7.3.1 Supply Voltage

The TPS61199 has a built-in linear regulator to supply the device analog and logic circuitry. The VDD pin, output of the regulator, must be connected to a 2.2-µF bypass capacitor. VDD only has a current sourcing capability of 15 mA. VDD voltage is ready after the EN pin is pulled high.

7.3.2 Boost Controller

A boost controller is shown at the top of the Functional Block Diagram. The TPS61199 regulates the output voltage with current mode pulse width modulation (PWM) control. The control circuitry turns on an external switch FET at the beginning of each switching cycle. The input voltage is applied across the inductor and stores the energy as the inductor current ramps up. During this portion of the switching cycle, the load current is provided by the output capacitor. When the inductor current rises to the threshold set by the Error Amplifier (EA) output, the switch FET is turned off and the external Schottky diode is forward biased. The inductor transfers stored energy to replenish the output capacitor and supply the load current. This operation repeats each switching cycle. The switching frequency is programmed by the external resistor.

A ramp signal from the oscillator is added to the current ramp to provide slope compensation, shown in the Oscillator and Slope Compensation block. The duty cycle of the converter is then determined by the PWM Logic block which compares the EA output and the slope compensated current ramp. The feedback loop regulates the OVP pin to a reference voltage generated by the minimum voltage across the IFB pins. The output of the EA is connected to the COMP pin. An external RC compensation network must be connected to the COMP pin to optimize the feedback loop for stability and transient response.

The device consistently adjusts the boost output voltage to account for any changes in LED forward voltages. In the event that the boost controller is not able to regulate the output voltage due to the minimum pulse width (t_skip in Electrical Characteristics), the device enters pulse skip mode. In this mode, the device keeps the power switch off for several switching cycles to prevent the output voltage from rising above the regulated voltage. This operation typically occurs in light load condition or when the input voltage is higher than the output voltage.

7.3.3 Switching Frequency

The TPS61199 switching frequency can be programmed between 300 kHz to 800 kHz by a external resistor (R7 in Typical Application). Table 1 shows the recommended values for the resistance.

7.3.4 Enable and Undervoltage Lockout

The TPS61199 is enabled with the soft-start when the EN pin voltage is higher than 2 V; a voltage of less than 0.8 V disables the device.

An undervoltage lockout protection feature is provided. When the voltage at VIN pin is less than 7 V, the device is switched off. The device resumes the operation once the voltage at VIN pin recovers adjusted for hysteresis (see V_{VIN_SYS} in Electrical Characteristics).

7.3.5 Start-Up

The TPS61199 has integrated soft-start circuitry to avoid any inrush current during start-up. During the start-up period, the output voltage rises step-by-step from the minimum voltage of LED string in 100-mV increments, shown in Figure 6. The soft-start time depends on the load and the output capacitor.

7.3.6 Unused LED String

If the application requires fewer than eight LED strings, the TPS61199 simply requires shorting the unused IFB pin to ground. The device detects the voltage less than 0.3 V and immediately disables the string during start-up. Refer to Figure 11.

7.3.7 Program LED Full-Scale Current

The eight current sink regulators embedded in the TPS61199 can be configured to provide up to a maximum of 70 mA per string. The current must be programmed to the expected full-scale LED current by the ISET pin resistor (R6 in Typical Application) using Equation 2.

Equation 2.

where

• K_ISET = Current multiple (1990 typical, in Electrical Characteristics)
• V_ISET = ISET pin voltage (1.229 V typical, in Electrical Characteristics)

7.3.8 PWM Dimming

LED brightness dimming is set by applying an external PWM signal of 100 Hz to 22 kHz to the PWM pin. Varying the PWM duty cycle from 0% to 100% adjusts the LED from minimum to maximum brightness respectively. The minimum on time of the LED string is 1 µsec; thus the TPS61199 has a dimming ratio of 5000:1 at 200 Hz. Refer to Figure 2 for dimming ratio in other dimming frequency.

When the PWM voltage is pulled low, the device will turn off the LED strings and keep the boost converter output at the same level as when PWM is high. Thus, the TPS61199 limit the output ripple due to the load transient that occurs during PWM dimming.

7.3.9 Drive High Current LED

For applications requiring LEDs rated for more than 70 m A, it is acceptable to tie two or more IFB pins together as shown in Figure 12.

7.4.1 Protection

1. Switch current limit protection using the ISNS pin

The TPS61199 monitors the inductor current through the voltage across a sense resistor (R1 in Typical Application) in order to provide current limit protection. During the switch FET on period, when the voltage at ISNS pin rises above 160 mV (V_ISNS in Electrical Characteristics), the device turns off the FET immediately and does not turn it back on until the next switch cycle. The switch current limit is equal to 160 mV / R1.



2. LED open protection

When one of the LED strings is open, the boost output rises to the clamp threshold voltage (see 5. Output overvoltage protection using the OVP pin ). The device detects the open string by sensing no current on the corresponding IFB pin. As a result, the device deactivates the open IFB pin and removes it from the voltage feedback loop. Afterwards, the output voltage returns to the voltage required for the connected WLED strings. The IFB pin currents of the connected strings remain in regulation during this process.

If all the LED strings are open, the device repeatedly attempts to restart until the fault is cleared.



3. LED short-across protection using the FBP pin

If one or several LEDs short in one string, the corresponding IFB pin voltage rises but continues to sink the LED current, causing increased device power dissipation. To protect the device, the TPS61199 provides a programmable LED short-across protection feature with threshold voltage that can be programmed by properly sizing the resistor on the FBP pin (see R5 in Typical Application) using Equation 3.



Equation 3.



If any IFB pin voltage exceeds the threshold (V_{LED_short}), the device turns off the corresponding current sink and removes this IFB pin from the output voltage regulation loop. Current regulation of the remaining IFB pins is not affected.

If the voltage on all the IFB pins exceed the threshold, the device repeatedly attempts to restart until the fault is cleared.



4. IFB overvoltage protection

When any of IFB pin reaches the threshold (V_{OVP_IFB}), the device stops switching immediately to protect from damage. The device re-starts when IFB pin voltage falls below the threshold. The time delay depends on how quickly IFB voltage can fall. It is usually determined by the amount of output capacitance and load.



5. Output overvoltage protection using the OVP pin:

Use a resistor divider to program the clamp threshold voltage as follows:

1. Compute the maximum output voltage by multiplying the maximum forward voltage (V_FWD(MAX)) and number (n) of series LEDs. Add 1V to account for regulation and resistor tolerances and load transients.


Equation 4.


2. The recommended bottom feedback resistor (R3 in Typical Application ) at 10 k. Calculate the top resistor (R2 in Typical Application) using Equation 5:


Equation 5.

When the device detects that the OVP pin exceeds 2.95 V, indicating that the output voltage has exceeded the clamp threshold voltage, the device clamps the output voltage to the set threshold.

When the OVP pin voltage is higher than 3 V, indicating that the output is higher than the clamp threshold voltage due to transients or high voltage noise spike coupling from external circuits, the device shuts down the boost controller until the output drops below the clamp threshold voltage.



6. Output short-to-ground protection

When the inductor peak current reaches twice the switch current limit in each switch cycle, the device immediately disables the boost controller until the fault is cleared. This protects the device and external components from damage if the output is shorted to ground.



7. Thermal Protection

When the device junction temperature is over 150°C, the thermal protection circuit is triggered and shuts down the device immediately. The device automatically restarts when the junction temperature falls back to less than 150°C, with approximate 15°C hysteresis.