LP3359

LP3359-datasheet

Ver.1.1

Nov.-2005

5

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:

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

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.

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