TLED2043 Datasheet, PDF (10/16 Pages) Texas Instruments – 4-CHANNEL WHITE LED DRIVER

TLED2043
4-CHANNEL WHITE LED DRIVER
SLDS147C – JUNE 2002 – REVISED NOVEMBER 2002
TYPICAL CHARACTERISTICS
DEVICE OPERATION/APPLICATION HINTS
Refer to the functional block diagram and typical application circuit (see Figure 8)
White LEDs commonly are used to produce the backlight for small color LCDs found in portable electronics,
such as cell phones and PDAs. Because the LED’s brightness and chromaticity are dependent on the LED’s
bias current, precise control of this current is necessary for proper illumination of the LCD. The TLED2043 is,
essentially, four voltage-controlled current sinks that are regulated in unison, with each current sink setting the
bias current for one white LED. The basic circuitry and operation of the TLED2043 can be divided into three main
operating blocks described below.
output FETs
The TLED2043 has four n-channel MOSFETs to drive four external LEDs. The FETs have a specified maximum
r_on of 6 Ω, while delivering 20 mA of load current. To determine the biasing current for each LED, the gate of
each FET is driven by a buffer whose output is controlled by the current-control amplifier.
current control amplifier
The output of the Current Control Amplifier simultaneously drives four unity-gain amplifiers connected to the
gates of the output FETs. To determine the drain current of each FET (the LED current), the noninverting input
(IN+) of this amplifier typically is coupled to the battery voltage via Switch 1, while the inverting input (IN–)
typically is coupled to a fixed reference voltage that remains constant as battery voltage changes. Therefore,
the output of the Current Control Amplifier and the corresponding LED currents are proportional to the difference
between the battery and reference voltages. As battery voltage drops with use, so does the LED current,
allowing for lower power consumption to conserve battery power.
switch 1, 2, 3
The TLED2042 has three switches that are opened/closed in unison via an active-low switch-enable (SW) pin;
the switches are closed with a low input applied and opened with a high input. Each switch has a maximum loss
of 0.1 V across it; thus, the output of each switch is specified to be (V_IN – 0.1 V) < V_OUT < V_IN.
During periods of inactivity when the LEDs are not needed, significant power savings can be achieved (from
a maximum of 1 mA to 1 µA) by opening the switches. The purpose of each switch is described below:
• Switch 1: used to remove power to the TLED2043 when the LEDs are not needed.
• Switch 2 (or 3): used to remove power to the series voltage reference (V_REF in the typical application
Circuit). Without this switch, V_REF current flows continuously (through R1 and R2), even when power is
removed from the TLED2043. (Note that, if a shunt reference like the TLV431 is used, current still flows
through the reference, even if Switch 2 is open).
• Switch 3: Though not used in the typical application circuit here, this switch can be used to connect
various external circuitries to the current-control amplifier. One example is to connect an external
voltage to either input of the current-control amplifier to implement an analog LED brightness control
(see next section).
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