© 2010 Fairchild Semiconductor Corporation www.fairchildsemi.com
FAN5776 • Rev. 1.1.0 11
FAN5776 — Synchronous Boost and Series/Parallel 10-LED Driver
Circuit Description
Overview
The FAN5776 is a 1.8 MHz synchronous step-up DC-DC
converter with integrated constant-current high-side LED
drivers capable of driving one to five LED strings up to 5 x
25 mA LED current.
The device starts when at least one LED string is utilized and
the appropriate EN pin is enabled. The device is disabled in
30 ms by setting both EN pins LOW.
The VOUT voltage is internally set to 290 mV above the highest
LED string voltage, and it is sampled at every falling LED PWM
cycle. For 100% duty cycle, the LED-pin voltage is sampled
and the VOUT voltage is refined every 10 ms.
The LED strings can be disabled by connecting them to VOUT
or shorting them to GND. They can also be left disconnected. If
the LED string is temporarily disabled or shorted, the device
must be re-enabled to enable the string again.
The LED drivers work independently and allow multiple LED
voltages, such that many types of LEDs can be driven at the
same time and some strings can be used to drive a single LED
while other channels are driving two LEDs in series. The VOUT
voltage is defined by the highest LED voltage and the LED
driver dropout voltage is increased to provide the LED string a
specific voltage. If the voltage difference between the LED
strings is large, the system efficiency may decrease.
LED Current
The LED string current is set by the resistor, RSET, between the
ISET and GND pins. The same current is applied to across all
strings such that total output current: IOUT = 5 x ILED = 5 x
20 mA = 100 mA if RSET = 25 kand all LED strings are used.
In general, the LED string current can be calculated as follows:
Figure 26. LED Current vs. RSET Value
Startup
The three different startup functions depend on the system
configuration:
1. All LED strings are utilized: Setting one or both EN pins
HIGH enables the device and VOUT rises to 7.5 V.
FAN5776 starts to step up or down to the appropriate
regulated voltage.
2. At least one LED string in a group is shorted to GND. VOUT
rises to 7.5 V while the shorted LED string is disabled and
the device starts to step up or down to regulated voltage.
3. At least one LED string is floating or connected to VOUT.
VOUT rises to 9.0 V, the floating LED string is disabled, and
the device starts to step down to regulated voltage.
These functions work for each group independently. If all five
strings are utilized and EN13 is HIGH, VOUT rises to 7.5 V (case
1) and goes to the highest voltage required by LED1-3. Then
EN45 is raised and VOUT is stepped up again to 7.5 V and
regulates to highest voltage required by LED1-5.
If VOUT cannot reach 7.5 V within 1.2 ms after an enable cycle,
the device stays disabled and a new enable cycle is required.
PWM Dimming
A LED PWM signal of 100 Hz to 800 Hz can be applied to
EN13 and EN45 pins to control LED1-3 and LED4-5 light
intensity. The LED current is a linear function of the LED
PWM duty cycle from 100% down to 0.4%. The FAN5776
can be started by a PWM signal with a low duty cycle to
enable smooth startup. EN13 and EN45 pins can be
operated either synchronously or asynchronously, which
makes it possible to use the device to backlight two
separate displays at the same time.
Under-Voltage Lockout (UVLO)
The Under-Voltage Lockout circuitry turns off all MOSFETs and
the device remains in a very low quiescent current state until
VIN has risen above the UVLO threshold.
Short-Circuit Protection (SCP)
The LED driver output current is limited to 0.5 µA or less when
a LED number pin voltage is below 1.0 V. This limit shall be
applied within one LED PWM cycle, or 10 ms, whichever
elapses first.
Over-Voltage Protection (OVP)
When the regulator is active, it monitors the VOUT pin. If the
VOUT voltage reaches 9.0 V, the regulator stops switching until
the capacitor at VOUT discharges below 8.5 V.
LED-Open Detection
If VOUT is detected above >9.0 V, the LED voltages are
scanned. All LED pins with voltage greater than VOUT - 0.5 V
are disabled. If all LED pins’ voltages exceed 8.5 V and VOUT is
greater than 9.0 V, device is disabled and a new startup cycle
is required.
Over-Current Protection (OCP)
The PWM converter is protected against overload through
cycle-by cycle current limit using a fixed internal limit.
Thermal Shutdown
When the die temperature exceeds 150°C, reset occurs and
remains in effect until the die cools to 125°C; at which time,
the circuit enters the normal soft-start sequence.
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