LED7708TR datasheet(18/53 Pages) STMICROELECTRONICS

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Device description

LED7708

18/53

Doc ID 022698 Rev 1

Device description

The LED7708 is an LED driver that integrates a boost controller, sixteen current generators

and a 4-wire serial interface. It has been specifically designed for driving the LED backlight

in medium to large LCD panels.

The device is controlled by a 4-wire serial interface and can operate both as standalone

driver and as master driver in conjunction with other slave devices.

Basic and advanced dimming functions are supported in order to meet different application

requirements. Regardless of the dimming control technique, the LED7708 can optimize the

power dissipation by controlling the output voltage of the boost converter. Two internal linear

regulators derive the device supply (3.3 V) and the gate driver supply (5 V) from a single

power rail.

7.1

Device supply

The LED7708 integrates two low drop-out linear regulators to derive the +3.3 V (typ.) main

supply and the +5 V supply for the gate driver. The VIN pin is the input terminal for both

linear regulators. The output of the 3.3 V LDO is the LDO3 pin, the output of the 5 V LDO is

the VDR pin. The 3.3 V LDO (LDO3 pin) is active if the LREN (linear regulator enable) pin is

left floating or externally set high (see Section 5: Electrical characteristics for the thresholds

involved).

The serial interface I/O stage is supplied through the VSI pin. This pin is used to enable the

serial interface and it allows the setting of the logic level of the serial interface signals (3.3 V

or 5 V) at the same time. The serial interface can be powered by the internal 3.3 V LDO (by

simply connecting the LDO3 pin to the VSI pin) or by an external source, usually the same

supply rail as the host controller.

When an external +3.3 V source is available, it is possible to bypass the 3.3 V LDO by

connecting VIN, VDR and LDO3 together to the +3.3 V rail; in this case the gate driver is

supplied at 3.3 V too and a super-logic level MOSFET is required as the power switch.

The 3.3 V regulator is self-protected against overcurrent and thermal damage thanks to a

foldback mechanism. The continuous current capability of the 3.3 V LDO is calculated to not

exceed the junction temperature limit in worst power dissipation conditions.

A 1 µF MLCC on the LDO3 pin is required for ripple filtering and LDO output stability.

The LDO3 pin is internally connected to the undervoltage lockout (UVLO) protection; the

upper and lower UVLO thresholds are reported in Section 5: Electrical characteristics.

When the voltage at the LDO3 pin falls below the lower UVLO threshold, the device is

turned off (non-latched condition). The device turns on when the voltage at the LDO3 pin

crosses the upper UVLO threshold. Crossing the lower and upper thresholds in sequence

performs the so called “power-on reset” (POR). The POR sequence is used, as well as

toggling the internal DEN bit (see internal registers), when the device is asked to restore

normal operation after a latched fault condition. The 5 V linear regulator, whose output is

connected to the VDR pin, is turned off when the device is disabled, while the 3.3 V linear

regulator is controlled by the LREN pin only.

LED7708TR 数据表(PDF) 18 Page - STMicroelectronics

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