SC652

13

Applications Information (continued)

average current. Providing a DC current through the

LEDs instead of a pulsed current provides an efficiency

advantage over other PWM controlled systems by allow-

ing the charge pump to remain in 1x mode longer

because the maximum current is equal to the average

current.

PWM Sampling

The sampling system that translates the PWM signal to

a DC current requires the EN/PWM pin to have a

minimum high time t

HIGH_MIN

to set the DC level. High

time less than t

HIGH_MIN

impacts the accuracy of the target

I

BL

. The minimum duty cycle needed to support the

minimum high time specification varies with the applied

PWM frequency (see figure 1). Note that use of a lower

PWM frequency, from 200Hz to 10kHz, will support

lower minimum duty cycle and an extended backlight

dimming range.

0

1

2

3

4

5

010

20

30

40

50

PWM Frequency (kHz)

Figure 1 — Minimum Duty Cycle

Shutdown Mode

The device is disabled when the EN/PWM pin is held low

for 7ms or longer.

Protection Features

The SC652 provides several protection features to safe-

guard the device from catastrophic failures. These features

include:

Output Open Circuit Protection

Over-Temperature Protection

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Charge Pump Output Current Limit

LED Float Detection

Output Open Circuit Protection

Over-Voltage Protection (OVP) at the OUT pin prevents

the charge pump from producing an excessively high

output voltage. In the event of an open circuit between

the OUT pin and all current sinks (no loads connected),

the charge pump runs in open loop and the voltage rises

up to the 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 cata-

strophic failure. When the junction temperature exceeds

165°C, the device goes into thermal shutdown with all

outputs disabled until the junction temperature is

reduced. All register 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 45mA. The

typical output current is limited to 400mA 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 result-

ing open loop. Float detect ensures device protection

but does not ensure optimum performance.

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