SC657 Datasheet, PDF (13/22 Pages) Semtech Corporation – Backlight Driver for 5 LEDs with SemPulseTM Interface

SC657
Applications Information (continued)
OVP limit. OVP operation is hysteretic, meaning the charge
pump will momentarily turn off until V_OUT is sufficiently
reduced. The maximum OVP threshold is 6.0V, allowing
the use of a ceramic output capacitor rated at 6.3V.
Over-Temperature Protection
The Over-Temperature (OT) protection circuit prevents the
device from overheating and experiencing a catastrophic
failure. When the junction temperature exceeds 160­°C, the
device goes into thermal shutdown with all outputs dis-
abled until the junction temperature is reduced. All regis-
ter information is retained during thermal shutdown.
Hysteresis of 20°C is provided to ensure that the device
cools sufficiently before re-enabling.
Charge Pump Output Current Limit
The device limits the charge pump current at the OUT pin.
If the OUT pin is shorted to ground, or V_OUT is lower than
2.5V, the typical output current limit is 60mA. The output
current is limited to 300mA when over loaded resistively
with V_OUT greater than 2.5V.
LED Float Detection
Float detect is a fault detection feature of the LED back-
light outputs. If an output is programmed to be enabled
and an open circuit fault occurs at any backlight output,
that output will be disabled to prevent a sustained output
OVP condition from occurring due to the resulting open
loop. Float detect ensures device protection but does not
ensure optimum performance. Unused LED outputs must
be disabled to prevent an open circuit fault from
occurring.
PCB Layout Considerations
Following fundamental layout rules is critical for achieving
the performance specified in the Electrical Characteristics
table. The following guidelines are recommended when
developing a PCB layout:
• Place all capacitors (C1, C2, CIN, and COUT) as
close to the device as possible.
• All charge pump current passes through the IN/
OUT and the bucket capacitor connection pins.
Ensure that all connections to these pins make
the of wide traces so that the resistive drop on
each connection is minimized.
• Make all ground connections to a solid ground
plane as shown in the example layout .
Figure 2 — Suggested Layout
Thermal Management
Although the SC657 can provide up to 125mA output
current, the maximum thermal temperature and the
thermal resistance (θ_JA) of the package and layout may
limit the output current. Thermal resistance can be
lowered by following the recommended layout guidelines
in PCB Layout Considerations, as illustrated in Figure 2.
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