LM3538

SNVS674A – JUNE 2011 – REVISED MAY 2013

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B0 GROUP B BRIGHTNESS

Bits [2:0] set the Group B Brightness Levels, as shown in Table 6 below:

Table 6. Group B Brightness Levels

dxb[2]

dxb[1]

dxb[0]

Group B LED Current (typ.)

1

1

1

25.0mA

1

1

0

17.5mA

1

0

1

15.0mA

1

0

0

12.5mA

0

1

1

10.0mA

0

1

0

7.5mA

0

0

1

5.0mA

0

0

0

2.5mA

Application Information

LED CONFIGURATIONS

The LM3538 has a total of 6 current sinks capable of sinking 150mA of total diode current. These 6 current sinks

are configured to operate in one or two independently controlled lighting regions. Group A has six dedicated

current sinks, while Group B has 0 by default. However, drivers D5 and D6 can be assigned to either Group A or

Group B one-by-one through a setting in the configuration register. With this added flexibility, the LM3538 is

capable of supporting applications requiring 4 or 5 LEDs for main display lighting, while still providing additional

current sink(s) that can be used for a wide variety of lighting functions.

PARALLEL CONNECTED AND UNUSED OUTPUTS

Connecting the outputs in parallel does not affect internal operation of the LM3538 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.

If some of the drivers are not going to be used, make sure that the enable bits in the DIODE ENABLE register

are set to '0' to ensure optimal efficiency.

THERMAL PROTECTION

Internal thermal protection circuitry disables the LM3538 when the junction temperature exceeds 160°C (typ.).

This feature protects the device from being damaged by high die temperatures that might otherwise result from

excessive power dissipation. The device will recover and operate normally when the junction temperature falls

below 155°C (typ.). It is important that the board layout provide good thermal conduction to keep the junction

temperature within the specified operating ratings.

CAPACITOR SELECTION

The LM3538 circuit requires 11 external capacitors for proper operation. Surface-mount multi-layer ceramic

capacitors are recommended. These capacitors are small, inexpensive and have very low equivalent series

resistance (ESR <20mOhm typ.). Tantalum capacitors, OS-CON capacitors, and aluminum electrolytic capacitors

are not recommended for use with the LM3538 due to their high ESR, as compared to ceramic capacitors.

For most applications, ceramic capacitors with X7R or X5R temperature characteristic are preferred for use with

the LM3538. These capacitors have tight capacitance tolerance (as good as ±10%) and hold their value over

temperature (X7R: ±15% over -55°C to 125°C; X5R: ±15% over -55°C to 85°C).

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