LP3359-MSF POWER [Lowpower Semiconductor inc], LP3359-MSF Datasheet - Page 5

• Manufacturer Part Number: LP3359-MSF
• Description: High performance, Current Source For Parallel White-LED Driver
• Manufacturer: POWER [Lowpower Semiconductor inc]

Applications Information
Enable/Shutdown
When the voltage on the active-high-logic enable pin is
low, the LP3359 will be in shutdown. While disabled, the
LP3359 typically draws 0.01µA. There is no internal
pull-up or pull-down on the PWM pin of the LP3359, Do
not let PWM pin floating.
Output Current Capability
The LP3359 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
external resistor can be used to set the output current, as
approximated with the following the equation:
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:
V
defined by the following equation:
ILEDX is the desired diode current, and kHR, typically
15mV/mA in the LP3359, 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 LP3359 with
insufficient headroom voltage across all current sources
LP3359-datasheet Ver.1.1 Nov.-2005
LEDX
is the diode forward voltage, and VHR is
±
0.3% between adjacent sources. An
should be avoided.
Brightness Control
(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.
If the PWM frequency is much less than 100Hz, flicker
may be seen in the LEDs. For the LP3359, 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.
(2)Using a DC Voltage Added to R
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
5
-
5
SET
LP3359