LM3535 datasheet(21/26 Pages) NSC

21 / 26 page

PWM bit set to ‘1’ (Configuration Register bit [0]). Figure 20

shows how the different blocks (PWM and ALS) influence the

LED current.

30082446

FIGURE 20. Current Control Block Diagram

ALS + PWM Example

In this example, the APDS-9005 sensor detects that the am-

bient light has changed to 1 kLux. The voltage at the ALS input

is now around 875mV and the ambient light falls within Zone

5. This causes the LED brightness to be a function of Zone

Target Register 5 (loaded with 0x7F). Now assume the PWM

input is also driven with a 50% duty cycle pulsed waveform.

The LED current now becomes:

LED CONFIGURATIONS

The LM3535 has a total of 8 current sinks capable of sinking

200mA of total diode current. These 8 current sinks are con-

figured to operate in three independently controlled lighting

regions. GroupA has four dedicated current sinks, while

GroupB and GroupC each have one. To add greater lighting

flexibility, the LM3535 has two additional drivers (D53 and

D62) that can be assigned to either GroupA or GroupB

through a setting in the general purpose register.

At start-up, the default condition is four LEDs in GroupA, three

LEDs in GroupB and a single LED in GroupC (NOTE: GroupC

only consists of a single current sink (D1C) under any con-

figuration). Bits 53A and 62A in the general purpose register

control where current sinks D53 and D62 are assigned. By

writing a '1' to the 53A or 62A bits, D53 and D62 become as-

signed to the GroupA lighting region. Writing a '0' to these bits

assigns D53 and D62 to the GroupB lighting region. With this

added flexibility, the LM3535 is capable of supporting appli-

cations requiring 4, 5, or 6 LEDs for main display lighting,

while still providing additional current sinks that can be used

for a wide variety of lighting functions.

MAXIMUM OUTPUT CURRENT, MAXIMUM LED

VOLTAGE, MINIMUM INPUT VOLTAGE

The LM3535 can drive 8 LEDs at 25mA each (GroupA ,

GroupB, GroupC) from an input voltage as low as 3.2V, so

long as the LEDs have a forward voltage of 3.6V or less (room

temperature).

The statement above is a simple example of the LED drive

capability of the LM3535. The statement contains the key ap-

plication parameters that are required to validate an LED-

drive design using the LM3535: LED current (I

LEDx), number

of active LEDs (N

x), LED forward voltage (VLED), and mini-

mum input voltage (V

IN-MIN).

The equation below can be used to estimate the maximum

output current capability of the LM3535:

LED_MAX = [(1.5 x VIN) - VLED - (IADDITIONAL × ROUT)] /

[(N

x x ROUT) + kHRx] (eq. 1)

LED_MAX = [(1.5 x VIN ) - VLED - (IADDITIONAL × 2.4Ω)] /

[(N

x x 2.4Ω) + kHRx]

ADDITIONAL is the additional current that could be delivered to

the other LED Groups.

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