6
2007 Semtech Corp.
www.semtech.com
SC4505
POWER MANAGEMENT
The SC4505 contains a 1MHz fixed-frequency, current-
mode boost converter, and two independent LED current
regulators. The LED current setpoints are chosen using
external resistors, and the PWM controller operates
independently to keep the two currents in regulation.
Since the SC4505 receives feedback from both of the
LED current regulators, either or both LED strands can
be on at any given time. Additionally, different numbers
of LED can be used in the two strands with no resistor
ballasting, or preset output voltage setpoint.
A typical application would use 3-6 backlight LED, driven
with 20mA, and 2-6 flash LED, driven with 20mA during
torch mode (video recording), and driven with 100mA
during flash mode (for photographs). Usually only the
backlight LED are used, but during some cases both
strands must be on at the same time. As the output
voltage is different for each case, a designer is often
forced to use lossy ballast resistors to balance the LED
currents, or to use two separate converter ICs, greatly
increasing component count and BOM cost.
The SC4505 solves these issues by controlling the boost
converter set point based on instantaneous requirements
of the two current regulators. Therefore, only a single
inductor and power switch is needed to provide power
to the entire lighting subsystem, increasing efficiency and
reducing part count. A digital interface allows independent
control of all LED currents with no converter
“babysitting”. Interface to output control is high-
bandwidth, supporting digital PWM dimming on any
channel at 50Hz to 50kHz dimming frequency, while
aggressively shutting the entire IC down to less than
1µA shutdown mode when all LED strands are turned
off. In shutdown mode, leakage through the current
regulator outputs is also < 1µA, keeping the output
capacitor charged and ready for instant activation of the
LED strands.
1 MHz switching speed provides high output power using
a tiny 1.0mm high inductor, maximizing efficiency for
space-constrained and cost-sensitive applications. In
addition, converter and output capacitor are protected
from open-LED conditions by overvoltage protection, and
flash LED are protected from burnout by a user-settable
time-out feature.
Overview
The states of the two LED current regulators are chosen
by a three-bit digital input. Either or both of the current
regulators can be on at any given time. The converter
automatically shuts down to zero-current shutdown mode
if all pins are low.
EN_BL – Enable Backlight regulator (I
O1
).
EN_FL – Enable Flash/Torch regulator (I
O2
)
FL_TRB – Select flash or torch mode for I
O2
. The current
in torch mode is approximately 1/5 of the current in flash
mode. Flash mode when FL_TRB = 1.
