LM3537

SNVS634B – JUNE 2011 – REVISED MAY 2013

www.ti.com

APPLICATION INFORMATION

LED CONFIGURATIONS

The LM3537 has a total of 8 current sinks capable of sinking 180mA of total diode current. These 8 current sinks

are configured to operate in one or two independently controlled lighting regions. GroupA has eight dedicated

current sinks, while GroupB has 0 by default. However, drivers D5 to D8 can be assigned to either GroupA or

GroupB one-by-one through a setting in the configuration register. With this added flexibility, the LM3537 is

capable of supporting applications requiring from 4 to 7 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 LM3537 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 tables 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 LM3537 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 LM3537 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 <20 m

Ω typ.). Tantalum capacitors, OS-CON capacitors, and aluminum electrolytic capacitors

are not recommended for use with the LM3537 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 LM3537. 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).

Capacitors with Y5V or Z5U temperature characteristic are generally not recommended for use with the LM3537.

Capacitors with these temperature characteristics typically have wide capacitance tolerance (+80%, -20%) and

vary significantly over temperature (Y5V: +22%, -82% over -30°C to +85°C range; Z5U: +22%, -56% over +10°C

to +85°C range). Under some conditions, a nominal 1µF Y5V or Z5U capacitor could have a capacitance of only

0.1µF. Such detrimental deviation is likely to cause Y5V and Z5U capacitors to fail to meet the minimum

capacitance requirements of the LM3537.

Table 7 below lists recommended external capacitors from some leading ceramic capacitor manufacturers. It is

strongly recommended that the LM3537 circuit be thoroughly evaluated early in the design-in process with the

mass-production capacitors of choice. This will help ensure that any variability in capacitance does not negatively

impact circuit performance.

26

Submit Documentation Feedback

Copyright © 2011–2013, Texas Instruments Incorporated

Product Folder Links: LM3537