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NCP5008DMR2-NCP5009DMR2
White LED Driver
AND8126/D
NCP5008/NCP5009
Compensation Methods
Prepared by: Pierre GenestON Semiconductor
Applications Engineer
ABSTRACTThe NCP5008 and NCP5009 are high efficiency boost
converters operating in current loop control mode to drive
Light Emitting Diode (LED). These products are dedicated
to supply any LEDs for any portable application. A digital
programming current level allows the user to set 8 different
levels of brightness intensity. The photo sense input is an
innovative function: it adapts the brightness backlight in
function of the ambient light in order to save the battery
lifetime duration. The devices are designed to operate in
open−loop configuration to enable the digital and the
photo−sense brightness control.
This application note describes alternate application
schematics to increase the output current accuracy.
INTRODUCTIONThis manual describes several possible applications
built around the NCP5008 and NCP5009. We thank you
for choosing an ON Semiconductor part, and we hope
this guide will help you to benefit from these devices’
advanced features. Please use the NCP5009 data sheet as a
reference source. For an up−to−date document, refer to the
NCP5009 data sheet which is available on our web page:
http://www./home.
Among the electrical parameters, the power supply must be
capable to yield 200 mA minimum continuous DC current to
operate the system properly. Peak current up to 300 mA can
be developed when the driver operates at full power.
As opposed to classical Boost DC−DC converter, which
operates in closed loop mode, the NCP5009 is operating in
open−loop mode to avoid the need for a sense resistor in
series with the white LEDs. This feature significantly
simplifies the implementation when the LED network and
the driver are located on separate boards because only one
connection is required. The second advantage is that the
current flowing through the photo sensor can be
subtracted from the DC reference current. The external
reference current is generated by connecting a
between the Iref pin and the ground. It can be internally
modified by a digital programming word or by the current
flowing through the photo sensor. The modified current is
used to control the inductor’s current peak value.
The consequence of open−loop operation, is that the
current actually flowing through the LED is indirectly
controlled, leading to lower accuracy as opposed to closed
loop operation. The main cause of variation is the power
supply, because the amount of energy stored in the inductor
is proportional to it. This may be an issue when the device
is supplied from a nonconstant DC source, such as a battery.
