EUP3595
Application Information
Brightness Control
Enable/Shutdown
(1)Using a PWM Signal to EN/PWM Pin
Brightness control can be implemented by pulsing a
signal at the PWM pin. The RSET value should be
selected using the RSET equation. LED brightness is
proportional to the duty cycle (D) of the PWM signal.
For linear brightness control over the full duty cycle
adjustment range, the PWM frequency (f) should be
limited to accommodate the turn-on time (TON = 20µs)
of the de- vice.
When the voltage on the active-high-logic enable pin
is low, the EUP3595 will be in shutdown. While
disabled, the EUP3595 typically draws 0.01µA. There
is no internal pull-up or pull-down on the PWM pin
of the EUP3595, Do not let PWM pin floating.
Output Current Capability
The EUP3595 is capable of providing up to 25mA of
current to each of the four outputs given an input
voltage of 3.0V to 5.5V. The outputs have a typical
current matching of ± 0.3% between adjacent sources.
An external resistor can be used to set the output
current, as approximated with the following the
equation:
∗
D
(
1/ f
)
> T
ON
f
= D
/ T
ON
MAX
MIN
If the PWM frequency is much less than 100Hz,
flicker may be seen in the LEDs. For the EUP3595,
zero duty cycle will turn off the LEDs and a 50%
duty cycle will result in an average ILED being half of
the programmed LED current. For example, if RSET is
set to program 15mA, a 50% duty cycle will result in
an average ILED of 7.5mA, LED being half the
programmed LED current. RSET should be chosen not
to exceed the maximum current delivery capability of
the device.
R
= 100 ×
(
1.18V / I
)
LEDX
SET
In order for the output currents to be regulated
properly, sufficient headroom voltage (VHR) must be
present. The headroom voltage refers to the minimum
amount of voltage that must be present across the
current source in order to ensure the desired current is
realizable. To ensure the desired current is obtained,
apply the following equations to find the minimum
input voltage required:
(2)Using a DC Voltage Added to RSET
Using an analog input voltage VADJ via a resister RADJ
connects to the RSET pin can also be used to achieve
setting LED current. Figure 3 shows this application
circuit. For this application the LED's current can be
derived from the following Equation. Figure 4 and
table 2 shows the relation between VADJ and ILED of a
typical application example, where the VADJ from 0 to
2.5V, RSET equals 11.5kΩ and RADJ equals 12.5kΩ.
V
− V
≥ V
IN
LEDX HR
VLEDX is the diode forward voltage, and VHR is
defined by the following equation:
V
= K
×
(
0.95× I
)
LEDX
HR
HR
ILEDX is the desired diode current, and kHR, typically
15mV/mA in the EUP3595, is a proportionality
constant that represents the ON-resistance of the
internal current mirror transistors. For worst-case
design calculations, using a kHR of 20mV/mA is
recommended. (Worst-case recommendation accounts
for parameter shifts from part-to-part variation and
applies over the full operating temperature range).
Changes in headroom voltage from one output to
the next, possible with LED forward voltage
mismatch, will result in different output currents and
LED brightness mismatch. Thus, operating the
EUP3595 with insufficient headroom voltage across
all current sources should be avoided.
V
R
1
1
ADJ
ADJ
I
=100× 1.18×
+
−
LED
R
R
SET
ADJ
Table 1. ILEDX, RSET and VHR MIN
-
kHR= 20 mV/mA (worst-case)
Figure3. The Application Circuit of Brightness
which Uses a DC Voltage Into RSET
IOUT
RSET
VHEADROOM
12kΩ
7.5kΩ
4.7kΩ
10mA
15mA
25mA
200mV
300mV
500mV
DS3595 Ver1.0 Feb. 2007
6
EUTECH [ EUTECH MICROELECTRONICS INC ]
