ACT8740
Rev PrB, 25-Feb-08
WLED BIAS DC/DC CONVERTER
FUNCTIONAL DESCRIPTION
Inductor Selection
General Description
REG2 was designed to provide excellent perform-
ance across a wide range of applications, but was
optimized for operation with inductors in the 10µH
to 22µH range, although larger inductor values of
up to 68µH can be used to achieve the highest pos-
sible efficiency.
REG2 is a highly efficient step-up DC/DC converter
that employs a fixed frequency, current-mode,
PWM architecture. This regulator is optimized for
white-LED bias applications consisting of up to
seven white-LEDs.
Enabling and Disabling REG2
Optimizing for Smallest Footprint
Enable/disable control of REG2 is achieved through
the ACT8740’s I2C serial interface. Enable REG2 by
setting REG2/ON[ ] to [1], disable REG2 by clearing
REG2/ON[ ] to [0]. When disabled, REG2’s quies-
cent supply current drops to just 1µA. As with all
non-synchronous step-up DC/DC converters,
REG2’s application circuit produces a DC current
path between the input and the output in shutdown
mode. Although the forward drop of the WLEDs
makes this leakage current very small in most appli-
cations, it is important to consider the effect that this
may have in your application particularly when us-
ing fewer than three WLEDs.
REG2 is capable of operating with very low inductor
values in order to achieve the smallest possible
footprint. When solution size is of primary concern,
best results are achieved when using an inductor
that ensures discontinuous conduction mode (DCM)
operation over the full load current range.
Optimizing for Highest Efficiency
REG2 achieves excellent efficiency in applications
that demand the longest possible battery life. When
efficiency is the primary design consideration, best
results are achieved when using an inductor that
results in continuous conduction mode (CCM) op-
eration and achieves very small inductor ripple cur-
rent.
Over-Voltage Protection
REG2 features internal over-voltage protection
(OVP) circuitry which protects the system from LED
open-circuit fault conditions. If the voltage at OV
ever reaches the over-voltage threshold, REG2 will
regulate the top of the LED strong to the OVP
threshold voltage. By default, the ACT8740’s OVP
threshold is set at 28.5V, although it may be pro-
grammed to a lower value by writing to the
REG2/VSET[ ] register.
Output Capacitor Selection
REG2 was designed to operate with output capaci-
tors ranging from 0.47µF to 10µF, providing design
flexibility. A 1µF output capacitor is suitable for most
applications, although larger output capacitors may
be used to minimize output voltage ripple, if
needed. Ceramic capacitors are recommended for
most applications.
Power-OK Bit
Rectifier Selection
REG2 features a Power-OK status bit that can be
read by the system microprocessor via the I2C inter-
face. If the voltage at OV is greater than the OVP
threshold, REG2/OK[ ] will clear to 0.
REG2 requires a Schottky diode to rectify the induc-
tor current. Select a low forward voltage drop
Schottky diode with a forward current (IF) rating that
is sufficient to support the maximum switch current
and a sufficient peak repetitive reverse voltage
(VRRM) to support the output voltage.
Compensation and Stability
REG2 utilizes current-mode control and an internal
compensation network to optimize transient per-
formance, ease compensation, and improve stabil-
ity over a wide range of operating conditions. REG2
is a flexible regulator, and its external components
can be chosen to achieve the smallest possible
footprint or to achieve the highest possible effi-
ciency.
Setting the LED Bias Current
The LED bias current is set by a resistor con-
nected from FB2 and ground, and the regulator is
satisfied when the LED current is sufficient to gen-
erate 250mV across this resistor. Once the bias
current is programmed, the LED current can be
Innovative Products. Active Solutions.
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