7.3.1 Supply Voltage

The TPS61187 device has a built-in linear regulator to supply the device 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.

7.3.2 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 shown in Typical Application – Phase Shift PWM Mode are used. The output voltage of the boost regulator is automatically set by the device to minimize voltage drop across the IFB pins. The device regulates the lowest IFB pin to 350 mV and consistently 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 WLED forward voltage drops (for example, 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. Because the TPS61187 integrates a 2-A, 40-V power MOSFET, the boost converter can provide up to a 38-V output voltage.

The TPS61187 switching frequency can be programmed between 300 kHz to 1 MHz by the resistor value on the FSLCT pin according to Equation 1:

See Figure 3 for boost converter switching frequency adjustment resistor R_FSLCT selection.

The adjustable switching frequency feature provides the user with the flexibility of choosing a faster switching frequency, as well as an inductor with smaller inductance and footprint or slower switching frequency, and therefore, potentially higher efficiency due to lower switching losses. Use Equation 1 or refer to Table 1 to select the correct value:

7.3.3 LED Current Sinks

The six current sink regulators embedded in the TPS61187 can be collectively configured to provide up to a maximum of 30 mA each. These six specialized current sinks are accurate to within ±2% maximum for currents at 20 mA, with a string-to-string difference of ±1.5% typical.

The IFB current must be programmed to the highest WLED current expected using the ISETH pin resistor and Equation 2.

7.3.4 Enable and Start-up

The internal regulator which provides VDDIO wakes up as soon as V_IN is applied even when EN is low. This allows the device to start when EN is tied to the VDDIO pin. VDDIO does not come to full regulation until EN is high. The TPS61187 checks the status of all current feedback channels and shuts down any unused feedback channels. It is recommended to short the unused channels to ground for faster start-up.

After the device is enabled, if the PWM pin is left floating, the output voltage of the TPS61187 regulates to the minimum output voltage. Once the device detects a voltage on the PWM pin, the TPS61187 begins to regulate the IFB pin current, as pre-set per the ISETH pin resistor, according to the duty cycle of the signal on the PWM pin. The boost converter output voltage rises to the appropriate level to accommodate the sum of the white LED string with the highest forward voltage drops plus the headroom of the current sink at that current.

Pulling the EN pin low shuts down the device, resulting in the device consuming less than 11 µA in shutdown mode.

7.3.5 IFB Pin Unused

The TPS61187 has open/short string detection. For an unused IFB string, simply short it to ground or leave it open. TI recommend shorting unused IFB pins to ground for faster start-up. After EN is pulled high, the TPS61187 outputs about 40–µA current to each current channel for 4 ms and measures the voltage on each channel. If the voltage on any channel is less than 600 mV, the channel is turned off and removed from the boost control loop as unused channel.

7.4.1 Brightness Dimming Control

The TPS61187 has auto-phase-shifted PWM dimming control with the PWM control interface.

The internal decoder block detects duty information from the input PWM signal, saves it in an eight bit register and delivers it to the output PWM dimming control circuit. The output PWM dimming control circuit turns on/off six output current sinks at the PWM frequency set by RFPWM and the duty cycle from the decoder block.

The TPS61187 also has direct PWM dimming control with the PWM control interface. In direct PWM mode, each current sink turns on/off at the same frequency and duty cycle as the input PWM signal. See the Mode Selection – Phase-Shift PWM Or Direct PWM Dimming for dimming mode selection.

When in phase-shifted PWM mode, TI recommends insertion of a series resistor of 10 kΩ to 20 kΩ value close to the PWMIN pin. This resistor together with an internal capacitor forms a low pass R-C filter with 30-ns to 60-ns time constant. This prevents possible high frequency noises being coupled into the input PWM signal and causing interference to the internal duty cycle decoding circuit. However, it is not necessary for direct PWM mode because the duty cycle decoding circuit is disabled during the direct PWM mode.

7.4.2 Adjustable PWM Dimming Frequency and Mode Selection (R_FPWM/MODE)

The TPS61187 can operate in auto phase shift mode or direct PWM mode. Tying the RFPWM/MODE pin to VDDIO forces the device to operate in direct PWM mode. A resistor between the RFPWM/MODE pin and ground sets the device into auto-phase-shift mode and the value of the resistor determines the PWM dimming frequency. Use Equation 3 or refer to Table 2 to select the correct value:

7.4.3 Mode Selection – Phase-Shift PWM Or Direct PWM Dimming

The phase-shift PWM dimming method or direct PWM dimming method can be selected through the RFPWM pin. By attaching an external resistor to the RFPWM pin, the default phase shift PWM mode can be selected. To select direct PWM mode, the RFPWM pin needs to be tied to the VDDIO pin. The RFPWM/MODE pin can be noise sensitive when R2 has high impedance. In this case, careful layout or a parallel bypassing capacitor improves noise sensitivity but the value of the parallel capacitor may not exceed 33 pF for oscillator stability.

7.4.4 Overvoltage Clamp and Voltage Feedback (OVC / FB)

The correct divider ratio is important for optimum operation of the TPS61187. Use the following guidelines to choose the divider value. It can be noise sensitive if R_upper and R_down have high impedance. Careful layout is required. Also, choose lower resistance values for R_upper and R_down when power dissipation allows.

1. Determine the maximum output voltage, V_O, for the system according to the number of series WLEDs.
2. Select an R_upper resistor value (1 MΩ for a typical application; a lower value such as 100 kΩ for a noisy environment).
3. Calculate R_down using Equation 5

When the device detects that the OVC pin exceeds 1.95 V typical, indicating that the output voltage is over the set threshold point, the OVC circuitry clamps the output voltage to the set threshold.

7.4.5 Current-Sink Open Protection

For the TPS61187, if one of the WLED strings is open, the device automatically detects and disables that string. The device detects the open WLED string by sensing no current in the corresponding IFB pin. As a result, the device deactivates the open IFB pin and removes it from the voltage feedback loop. Subsequently, the output voltage drops and is regulated to the minimum voltage required for the connected WLED strings. The IFB current of the connected WLED strings remains in regulation.

If any IFB pin voltage exceeds the IFB overvoltage threshold (13.5 V typical), the device turns off the corresponding current sink and removes this IFB pin from the 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 large voltage differences among WLED strings.

The device only shuts down if it detects that all of the WLED strings are open. If an open string is reconnected again, a power-on reset (POR) or EN pin toggling is required to reactivate a previously deactivated string.

7.4.6 Overcurrent and Short-Circuit Protection

The TPS61187 has a pulse-by-pulse over-current limit of 2 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 during on overload conditions. When there is a sustained overcurrent condition, the device turns off and requires a POR or EN pin toggling to restart. Under severe overload and/or short-circuit conditions, the boost output voltage can be pulled below the required regulated voltage to keep all of the white LEDs operating. Under thses conditions, the current flows directly from input to output through the inductor and schottky diode. To protect the TPS61187, the device shuts down immediately. The device restarts after input POR or EN pin toggling.

7.4.7 Thermal Protection

When the junction temperature of the TPS61187 is over 150°C, the thermal protection circuit is triggered and shuts down the device immediately. Only a POR or EN pin toggling clears the protection and restarts the device.