supply voltages as low as 3.3 V.
For reference, typical LED forward voltages are:
3.5 – 4.0 V
3.0 – 4.0 V
1.8 – 2.2 V
2.0 – 2.1 V
1.9 – 2.65 V
1.6 – 2.25 V
1.2 – 1.5 V
Pattern Layout. This device has a common logic-
ground and power-ground terminal. If ground pattern lay-
out contains large common-mode resistance, and the volt-
age between the system ground and the LATCH ENABLE
or CLOCK terminals exceeds 2.5 V (because of switching
noise), these devices may not operate correctly.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000