SC657 datasheet(13/22 Pages) SEMTECH

13 / 22 page

Applications Information (continued)

SC657

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 60°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.

Thermal Management

Although the SC657 can provide up to 25mA 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.

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 (C, 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

•

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