5
Date: 6/20/06 Rev T SP7685, 1.2A Buck/BoostCharge Pump LED Driver © 2006 Sipex Corporation
In the Torch mode, (Flash = GND) the Flash
pin is set to logic low and the SP7685 FB pin
regulates to 50mV output:
VFB = 50mV (Torch Mode)
When in Flash mode, (Flash = V
IN
), the FB
regulation voltage is set by the resistor RSET
connected between the RSET pin and SGND
and the equation:
VFB = RSET * 14μA / 5 (Flash Mode)
Where 14μA is an internal regulated
current and 5 is an internal factor used to
scale the VSET voltage to the VFB voltage.
Typical values of RSET are 140KΩ to
35KΩ for a range of VFB = 400mV to
100mV in Flash mode.
The output current is then set in either Flash
or Torch mode by the equation:
I
OUT
= V
FB
/ R
SENSE
FLASH TIMEOUT PROTECTION
Due to the high currents typically available
in Flash mode, it is necessary to protect the
white LED from damage if left on too long.
The SP7685 has a timeout in Flash mode of
approximately 2.6 seconds after which it will
shut down operation. Operation will not
begin again in Flash mode until the Enable
pin or Flash pin have been set Low and then
High again.
OVERTEMPERATURE PROTECTION
When the temperature of the SP7685 rises
above 145
˚
C, the overtemperature protec-
THEORY OF OPERATION
tion circuitry turns off the output switches
to prevent damage to the device. If the
temperature drops back down below 135
degrees Celsius, the part automatically
recovers and executes a soft start cycle.
OVERVOLTAGE PROTECTION
The SP7685 has over voltage protec-
tion. If the output voltage rises above
the 5.5V threshold, the over voltage
protection shuts off all of the output
switches to prevent the output voltage
from rising further. When the output
decreases below 5.5V, the device re-
sumes normal operation.
OVERCURRENT PROTECTION
The over current protection circuitry moni-
tors the average current out of the V
OUT
pin. If the average output current exceeds
approximately 1.6 Amps, then the
overcurrent protection circuitry shuts off
the output switches to protect the chip.
BRIGHTNESS CONTROL USING PWM
Dimming control can be achieved by ap-
plying a PWM control signal to the EN pin.
The brightness of the white LEDs is con-
trolled by increasing and decreasing the
duty cycle of the PWM signal. While the
operating frequency range of the PWM
control is from 60Hz to 700Hz, the recom-
mended maximum brightness frequency
range of the PWM signal is from 60Hz to
200Hz. A repetition rate of at least 60Hz is
required to prevent flicker.
