EUP2794
Application Information
Circuit Description
The EUP2794 has four regulated current sources
connected to the device’s 1.5x charge pump output
(P
OUT
). At input voltages below 3.82V (typ.), the
charge-pump provides the needed voltage to drive
high forward voltage drop White LEDs. It does this
by stepping up the P
OUT
voltage 1.5 times the input
voltage. The charge pump operates in Pass Mode,
providing a voltage on P
OUT
equal to the input voltage,
when the input voltage is at or above 3.82V (typ.).
The device can drive up to 80mA through any
combination of LEDs connected to the constant
current outputs D1-D4. To set the LED drive current,
the device uses a resistor connected to the ISET pin to
set a reference current. This reference current is then
multiplied and mirrored to each constant current
output. The LED brightness can then be controlled by
analog and/or digital methods. Applying an analog
voltage in the range of 0V to 3.0V to the Brightness
pin (BRGT) adjusts the dimming profile of the LEDs.
The digital technique uses a PWM (Pulse Width
Modulation) signal applied to the Shutdown pin (SD).
(see
I
SET
and BRGT PINS
section).
Capacitor Selection
The EUP2794 requires 4 external capacitors for
proper operation. Surface-mount multi-layer ceramic
capacitors are recommended. These capacitors are
small, inexpensive and have very low equivalent
series resistance (ESR,
≒
15m
Ω.
typ.). Tantalum
capacitors, OS-CON capacitors, and aluminum
electrolytic capacitors are generally not recommended
for use with the EUP2794 due to their high ESR, as
compared to ceramic capacitors. For most
applications, ceramic capacitors with X7R or X5R
temperature characteristic are preferred for use with
the EUP2794. These capacitors have tight capacitance
tolerance (as good as ±10%), hold their value over
temperature (X7R: ±15% over -55°C to 125°C; X5R:
±15% over -55°C to 85°C), and typically have little
voltage coefficient.
I
SET
and BRGT Pins
An external resistor, R
SET
, is connected to the I
SET
pin
to set the current to be mirrored in each of the LED
outputs. The internal current mirror sets each LED
output current with a 10:1 ratio to the current through
R
SET
. The current mirror circuitry matches the current
through each LED to within 0.6%.
In addition to R
SET
, a voltage may be applied to the
V
BRGT
in to vary the LED current. By adjusting
current with the Brightness pin (BRGT), the
brightness of the LEDs can be smoothly varied.
Applying a voltage on BRGT between 0 to 3 volts
will linearly vary the LED current. Voltages above
3V do not increase the LED current any further. The
voltage on the V
BRGT
pin is fed into an internal
resistor network with a ratio of 0.385. The resulting
voltage is then summed with a measured offset
voltage of 0.188V, which comes from the reference
voltage being fed through a resistor network (See
Functional Block Diagram).
The brightness control
circuitry then uses the summed voltage to control the
voltage across R
SET
. An equation for approximating
the LED current is:
V
+
V
*0.385
BRGT
I
=
OFFSET
R
LED
SET
Soft-Start
Soft start is implemented internally by limiting the
current flow through the charge pump switches.
During soft start, the input current and LED current
will ramp up slowly and avoid current inrush.
Shutdown Mode
The shutdown pin (SD) disables the part and reduces
the quiescent current less than 1µA (typ.).The
EUP2794 has an active-low shutdown pin (LOW =
shutdown, HIGH = operating). The EUP2794 SD pin
can be driven with a low-voltage CMOS logic signal
(1.5V logic, 1.8V logic, etc). There is no internal
pull-up or pull-down on the SD pin of the EUP2794.
(
)
*
(
Mirror Ratio
)
I
0.188
+
V
*0.385
BRGT
=
R
LED
SET
(
)
* 10 Amps
1
DS2794 Ver0.5 Mar. 2006
8
EUTECH [ EUTECH MICROELECTRONICS INC ]
