TPS61180/1/2

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SLVS801E – DECEMBER 2007 – REVISED APRIL 2013

DETAILED DESCRIPTION

Recently, WLEDs have gained popularity as an alternative to CCFL for backlighting media size LCD displays.

The advantages of WLEDs are power efficiency and low profile design. Due to the large number of WLEDs, they

are often arranged in series and parallel, and powered by a boost regulator with multiple current sink regulators.

Having more WLEDs in series reduces the number of parallel strings and therefore improves overall current

matching. However, the efficiency of the boost regulator declines due to the need for high output voltage. Also,

there have to be enough WLEDs in series to ensure the output voltage stays above the input voltage range.

Otherwise, a buck-boost (for example, SEPIC) power converter has to be adopted which could be more

expensive and complicated.

The TPS61180/1/2 family of ICs have integrated all the key function blocks to power and control up to 60

WLEDs. The devices include a 40V/1.5A boost regulator, six 25mA current sink regulators and protection circuit

for over-current, over-voltage and short circuit failures. The key advantages of the devices are small solution

size, low output AC ripple during PWM dimming control, and the capability to isolate the input and output during

fault conditions.

SUPPLY VOLTAGE

The TPS61181/2 ICs have built-in LDO linear regulator to supply the IC analog and logic circuit. The LDO is

powered up when the EN pin is high. The output of the LDO is connected to the Cin pin. A 0.1

μF bypass

capacitor is required for LDO’s stable operation. Do not connect the Cin pin to the EN pin because this prevents

the IC from starting up. In addition, avoid connecting the Cin pin to any other circuit as this could introduce noise

into the IC supply voltage.

The TPS61180 has no built-in LDO linear regulator, and therefore requires an external supply voltage in the

range of 2.7V to 3.6V connected to the Cin pin. The benefit of using external supply is to reduce the power

losses incurred by the LDO as it provides the IC supply current. This loss could become a significant percentage

of total output power under light load condition. The Cin pin has 2.2V (typical) under-voltage lock out which turns

off the IC when the Cin pin voltage is below this threshold.

BOOST REGULATOR

The boost regulator is controlled by current mode PWM, and loop compensation is integrated inside the IC. The

internal compensation ensures stable output over the full input and output voltage range. The TPS61180/1

switches at 1.0MHz, and the TPS61182 switches at 1.3MHz. The switching frequencies of the two devices,

including their tolerance, due not over-lap. Therefore, in the unlikely event that one device creates

electromagnetic inference to the system; the other device, switching at a different frequency, can provide an

alternative solution.

The output voltage of the boost regulator is automatically set by the IC to minimize the voltage drop across the

IFB pins. The IC automatically regulates the lowest IFB pin to 400mV, and consistently adjusts the boost output

voltage to account for any changes of the LED forward voltages.

When the output voltage is too close to the input, the boost regulator may not be able to regulate the output due

to the limitation of minimum duty cycle. In this case, increase the number of WLED in series or include series

ballast resistors in order to provide enough headroom for the boost operation.

The TPS61180/1/2 boost regulators cannot regulate their outputs to voltages below 15V.

Copyright © 2007–2013, Texas Instruments Incorporated

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Product Folder Links: TPS61180/1/2