11

SW

FSW

5

10

F

=

R

´

TPS61183

SLVSAB4A – JUNE 2010 – REVISED JULY 2010

www.ti.com

DETAILED DESCRIPTION

NORMAL OPERATION

The TPS61183 is a high efficiency, high output voltage white LED driver for notebook panel backlighting

applications. The advantages of white LEDs compared to CCFL backlights are higher power efficiency and lower

profile design. Due to the large number of white LEDs required to provide backlighting for medium to large

display panels, the LEDs must be arranged in parallel strings of several LEDs in series. Therefore, the backlight

driver for battery powered systems is almost always a boost regulator with multiple current sink regulators. For

normal operation there must be enough white LEDs in series to ensure the output voltage stays above the input

voltage range. Having more white LEDs 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.

The TPS61183 IC has integrated all of the key function blocks to power and control up to 60 white LEDs. The

device includes a 40 V / 2.0 A boost regulator, six 30 mA current sink regulators, and a protection circuit for

over-current, over-voltage, Open LED, Short LED, and output short circuit failures.

The TPS61183 integrates programmable PWM dimming methods with the PWM interface to control output

dimming frequency independently with input frequency. An optional direct PWM mode is user selectable through

the RFPWM/MODE selection function.

SUPPLY VOLTAGE

The TPS61183 IC has a built-in linear regulator to supply the IC analog and logic circuit. The VDDIO pin, output

of the regulator, is connected to a 1 µF bypass capacitor for the regulator to be controlled in a stable loop.

VDDIO does not have high current sourcing capability for external use but it can be tied to the EN pin for start

up.

BOOST REGULATOR AND PROGRAMMABLE SWITCH FREQUENCY (FSCLT)

The fixed-frequency PWM boost converter uses current-mode control and has integrated loop compensation.

The internal compensation ensures stable output over the full input and output voltage ranges assuming the

recommended inductance and output capacitance values in Equation 1 are used. The output voltage of the boost

regulator is automatically set by the IC to minimize voltage drop across the IFB pins. The IC regulates the lowest

IFB pin to 350 mV, and constantly adjusts the boost output voltage to account for any changes in LED forward

voltages. If the input voltage is higher than the sum of the white LED forward voltage drops (e.g., at low duty

cycles), the boost converter is not able to regulate the output due to its minimum duty cycle limitation. In this

case, increase the number of WLEDs in series or include series ballast resistors in order to provide enough

headroom for the converter to boost the output voltage. Since the TPS61183 integrates a 2.0A/40V power

MOSFET, the boost converter can provide up to a 38 V output voltage.

The TPS61183 switching frequency can be programmed between 300 kHz to 1.0MHz by the resistor value on

the FSW pin according to Equation 1:

(1)

The adjustable switching frequency feature provides the user with the flexibility of choosing the switching

frequency. A faster switching frequency will allow for an inductor with smaller inductance and footprint while a

slower switching frequency could potentially yield higher efficiency due to lower switching losses. Use Equation 1

or refer to Table 1 to select the correct value:

833K

600 KHz

625K

800 KHz

499K

1 MHz

10

Submit Documentation Feedback

Copyright © 2010, Texas Instruments Incorporated

Product Folder Link(s): TPS61183