
IDTP95020
Product Datasheet
September 2, 2011 Revision 1.3 Final
103
2011 Integrated Device Technology, Inc.
LED Boost - Application
Figure 31 – LED_BOOST Application Schematic
VIN External Voltage is used to power the gate driver for
the external NFET, SW1.
LED_BOOST can be set via R1 and R2 to provide a
protection voltage between VIN and 40V for protecting
capacitor COUT in case the LED strings open. This voltage
should be set below the voltage rating of COUT.
The LED_BOOST converter monitors the current sense
elements in the sink blocks and reduces its output voltage
as necessary to keep the headroom voltage as low as
possible to minimize losses.
Input Capacitors
The input capacitors CIN and CEXT should be located as
physically close as possible to the power pin
(LED_BOOST_VIN)
and
power
ground
(LED_BOOST_GND).
Ceramic
capacitors
are
recommended their higher current operation and small
profile. Also, ceramic capacitors are inherently capable to
withstand input current surges from low impedance
sources such as batteries used in portable devices than
are tantalum capacitors. Typically, 10V or 16V rated
capacitors are required. See Table 165 for recommended
external components.
Output Capacitor
For proper load voltage regulation and operational
stability,a capacitor is required on the output after the
diode D1. The output capacitor connection to the ground
pin (LED_BOOST_GND) should be made as directly as
practically possible for maximum device performance.
Since the boost has been designed to function with very
low ESR capacitors, a ceramic capacitor is recommended
with a 50V rating for best performance.
Inductor Selection
Inductor manufacturer’s specifications list both the
inductor DC current rating, which is a thermal limitation,
and the peak current rating, which is determined by the
saturation characteristics. The inductor should not show
any appreciable saturation under normal load conditions.
Some inductors may meet the peak and average current
ratings yet result in excessive losses due to a high DCR.
Always consider the losses associated with the DCR and
its effect on the total converter efficiency when selecting
an inductor.
Selecting the Schottky Diode
To ensure minimum forward voltage drop and no recovery,
high voltage Schottky diodes are considered the best
choice for the boost converters. The output diode is sized
to maintain acceptable efficiency and reasonable