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GENERAL OVERVIEW
The SP7601 is a fixed frequency, Voltage-
mode, non-synchronous buck PWM controller
optimized for driving LEDs. Constant LED
current is achieved using resistor R SET as
shown in the page 1 schematic. A low 0.2V
reference voltage minimizes power dissipation
in R SET . A tight reference voltage tolerance of
+/-7%, 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 the
FB pin via a diode. Overcurrent protection
(OCP) is based on the high-side MOSFET’s
R DS ( ON ) and is programmable via a resistor
placed at the LX node.
Programming the LED Current
Use the following equation to program the LED
current:
the output current is to be set at 0.35A, then
R SET = 0.57 ? . If the output LED has a
corresponding V F of 3.5V, then the SP7601 will
step down the V IN to 3.5V. If two of these LEDs
are placed in series, then SP7601 will step
down the voltage to 7V. Superimposed on I LED
current is a current ripple that is equal in
magnitude to the inductor current ripple.
Current ripple will be nominally set to 10% of
I LED by proper sizing of the inductor. Note that
throughout this datasheet I LED and I OUT 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 to control dimming of the LED.
This signal turns the MOSFET gate drive on/off,
thereby modulating the average current
delivered to the LED. For turning the MOSFET
off, the minimum signal amplitude at the FB
node must be 1.2V; and to turn it back on it
Rset =
0.2 V
I LED
…………………………… (1)
must fall below 0.7V. The frequency of the
PWM signal can be up to 1kHz.
Where 0.2 is SP7601 reference voltage
The output voltage will adjust as needed to
ensure average I LED is supplied. For example, if
Figure1- 1kHz, 10% duty cycle dimming
signal applied to SP7601, ch1:LX, ch2:
dimming signal, ch4: I LED
Figure2- 1kHz, 80% duty cycle dimming
signal applied to SP7601, ch1:LX, ch2:
dimming signal, ch4: I LED
Sept25-07 Rev D
SP7601 29V High Voltage LED Driver
Page 4
? 2007 Exar Corporation
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