7.3.1 Supply Voltage

The TPS61196 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 1-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

The TPS61196 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 an external resistor.

A ramp signal from the oscillator is added to the current ramp to provide slope compensation, shown in Figure 23. 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 TPS61196 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_on(min), in the Electrical Characteristics), the TPS61196 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 switching frequency is programmed between 100 kHz to 800 kHz by an external resistor (R9 in Figure 23). To determine the resistance by a given frequency, use the curve in Figure 4 or calculate the resistance value by Equation 1. Table 1 shows the recommended resistance values for some switching frequencies.

Equation 1.

7.3.4 Enable and Undervoltage Lockout

The TPS61196 is enabled with the soft start-up when the EN pin voltage is higher than 1.8 V. A voltage of less than 1 V disables the device.

An undervoltage lockout (UVLO) protection feature is provided. When the voltage at the VIN pin is less than 6.5 V, the TPS61196 is powered off. The TPS61196 resumes the operation once the voltage at the VIN pin recovers above the hysteresis (V_{VIN_HYS}) more than the UVLO threshold of input falling voltage. If a higher UVLO voltage is required, use the UVLO pin as shown in Figure 19 to adjust the input UVLO threshold by using an external resistor divider. Once the voltage at the UVLO pin exceeds the 1.229-V threshold, the device is powered on, and a hysteresis current source of 3.9 µA is added. When the voltage at the UVLO pin drops lower than 1.229 V, the current source is removed. The resistors of R1, R2, and R5 can be calculated by Equation 2 from required V_START and V_STOP. To avoid noise coupling, the resistor divider R1 and R2 must be close to the UVLO pin. Placing a filter capacitor of more than 10 nF as shown in Figure 19 can eliminate the impact of the switching ripple and improve the noise immunity.

If the UVLO function is not used, pull up the UVLO pin to the VDD pin.

Figure 19. Undervoltage Lockout Circuit

Equation 2.

where

• I_HYS is 3.9 µA sourcing current from the UVLO pin.

When the UVLO condition happens, the FAULT pin outputs high impedance. As long as the UVLO condition removes, the FAULT pin outputs low impedance.

7.3.5 Power-up Sequencing and Soft Start-up

The input voltage, UVLO pin voltage, EN input signal and the input dimming PWM signal control the power up of the TPS61196. After the input voltage is above the required minimal input voltage of 7.5 V, the internal circuit is ready to be powered up. After the UVLO pin is above the threshold of 1.229 V and the EN signal is high, the internal LDO and logic circuit are activated. The device outputs a 20-ms pulse to detect the unused channels and remove them from the control loop. When any PWM dimming signal is high, the soft start-up begins. If the PWM dimming signals come before the EN signal is high, the soft start-up begins immediately after the detection of unused channels.

Figure 20. Power-Up Sequencing

The TPS61196 has integrated the soft-start circuitry working with an external capacitor at the REF pin to avoid inrush current during start-up. During the start-up period, the capacitor at the REF pin is charged with a soft-start current source. When the REF pin voltage is higher than the output feedback voltage at the OVP pin, the boost controller starts switching and the output voltage starts to ramp up. At the same time, the LED current sink starts to drive the LED strings. At the beginning of the soft start, the charge current is 200 µA. Once the voltage of the REF pin exceeds 2 V, the charge current changes to 10 µA and continues to charge the capacitor. When the current sinks are driving the LED strings, the IFB voltages are monitored. When the minimum IFB voltage is above 200 mV less than the setting voltage at the IFBV pin, the charge current is stopped, and the soft start-up is finished. The TPS61196 enters normal operation to regulate the minimum IFB voltage to the required voltage set by the resistor at the IFBV pin. The total soft start time is determined by the external capacitance. The capacitance must be within 1 µF to 4.7 µF for different start-up time and different output voltage.

Figure 21. Soft-Start Waveforms

7.3.6 Unused LED String

If the application requires less than six LED strings, the TPS61196 simply requires connecting the unused IFB pin to ground through a resistor between 20 kΩ and 36 kΩ. Once the TPS61196 is turned on, the TPS61196 uses a 60-µA current source to detect the IFB pin voltage. If the IFB voltage is between 1 V and 2.5 V, the device immediately disables this string during start-up.

7.3.7 Current Regulation

The six channel current sink regulators can be configured to provide up to 400 mA per string. The expected LED current is programmed by a resistor (R11 in Figure 23) at the ISET using Equation 3.

Equation 3.

where

• V_ISET is the ISET pin voltage of 1.229 V
• K_ISET is the current multiple of 3992.

To sink the set LED current, the current sink regulator requires a minimum headroom voltage at the IFB pins for working properly. For example, when the LED current is set to 130 mA, the minimum voltage required at the IFB pin must be higher than 0.35 V. For other LED currents, refer to Figure 5 for recommended minimum headroom voltage required. The TPS61196 regulates the minimum voltage of the IFB pins to the IFBV voltage. The IFBV voltage is adjustable with an external resistor (R10 in the Figure 23) at the IFBV pin. After choosing the minimum IFB voltage, the IFBV voltage must be set to this value and the setting resistance can be calculated by Equation 4.

Equation 4.

If a large LED current is set, the headroom voltage is required higher. This leads to more heat on the device. To maintain the total power dissipation in the range of the package limit, normally all strings can not sink large current in continuous mode but pulse mode. The backlight of an active shutter glass 3-D TV may work with large LED current in pulse mode.

7.3.8 PWM Dimming

LED brightness dimming is set by applying an external PWM signal of 90 Hz to 22 kHz to the PWM pins. Each LED string has an independent PWM input. Varying the PWM duty cycle from 0% to 100% adjusts the LED from minimum to maximum brightness respectively. The recommended minimum on time of the LED string is 10 µsec. Thus, the device has a minimum dimming duty cycle of 500:1 at 200 Hz.

When all PWM voltages are pulled low during dimming off, the TPS61196 turns off the LED strings and keeps the boost converter running at PFM mode. The output voltage is kept at the level which is a little bit lower than that when PWM is high. Thus, the device limits the output ripple due to the load transient that occurs during PWM dimming.

When all PWM voltages are pulled low for more than 20 ms, to avoid the REF pin voltage dropping due to the leakage current, the voltage of the REF pin is held by an internal reference voltage which equals to the REF pin voltage in normal dimming operation. Thus, the output voltage will be kept at the same level as the normal output voltage.

Since the output voltage in long time dimming off status is almost the same as the normal voltage for turning the LED on, the TPS61196 turns on the LED very fast without any flicker when recovering from long time dimming off to small duty cycle dimming on.

7.4.1 Protections

The TPS61196 has a full set of protections making the system safe to any abnormal conditions. Some protections will latch the TPS61196 in off state until its power supply is recycled or it is disabled and then enabled again. In latch off state, the REF pin voltage is discharged to 0 V.

7.4.1.3 LED Short-Cross 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 TPS61196 provides a programmable LED short-across protection feature by properly sizing the resistor on the FBP pin (R12 in Figure 23) using Equation 5.

Equation 5.

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.

7.4.1.7 Output Overvoltage Protection Using the OVP Pin

Use a resistor divider to program the maximum output voltage of the boost converter. To ensure the LED string can be turned on with setting current, the maximum output voltage must be higher than the forward voltage drop of the LED string. The maximum required voltage can be calculated by multiplying the maximum LED forward voltage (V_FWD(max)) and number (n) of series LEDs , and adding extra 1 V to account for regulation and resistor tolerances and load transients.

The recommended bottom feedback resistor of the resistor divider (R4 in Figure 23) is 10 kΩ. Calculate the top resistor (R3 in the Figure 23) using Equation 6, where V_OVP is the maximum output voltage of the boost converter.

Equation 6.

When the device detects that the voltage at the OVP pin exceeds overvoltage protection threshold of 3.02 V, indicating that the output voltage has exceeded the clamp threshold voltage, the TPS61196 clamps the output voltage to the set threshold. When the OVP pin voltage does not drop from the OVP threshold for more than 500 ms, the device is latched off until the input power or the EN pin voltage is re-cycled.

7.4.2 Indication For Fault Conditions

The TPS61196 has an open-drain fault indicator pin to indicate abnormal conditions. When the device is operating normally, the voltage at the FAULT pin is low. When any fault condition happens, it is in high impedance, which can be pulled to high level through an external resistor. The FAULT pin can indicate following conditions:

• Overvoltage condition at the OVP or the IFB pin
• LED short and open
• IFB short to ground
• ISET short to ground
• Diode open and short
• Output short circuit
• Overtemperature