ACT8712QLCHA-T View Datasheet(PDF) - Active-Semi, Inc
Part Name
Description
MFG CO.
ACT8712QLCHA-T
Active-Semi, Inc
'ACT8712QLCHA-T' PDF : 36 Pages View PDF
ACT8712
Rev0, 25-Feb-08
WLED BIAS DC/DC CONVERTER
FUNCTIONAL DESCRIPTION
General Description
REG3 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 REG3
Enable/disable control of REG3 is done through a
combination of the ACT8712’s system control cir-
cuitry, the ON2 pin, and the REG3/ON[ ] bit. REG3
may be enabled when at least one of the following
conditions exists:
1) A valid supply voltage is present at VIN
2) nMSTR is asserted low, or
3) ON1 is asserted high.
When any of these conditions exist, REG3 may be
enabled by either of the following conditions:
1) ON2 is asserted high, or
2) REG3/ON[ ] is set to [1].
When disabled, REG3's quiescent supply current
drops to just 1µA.
As with all non-synchronous step-up DC/DC con-
verters, REG3'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 applications, it is important to consider the
effect that this may have in your application particu-
larly when using fewer than three WLEDs.
Over-Voltage Protection
REG3 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, REG3 will
regulate the top of the LED strong to the OVP
threshold voltage. By default, the ACT8712's OVP
threshold is set at 28.5V, although it may be pro-
grammed to a lower value by writing to the
REG3/VSET[ ] register.
Power-OK Bit
REG3 feature a variety of status bits that can be
read by the system microprocessor. If the voltage at
OV is greater than the OVP threshold, REG3/OK[ ]
will clear to 0.
Compensation and Stability
REG3 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. REG3
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.
Inductor Selection
REG3 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.
Optimizing for Smallest Footprint
REG3 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
REG3 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.
Output Capacitor Selection
REG3 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
Innovative Products. Active Solutions.
ActivePMUTM is a trademark of Active-Semi.
I2CTM is a trademark of Philips Electronics.
- 27 -
www.active-semi.com
Copyright © 2008 Active-Semi, Inc.
Share Link: 