SC652 Datasheet, PDF (13/16 Pages) Semtech Corporation – Backlight Driver for 5 LEDs with Charge Pump and PWM Control

SC652
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
to set the DC level. High
HIGH_MIN
time less than t
impacts the accuracy of the target
HIGH_MIN
I . The minimum duty cycle needed to support the
BL
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.
5 ^{tHIGH_MIN = 1μs}
4
3
• 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 is
OUT
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.
2
1
0
0
10
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
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 is lower
OUT
than 2.5V, the typical output current limit is 45mA. The
typical output current is limited to 400mA when over
loaded resistively with V greater than 2.5V.
OUT
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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