XRP7604 Datasheet, PDF (8/13 Pages) Exar Corporation – 1A 29V Non-Sync. Buck High Power LED Driver

XRP7604
1A 29V Non-Sync. Buck High Power LED Driver
THEORY OF OPERATION
The XRP7604 is a fixed frequency, Voltage-
mode, non-synchronous buck PWM regulator
optimized for driving LEDs. Constant LED
current is achieved using resistor RFB as
shown in the page 1 schematic. A low 0.2V
reference voltage minimizes power dissipation
in RFB. A tight reference voltage tolerance of
±3%, over full operating conditions, helps
accurately program the LED current. High
switching frequency of 1.2MHz (nominal)
reduces the size of passive components.
Dimming and power sequencing is achieved
using a logic-level PWM signal applied to FB
pin via a diode. Overcurrent protection (OCP)
is based on high-side MOSFET’s Rds(on) and is
programmable via a resistor placed at LX
node.
DIM = L. The DIM signal needs to be greater
than 600mV minimum to turn-off the XRP7604
and less than 200mV to fully turn-on the
XRP7604. It is recommended to use a signal
with DIM = 1V or more for OFF and 0V for ON.
The user should note that the logic is reversed
relative to many other PWM controlled LED
drivers. In other words a logic level high at
20% duty cycle will result in approximately an
80% duty cycle for the LED. Recommended
modulation frequencies are from 100Hz to
200Hz with 10 – 90% duty cycle, 500Hz with
10 – 80% duty cycle, and 1000Hz with 10 -
70% duty cycle. Figures 15 & 16 show the
output response at the maximum PWM DIM
signal of 1000Hz. See figure 17 for 100Hz to
1000Hz duty cycle response for two Luxeon K2
LEDs in parallel at 0.75A total current.
PROGRAMMING THE LED CURRENT
Use the following equation to program the LED
current:
Equ.1: RFB = ^0.2V
I _LED
The output voltage will adjust as needed to
ensure average I_LED is supplied. For example if
the output current has to be set at 0.35A then
RFB=0.57 Ohm. If the output LED has a
corresponding Vf of 3.5V then XRP7604 will
step down the V_IN to 3.5V. If two of these
LEDs are placed in series then XRP7604 will
step down the voltage to 7V. Superimposed on
I_LED there is a current ripple that is equal in
magnitude to inductor current ripple. Current
ripple will be nominally set to 10% of I_LED by
proper sizing of inductor. Note that throughout
this datasheet I_LED and I_O will be used
interchangeably.
DIMMING SIGNAL
A logic-level PWM signal applied through a
small-signal diode to the feed-back (FB) pin
can be used for dimming control of the LED.
This external signal we call DIM turns the
MOSFET gate drive on/off, thereby modulating
the average current delivered to the LED. The
DIM signal connects to the VFB pin through a
1N4148 diode and will shutdown the XRP7604
when DIM = H and turn-on the XRP7604 when
© 2009 Exar Corporation
8/13
Ch1: DIM Signal
Ch2: VFB – 0.75A I_OUT/div
Fig. 14: 1.1KHz, 10% Duty Cycle Dimming Signal
Dimming Signal is ~70% LED Duty Cycle
Ch1: DIM Signal
Ch2: VFB – 0.75A I_OUT/div
Fig. 15: 1KHz, 70% Duty Cycle
Dimming Signal is ~10% LED Duty Cycle
Rev. 1.0.0