Characteristics and limits previously presented. The avail-

able diode output current, maximum diode voltage, and all

other specifications provided in the Electrical Characteristics

table apply to parallel output configurations, just as they do

to the standard 3-LED application circuit.

LED HEADROOM VOLTAGE (V

Three current sources are connected internally between

V

(V

current sources require a sufficient amount of headroom

voltage to be present across them in order to regulate prop-

erly. Minimum required headroom voltage is proportional to

the current flowing through the current source, as dictated by

the equation:

V

The parameter k

proportionality constant that represents the ON-resistance of

the internal current mirror transistors. For worst-case design

calculations, using a k

(Worst-case recommendation accounts for parameter shifts

from part-to-part variation and applies over the full operating

temperature range). Figure 4 shows how output current of

the LM3570 varies with respect to headroom voltage.

On the flat part of the graph, the currents regulate properly

as there is sufficient headroom voltage for regulation. On the

sloping part of the graph the headroom voltage is too small,

the current sources are squeezed, and their current drive

capability is limited. Changes in headroom voltage from one

output to the next, possible with LED forward voltage mis-

match, will result in different output currents and LED bright-

ness mismatch. Thus, operating the LM3570 with insufficient

headroom voltage across the current sources should be

avoided.

TABLE 1. I

k

I

OUT

R

SET

V

HEADROOM

10mA

12.4k

Ω

300mV

I

OUT

R

SET

V

HEADROOM

15mA

8.35k

Ω

450mV

25mA

6.25k

Ω

750mV

SOFT START

Soft start is implemented internally by ramping the reference

voltage more slowly than the applied voltage. During soft

start, the current through the LED outputs and the V

OUT

voltage will ramp up in proportion to the rate that the refer-

ence voltage is being ramped up.

ENABLE / SHUTDOWN

When the voltage on the active-high-logic enable pin (EN) is

low, the LM3570 will be in shutdown. While disabled, the

LM3570 typically draws 0.1µA. When the EN pin is uncon-

nected, the part automatically goes into shutdown due to an

internal 300k

Ω pull-down resistor that is tied between EN

and GND. When the part is in shutdown, it is important to

have the PWM pin also set to ground to avoid a leakage

current resulting from an internal 300k

Ω pull-down resistor

tied between the PWM pin and ground

PWM Pin

The PWM pin on the LM3570 is responsible for turning the

three constant current sources (D1-D3) on or off without

disabling the charge pump. This pin allows for PWM bright-

ness control on the diode outputs without affecting whatever

load is tied to the V

300k

Ω pull-down resistor that by default turns off the diode

outputs when no control signal is active.

I

PWM PIN

The following procedures illustrate how to set and adjust

output current levels using the PWM pin.

Brightness Control Using PWM

1.

Determine the maximum desired I

I

2.

Brightness control can be implemented by pulsing a

signal at the PWM pin. LED brightness is proportional to

the duty cycle (D) of the PWM signal. For linear bright-

ness control over the full duty cycle adjustment range,

the PWM frequency (f) should be limited to accommo-

date the turn-on time (T

ON

= 100µs) of the current

sources.

ON

f

If the PWM frequency is much less than 100Hz, flicker

may be seen in the LEDs. For the LM3570, zero duty

cycle will turn off the LEDs and a 50% duty cycle will

result in an average I

LED current. For example, if R

15mA, a 50% duty cycle will result in an average I

7.5mA.

CAPACITOR SELECTION

The LM3570 requires 4 external capacitors for proper opera-

tion (C

multi-layer ceramic capacitors are recommended. These ca-

pacitors are small, inexpensive and have very low equivalent

series resistance (

≤ 10mΩ typ.). Tantalum capacitors, OS-

CON capacitors, and aluminum electrolytic capacitors gen-

erally are not recommended for use with the LM3570 due to

20085008

FIGURE 4. I

3 LEDs, V

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