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LM3535

SNVS598B – AUGUST 2010 – REVISED MARCH 2018

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Copyright © 2010–2018, Texas Instruments Incorporated

8.2.2.3.1

Total Output Current Capability

The maximum output current that can be drawn from the LM3535 is 200 mA.

DRIVER TYPE

MAXIMUM Dxx CURRENT

DxA

25 mA per DxA pin

DxB

25 mA per DxB pin

D1C

25 mA

8.2.2.4 Parallel Connected and Unused Outputs

Connecting the outputs in parallel does not affect internal operation of the LM3535 and has no impact on the

Electrical Characteristics and limits previously presented. The available diode output current, maximum diode

voltage, and all other specifications provided in the Electrical Characteristics table apply to this parallel output

configuration, just as they do to the standard LED application circuit.

All Dx current sinks utilize LED forward voltage sensing circuitry to optimize the charge-pump gain for maximum

efficiency. Due to the nature of the sensing circuitry, TI recommends not leaving any of the Dx pins open when

the current sinks are enabled (ENx bits are set to 1). Leaving Dx pins unconnected forces the charge-pump into

3/2× mode over the entire VIN range negating any efficiency gain that could have been achieved by switching to

1× mode at higher input voltages.

If the D1B or D1C drivers are not going to be used, make sure that the ENB and ENC bits in the general purpose

register are set to 0 to ensure optimal efficiency.

8.2.2.5 Power Efficiency

Efficiency of LED drivers is commonly taken to be the ratio of power consumed by the LEDs (PLED) to the power

drawn at the input of the part (PIN). With a 3/2× – 1× charge pump, the input current is equal to the charge pump

gain times the output current (total LED current). The efficiency of the LM3535 can be predicted as follow:

PLEDTOTAL = (VLEDA × NA × ILEDA) + (VLEDB × NB × ILEDB) + (VLEDC × ILEDC)

(10)

PIN = VIN × IIN

(11)

PIN = VIN × (GAIN × ILEDTOTAL + IQ)

(12)

E = (PLEDTOTAL / PIN)

(13)

The LED voltage is the main contributor to the charge-pump gain selection process. Use of low forward-voltage

LEDs (3 V to 3.5 V) allows the LM3535 to stay in the gain of 1× for a higher percentage of the lithium-ion battery

voltage range when compared to the use of higher forward voltage LEDs (3.5 V to 4 V). See LED Forward

Voltage Monitoring for a more detailed description of the gain selection and transition process.

For an advanced analysis, TI recommends that power consumed by the circuit (VIN x IIN) for a given load be

evaluated rather than power efficiency.

8.2.2.6 Power Dissipation

The power dissipation (PDISS) and junction temperature (TJ) can be approximated with the equations below. PIN is

the power generated by the 3/2× – 1× charge pump, PLED is the power consumed by the LEDs, TA is the ambient

temperature, and RθJA is the junction-to-ambient thermal resistance for the DSBGA 20-bump package. VIN is the

input voltage to the LM3535, VLED is the nominal LED forward voltage, N is the number of LEDs and ILED is the

programmed LED current.

PDISS = PIN – PLEDA - PLEDB – PLEDC

(14)

PDISS= (GAIN × VIN × IGroupA + GroupB + GroupC ) – (VLEDA × NA × ILEDA) – (VLEDB × NB × ILEDB) – (VLEDC × ILEDC)

(15)

TJ = TA + (PDISS x RθJA)

(16)

The junction temperature rating takes precedence over the ambient temperature rating. The LM3535 may be

operated outside the ambient temperature rating, so long as the junction temperature of the device does not

exceed the maximum operating rating of 110°C. The maximum ambient temperature rating must be derated in

applications where high power dissipation and/or poor thermal resistance causes the junction temperature to

exceed 110°C.