sp7600 Exar Corporation, sp7600 Datasheet - Page 5

• Manufacturer Part Number: sp7600
• Description: 2amp, 29v Buck Converter For Led Driver Applications
• Manufacturer: Exar Corporation

_______________________________________________GENERAL OVERVIEW
The SP7600 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.
Programming the LED current
Use the following equation to program the LED current:
Where 0.2V is the SP7600 reference voltage
The output voltage will adjust as needed to ensure average I
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 SP7600 will step down the Vin to 3.5V. If two of these LEDs are placed in series then
SP7600 will step down the voltage to 7V. Superimposed on I
in magnitude to inductor current ripple. Current ripple will be nominally set to 10% of I
sizing of inductor. Note that throughout this datasheet I
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 SP7600 when DIM = H and turn-on the SP7600
when DIM = L. The DIM signal needs to be greater than 600mV minimum to turn-off the SP7600 and
less than 200mV to fully turn-on the SP7600. 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 1 & 2 show the output response at the maximum PWM DIM signal of
1000Hz. See figure 3 for 100Hz to 1000Hz duty cycle response for two Luxeon K2 LEDs in parallel
at 2A total current.
RFB
12/17/07 rev: A
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=
0
I
2 .
LED
V
…………………………… (1)
SP7600
5
LED
and Io will be used interchangeably.
LED
there is a current ripple that is equal
LED
is supplied. For example if the
©Copyright 2007 Exar Corporation
LED
SP7600
by proper