LM3537_13 Datasheet, PDF (26/33 Pages) Texas Instruments – 8-Channel WLED Driver with Four Integrated LDOs

LM3537
SNVS634B – JUNE 2011 – REVISED MAY 2013
APPLICATION INFORMATION
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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 mΩ 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.
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