ILED = ILED_FS u ZoneTarget5 u D = 25 mA u 100% u 50% | 12.5 mA

25

LM3535

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SNVS598B – AUGUST 2010 – REVISED MARCH 2018

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8.2.2.1.4.1

ALS + PWM Example

In this example, the APDS-9005 sensor detects that the ambient light has changed to 1 kLux. The voltage at the

ALS input is now approximately 875 mV 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:

(4)

8.2.2.2 LED Configurations

The LM3535 has a total of 8 current sinks capable of sinking 200 mA of total diode current. These 8 current sinks

are configured 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 configuration). 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 assigned 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 applications

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.

8.2.2.3 Maximum Output Current, Maximum LED Voltage, Minimum Input Voltage

The LM3535 can drive 8 LEDs at 25 mA each (GroupA , GroupB, GroupC) from an input voltage as low as 3.2 V,

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

The statement above is a simple example of the LED drive capability of the LM3535. The statement contains the

key application parameters that are required to validate an LED-drive design using the LM3535: LED current

(ILEDx), number of active LEDs (Nx), LED forward voltage (VLED), and minimum input voltage (VIN-MIN).

Equation 5 and Equation 6 can be used to estimate the maximum output current capability of the LM3535:

ILED_MAX = [(1.5 x VIN) – VLED – (IADDITIONAL × ROUT)] / [(Nx × ROUT) + kHRx]

(5)

ILED_MAX = [(1.5 x VIN ) - VLED – (IADDITIONAL × 2.4 Ω)] / [(Nx × 2.4 Ω) + kHRx]

(6)

IADDITIONAL is the additional current that could be delivered to the other LED groups.

ROUT – Output resistance. This parameter models the internal losses of the charge pump that result in voltage

droop at the pump output VOUT. Since the magnitude of the voltage droop is proportional to the total output

current of the charge pump, the loss parameter is modeled as a resistance. The output resistance of the LM3535

is typically 2.4

Ω (VIN = 3.6 V, TA = 25°C) — see Equation 7:

VVOUT = (1.5 × VIN) – [(NA × ILEDA + NB × ILEDB + NC × ILEDC) × ROUT]

(7)

kHR – Headroom constant. This parameter models the minimum voltage required to be present across the current

sinks for them to regulate properly. This minimum voltage is proportional to the programmed LED current, so the

constant has units of mV/mA. The typical kHR of the LM3535 is 4mV/mA — see Equation 8:

(VVOUT – VLEDx) > kHRx × ILEDx

(8)

Typical Headroom Constant Values kHRA = kHRB = kHRC = 4 mV/mA

(9)

Equation 5 is obtained from combining Equation 7 (the ROUT equation) with Equation 8 (the kHRx equation) and

solving for ILEDx. Maximum LED current is highly dependent on minimum input voltage and LED forward voltage.

Output current capability can be increased by raising the minimum input voltage of the application, or by

selecting an LED with a lower forward voltage. Excessive power dissipation may also limit output current

capability of an application.