8.3.1 Supply Voltage

The TPS61181A has built-in LDO linear regulator to supply the device analog and logic circuits. 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 stable operation of the LDO. Do not connect the Cin pin to the EN pin because this prevents the device from starting up. In addition, avoid connecting the Cin pin to any other circuit as this could introduce noise into the device supply voltage.

The V_BAT connects to the input of the internal LDO, and powers the device. The voltage on the V_BAT pin is also the reference for the pull-up circuit of the Fault pin. In addition, it serves as the input signal to the short-circuit protection. There is an undervoltage lockout on the V_BAT pin which disables the device when its voltage reduces to 4.2 V (typical). The device restarts when the V_BAT pin voltage recovers by 220 mV.

8.3.2 Boost Regulator

The boost regulator is controlled by current mode PWM, and loop compensation is integrated inside the device. The internal compensation ensures stable output over the full input and output voltage range. The TPS61181A switches at 1 MHz which optimize boost converter efficiency and voltage ripple with a small form factor inductor and output capacitor.

The output voltage of the boost regulator is automatically set by the device to minimize the voltage drop across the IFB pins. The device automatically regulates the lowest IFB pin to 400 mV, 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 TPS61181A boost regulator cannot regulate its output to voltages below 15 V.

8.3.3 Enable and Start-Up

A logic high signal on the EN pin turns on the device. For the TPS61181A, taking EN high turns on the internal LDO linear regulator which provides supply to the device current. Then, an internal resistor, R_start (start up charging resistor) is connected between the V_BAT pin and V_O pin to charge the output capacitor toward the V_BAT pin voltage. The Fault pin outputs high during this time, and thus the external isolation PFET is turned off. Once the V_O pin voltage is within 2 V (isolation FET start up threshold) of the V_BAT pin voltage, R_start is open, and the Fault pin pulls down the gate of the PFET and connects the V_BAT voltage to the boost regulator. This operation is to prevent the in-rush current due to charging the output capacitor.

Once the isolation FET is turned on, the device starts PWM switching to raise the output voltage above V_BAT. Soft-start is implemented by gradually ramping up the reference voltage of the error amplifier to prevent voltage overshoot and in-rush current. See the start-up waveform of a typical example, Figure 8.

Pulling the EN pin low immediately shuts down the device, resulting in the device consuming less than 50 μA in the shutdown mode.

8.3.4 Overcurrent, Overvoltage, and Short-Circuit Protection

The TPS61181A has pulse-by-pulse over-current limit of 1.5 A (minimum). The PWM switch turns off when the inductor current reaches this current threshold. The PWM switch remains off until the beginning of the next switching cycle. This protects the device and external components under overload conditions. When there is sustained over-current condition for more than 16 ms ( under 100% dimming duty cycle), the device turns off and requires POR or the EN pin toggling to restart.

Under severe overload and/or short circuit conditions, the V_O pin can be pulled below the input (V_BAT pin). Under this condition, the current can flow directly from V_BAT to the WLED through the inductor and schottky diode. Turning off the PWM switch alone does not limit current anymore. In this case, the TPS61181A detects the output voltage is 1.7 V (V_sc,short circuit detection threshold) below the input voltage, turns off the isolation FET, and shuts down the device. The device restarts after input power-on reset (V_BAT POR) or EN pin logic toggling.

During device start up, if there is short-circuit condition on the boost converter output, the output capacitor will not be charged to within 2 V of V_BAT through R_start. After 32 ms (V_{sc_dly}short circuit detection delay during start-up), the TPS61181A shuts down and does not restart until there is V_BAT POR or EN pin toggling. The isolation FET is never turned on under the condition.

If one of the WLED strings is open, the boost output rises to overvoltage threshold (39V typical). The TPS61181A detects the open WLED string by sensing no current in the corresponding IFB pin. As a result, the device removes the open IFB pin from the voltage feedback loop. Subsequently, the output voltage drops down and is regulated to a voltage for the connected WLED strings. The IFB current of the connected WLED strings keep in regulation during the whole transition. The device only shuts down if it detects that all of the WLED strings are open.

If the overvoltage threshold is reached, but the current sensed on the IFB pin is below the regulation target, the device regulates the boost output at the overvoltage threshold. This operation could occur when the WLED is turned on under cold temperature, and the forward voltages of the WLEDs exceed the overvoltage threshold. Maintaining the WLED current allows the WLED to warm up and their forward voltages to drop below the over-voltage threshold.

If any IFB pin voltage exceeds IFB overvoltage threshold (17 V typical), the device turns off the corresponding current sink and removes this IFB pin from V_O regulation loop. The current regulation of the remaining IFB pins is not affected. This condition often occurs when there are several shorted WLEDs in one string. WLED mismatch typically does not create such large voltage difference among WLED strings.

8.3.5 IFB Pin Unused

If the application requires less than 6 WLED strings, one can easily disable unused IFB pins. The TPS61181A simply requires leaving the unused IFB pin open or shorting it to ground. If the IFB pin is open, the boost output voltage ramps up to V_O overvoltage threshold during start up. The device then detects the zero current string, and removes it from the feedback loop. If the IFB pin is shorted to ground, the device detects the short immediately after device enable, and the boost output voltage does not go up to V_O overvoltage threshold. Instead, it ramps to the regulation voltage after soft start.

8.4.1 Current Program and PWM Dimming

The six current sink regulators can each provide a maximum of 30 mA. The IFB current must be programmed to maximum WLED current using the ISET pin resistor and Equation 1.

Equation 1.

where

• K_ISET = Current multiple (1000 typical)
• V_ISET = ISET pin voltage (1.229 V typical)
• R_ISET = ISET pin resistor

The TPS61181A has six built-in precise current sink regulators. The current matching among the current sinks at 20-mA current through is below 2.5%. This means the differential value between the maximum and minimum current of the six current sinks divided by the average current of the six is less than 2.5%.

The WLED brightness is controlled by the PWM signal on the DCTRL pin. The frequency and duty cycle of the DCTRL signal is replicated on the IFB pin current. Keep the dimming frequency in the range of 100 Hz to 1 kHz to avoid screen flickering and maintain dimming linearity. Screen flickering may occur if the dimming frequency is below the range. The minimum achievable duty cycle increases with the dimming frequency. For example, while a 0.1% dimming duty cycle, giving a 1000:1 dimming range, is achievable at 100 Hz dimming frequency, only 1% duty cycle, giving a 100:1 dimming range, is achievable with a 1-KHz dimming frequency, and 5% dimming duty cycle is achievable with 5-KHz dimming frequency. The device could work at high dimming frequency like 20 KHz, but then only 15% duty cycle could be achievable. The TPS61181A is designed to minimize the AC ripple on the output capacitor during PWM dimming. Careful passive component selection is also critical to minimize AC ripple on the output capacitor. See Application and Implementation for more information.