LM3537 Datasheet, PDF (26/30 Pages) Texas Instruments – LM3537 8-Channel WLED Driver with Four Integrated LDOs

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 con-
figured 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 flexi-
bility, the LM3537 is capable of supporting applications re-
quiring 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 op-
eration 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 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 cir-
cuitry to optimize the charge-pump gain for maximum effi-
ciency.
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 pow-
er 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 conduc-
tion 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 alu-
minum electrolytic capacitors are not recommended for use
with the LM3537 due to their high ESR, as compared to ce-
ramic 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. Ca-
pacitors with these temperature characteristics typically have
wide capacitance tolerance (+80%, -20%) and vary signifi-
cantly 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 de-
viation 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 evalu-
ated 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 perfor-
mance.
TABLE 7. Suggested Capacitors
Model
Type
Vendor
1 µF for C_OUT , C_LDO1, C_LDO2, C_LDO3, C_LDO4, C_SEN, C₁, C₂ and C_{IN_A (Note 22)}
C1005X5R1A105K
Ceramic X5R
TDK
LMK105BJ105KV-F
Ceramic X5R
Taiyo Yuden
GRM155R61A105K
Ceramic X5R
Murata
0.1 µF for C_{IN_B (Note 22)}
GRM155R61A104K
Ceramic X5R
Murata
LMK105BJ104KV-F
Ceramic X5R
Taiyo Yuden
C1005X5R1A104K
Ceramic X5R
TDK
2.2 µF for C_{IN_C}
JMK105BJ225MV-F
Ceramic X5R
Taiyo Yuden
GRM155R60J225ME15D
Ceramic X5R
Murata
Voltage Rating
Package Size
10V
0402
10V
0402
10V
0402
10V
0402
10V
0402
10V
0402
6.3V
6.3V
0402
0402
Note 22: The recommended voltage rating for these capacitors is 10V to account for DC bias capacitance losses.
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