7.3.1 Supply Voltage

The TPS61176 can support single-cell Li-ion battery input directly. It has a built-in linear regulator to generate supply for internal analog and logic circuits. The VLDO pin, output of the regulator, should be connected to a 1-µF bypass capacitor for the regulator to be controlled in a stable loop. VLDO pin does not have current sourcing capability for external use.

If the device is used in a multi-cell battery system, the battery cannot be connected to VIN pin directly. In this case, connect a 3.3-V or 5-V power rail to bias the VIN pin and connect the battery voltage to the inductor. The VIN pin only consumes less than 3 mA for normal operation. Refer to Application and Implementation for more details.

7.3.2 Boost Converter

The boost converter of the TPS61176 has a fixed switching frequency of 1 MHz and uses current-mode control. A 2-A, 40-V power MOSFET is integrated so the device has a strong output driving capability. A 0.47-µF to 1-µF capacitor should be connected at COMP pin to ensure stable output over the full input and output voltage ranges assuming the recommended inductance and output capacitance values shown in Recommended Operating Conditions are used. The COMP pin is very sensitive, so careful layout is required to make sure no noise is coupled to it.

The output voltage of the boost is automatically set by the device to minimize the voltage drop across the IFBx (IFB1 ~ IFB6) pins. Normally the voltage across each WLED string is not same, so the voltages at IFBx pins are different. The device regulates the lowest IFBx voltage to 450 mV, and consistently adjusts the boost output voltage to account for any change of WLED forward voltage drop. If the input voltage is higher than the strings' forward voltage drop (for example, at low duty cycles), the boost converter can’t regulate the output due to its minimum duty-cycle limitation. In this case, increasing the number of WLED diodes in series is helpful to provide enough headroom for the converter to boost the voltage.

7.3.3 Current Sinks

The six current sink regulators embedded in TPS61176 can output up to 35 mA current each. By regulating the current sinks, the TPS61176 controls the current of the WLED strings to realize brightness dimming. The full-scale current per channel is programmed by the resistor at ISET pin according to Equation 1.

Equation 1.

where

• I_{FBx_full}, full-scale current per channel
• K_ISET = 1024 (Current multiple)
• V_{ISET_full} = 1.04 V (ISET pin voltage under 100% dimming duty cycle)
• R_ISET = ISET pin resistor

7.3.4 IFBx Pin Unused

If fewer than six channels are used, a user can easily disable the unused channel(s) by shorting the corresponding IFBx pin(s) to ground. The TPS61176 detects IFBx pins short status during the start-up process and disables the unused channel(s) before the boost converter starts switching.

7.3.5 Enable and Start-up

The TPS61176 receives a PWM signal at PWM/EN pin to implement the dimming as well as to enable and disable the device. When a PWM signal (high logic or PWM pulse) is input, the device is enabled automatically; when the PWM signal is pulled low for more than 20 ms, the device is disabled and enters into shutdown mode. In shutdown mode, the boost converter stops switching, and the MODE/FAULT pin is internally pulled to VIN to turn off external isolation MOSFET for true shutdown. The input supply current at VIN pin is 4 µA (maximum) in shutdown mode. In order to avoid fault-triggered shutdown during dimming, the PWM dimming signal should have a higher frequency than 100Hz.

Once enabled by PWM input, the TPS61176 enters the start-up process. The internal regulator is enabled first to supply current to internal circuits. Then the device detects the R_MODE at MODE/FAULT pin to set the dimming mode. The TPS61176 can detect if the mode resistor is connected between VIN pin and MODE/FAULT pin or connected between MODE/FAULT pin and GND pin. If the mode resistor is detected to be between VIN pin and MODE/FAULT pin, which indicates an external isolation P-channel MOSFET is connected, the MODE/FAULT pin will be pulled down by an internal current sink to turn on the isolation MOSFET after the detection process. The device also checks the status of all IFBx pins (short-to-ground or not) to disable any unused channels. There is no special time sequence requirement of VIN and PWM signals for start-up. If PWM signal is input first, the device starts up when VIN powers up.

The dimming mode and IFBx status detection process lasts about 4 ms, during which the MODE/FAULT pin outputs a high voltage (V_IN – 0.6 V, typical) to keep the isolation MOSFET off. When the 4-ms detection window ends, an internal current sink pulls the MODE/FAULT pin low to turn on the isolation MOSFET. Another 4-ms time window then starts and, at the end of the window, the device detects the OVP pin voltage. If the OVP voltage V_OVP is still lower than V_{OVP_SC} ramp-up threshold (90 mV typical), which normally indicates output short-to-ground issue happens, the boost remains off, and the MODE/FAULT pin is pulled up to VIN immediately by an internal resistor to turn off the isolation MOSFET. In this case, the device restarts only after a power-on reset (POR) toggling or PWM toggling. POR toggling means the VIN pin voltage is pulled below UVLO falling threshold first and then pulled above UVLO rising threshold to restart the device; PWM toggling means pulling PWM/EN low for more than 20 ms to disable the device and then apply PWM signal (high logic or PWM pulse) to restart the device. If OVP voltage V_OVP is higher than V_{OVP_SC} ramp up threshold, indicating no short to ground issue is detected, boost starts switching to raise the output voltage. Soft start is implemented by gradually ramping up the reference voltage of the error amplifier to prevent voltage overshoot and in-rush current. The capacitor at COMP pin can adjust the soft-start speed. Larger capacitance leads to slower start-up. A 0.47-µF to 1-µF COMP capacitor is recommended.

7.3.6 Brightness Dimming Control

The TPS61176 receives the PWM dimming signal at PWM/EN pin. An internal PWM decoding circuit detects the on time and the period of the PWM signal and calculates the duty cycle information. The device controls the current-sink-regulator output current according to the duty cycle to realize the brightness dimming.

The device supports mixed dimming mode, which leverages the advantages of both analog dimming and PWM dimming modes. When the dimming duty cycle is high, analog-dimming mode is auto-implemented, increasing the electrical-to-optical efficiency and reducing the power budget for the backlight; when the dimming duty cycle is low, PWM dimming mode is auto-implemented, eliminating potential color shift effect which normally happens when the DC current of WLED diode goes low. The switch point between the analog dimming mode and PWM dimming mode can be programmed by the mode resistor connected at MODE/FAULT pin.

The TPS61176 provides four dimming mode options as shown in Table 2. Besides two different switch point options: 25% or 12.5%, the device also offers two different PWM dimming mode options: direct PWM dimming or 22-kHz fixed-frequency PWM dimming. Refer to Dimming Modes for the details of different dimming modes.

Different mode resistor values set the different dimming modes. 5% or higher precision resistor should be used for the mode resistor. When an isolation P-channel MOSFET is connected, the mode resistor must be connected between VIN pin and MODE/FAULT pin; when the isolation MOSFET is not connected, the mode resistor should be connected between MODE/FAULT pin and ground. If there is no resistor connected at MODE/FAULT pin, which is only allowed when the isolation MOSFET is not connected, default mode (Mode 1) will be selected. Refer to Application and Implementation section for more details.

7.4.1 Dimming Modes

7.4.1.1 Analog Dimming Mode

In analog dimming mode, the brightness dimming is realized by controlling the DC current of WLED diodes. Because the forward voltage of a WLED diode drops when its DC current reduces, the required output voltage can become lower when dimming duty cycle goes low, reducing the power budget for the backlight and allowing more system power saving.

In analog dimming mode, the current of IFBx is regulated according to Equation 2:

Equation 2.

where

• I_FBx, current per string
• V_ISET, (ISET pin voltage during analog dimming)
• K_ISET = 1024 (Current multiple)
• V_{ISET_full} = 1.04 V (ISET pin voltage with 100% dimming duty cycle)
• R_ISET = ISET pin resistor
• Duty = duty cycle of the PWM signal

7.4.1.2 PWM Dimming

When the dimming duty cycle is below the switch point, PWM dimming mode is automatically implemented. In this mode, the current sink regulators are turned on and off according to the PWM duty cycle information, so the illumination of WLEDs is intermittent. At frequencies higher than a human eye critical flicker frequency, the brightness is the same as the average brightness of the fluctuating light, thus controlling the duty cycle can realize the brightness dimming.

While a current sink regulator is turned on during PWM dimming, its output current is equal to the DC current at the switch point. For example, if the switch point is set to 25%, the regulator's output current during the ON phase is equal to I_{FBx_full} × 25%, and the ON phase's duty cycle Duty' is equal to Duty / 25%, where Duty is the input PWM signal's duty cycle information. Then the average current during PWM dimming can be still equal to I_{FBx_full} × Duty. This design is in order to keep the brightness consistency between analog dimming and PWM dimming and avoid any abrupt brightness change around the switch point. If the switch point is set to 12.5%, the regulator's output current during the ON phase is equal to I_{FBx_full} × 12.5%, and the ON phase duty cycle Duty is equal to Duty / 12.5% (see Figure 8).

Figure 8. Mixed Dimming Mode

Generally, the average current of an LED string in PWM dimming mode is equal to

Equation 3.

where

• I_{FBx_PWM}, average current per string in PWM dimming mode
  
• V_{ISET_full} = 1.04V (ISET pin voltage with 100% dimming duty cycle)
• K_ISET = 1024 (Current multiple)
• R_ISET = ISET pin resistor
• Duty = duty cycle of the PWM signal

The frequency of the current sink regulators ON and OFF control depends on which PWM dimming mode is set. The TPS61176 provides two different PWM dimming modes: direct PWM dimming mode and 22-kHz fixed frequency PWM dimming mode.

In direct PWM dimming mode, the current sinks are turned ON and OFF with the same frequency detected from the input PWM signal. The advantage of this mode is the dimming frequency can be adjusted freely. In addition, it is easy to achieve high dimming resolution in direct PWM dimming mode: with lower input PWM frequency, the higher dimming resolution can be detected and output. For example, when the input PWM frequency is 100 Hz, 14-bit resolution can be achieved; when the input PWM frequency is 20 kHz, 9-bit resolution is achieved. So if high resolution is required, 100-Hz or 200-Hz dimming frequency is recommended. The TPS61176 is designed to minimize the AC ripple on the output capacitor during PWM dimming. Careful passive component selection is also crucial to minimize AC ripple on the output capacitor. In order to further avoid the potential audible noise, input PWM frequency out of audible frequency range is recommended. See Application and Implementation for more information.

In 22-kHz fixed frequency PWM dimming mode, current sinks are turned on and off according to the duty cycle information detected from the input PWM signal but with an internally fixed frequency. This mode facilitates the application where the input PWM signal frequency cannot be adjusted outside the audio frequency range. Thus in this mode the audible noise is eliminated completely.

The human eye is much more sensitive to the brightness change at low brightness compared to at high brightness, so in order to improve the visual experience and avoid any potential flickering perception, high-resolution dimming is implemented in PWM dimming mode. The TPS61176 can achieve up to 14-bit dimming resolution during the PWM dimming. Generally, higher resolution can be achieved with lower input PWM frequency. Refer to Table 3 for detailed dimming resolution information.

Table 3. Dimming Resolution Information in PWM Dimming Mode

DIMMING MODE	INPUT PWM FREQUENCY	DIMMING RESOLUTION IN PWM DIMMING MODE
Mode 1	100 Hz ~ 4.5 kHz	12-bit
	4.5 kHz ~ 9 Hz	11-bit
	9 kHz ~ 18 kHz	10-bit
	18 kHz ~ 20 kHz	9-bit
Mode 2	100 Hz ~ 1 kHz	14-bit
	1 kHz ~2 kHz	13-bit
	2 kHz ~ 4 kHz	12- bit
	4 kHz ~ 8 kHz	11-bit
	8 kHz ~ 16 kHz	10-bit
	16 kHz ~ 20 kHz	9 -bit
Mode 3	100 Hz ~ 5 kHz	12-bit
	5 kHz ~ 10 kHz	11-bit
	10 kHz ~ 20 kHz	10-bit
Mode 4	100 Hz ~ 1.2 kHz	14-bit
	1.2 kHz ~2.4 kHz	13-bit
	2.4 kHz ~ 4.8 kHz	12- bit
	4.8 kHz ~ 9.6 kHz	11-bit
	9.6k Hz ~ 20 kHz	10-bit

7.4.2 Overvoltage Protection

The output voltage of the boost converter is detected by the OVP pin. The overvoltage-protection threshold can be programmed by an external resistor divider (R3 and R4 in the Typical Application), allowing the usage of low voltage-rating Schottky diode in a low output-voltage application. The correct divider ratio is important for optimum operation of the device. 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.

Equation 4.

where

• V_{OVP_clamp} = 1.5 V

When the overvoltage threshold V_{OVP_clamp} is reached, the TPS61176 detects if there are any LED strings open, first by sensing whether there is current on IFBx pin. If any string is open, the corresponding current sink is disabled and removed from regulation. Subsequently, the output voltage drops down and is regulated to a voltage for the connected WLED strings. The IFBx current of the connected WLED strings keeps in regulation during the whole transition. If an open string is reconnected again, a POR toggling or PWM toggling is required to reactivate a previously deactivated string. The TPS61176 shuts down and keeps off when it detects that all of the WLED strings are open. In this case, a POR toggling or PWM toggling is required to restart the device. If there isn’t any string open, the TPS61176 regulates the boost output at the over-voltage threshold.

If the output voltage cannot be regulated at the value set by Equation 4 and keeps rising, once the OVP pin voltage exceeds V_{OVP_sd} rising threshold (1.6 V typical), the boost stops switching. When the OVP voltage falls below V_{OVP_sd} falling threshold (1.55 V typical), the boost recovers to switch. During the process, the IFBx current of the connected WLED strings keeps in regulation.

7.4.3 Current Sink Open Protection

If any IFBx pin voltage exceeds the 1st level IFB overvoltage threshold (8.5 V typical) when its current sink is turned on, the TPS61176 turns off this current sink and removes it from output regulation loop. The current regulation of the remaining IFBx pins is not affected. This situation often occurs when there are several shorted WLED diodes in one string. WLED mismatch typically does not create such large voltage difference among WLED strings. The TPS61176 shuts down when it detects that all of the IFBx pin voltages exceed the threshold. In this case, a POR toggling or PWM toggling is required to restart the device.

If any IFBx pin voltage exceeds the 2nd level IFB overvoltage threshold (18 V typical), no matter whether the current sink is turned on or off, the TPS61176 shuts down immediately to avoid potential over stress damage at IFBx pin. A POR toggling or PWM toggling is required to restart the device.

7.4.4 Overcurrent and Short Circuit Protection

The TPS61176 has a pulse-by-pulse overcurrent limit of 2 A (minimum). The boost power MOSFET is turned off when the inductor current reaches this current limit threshold, and it remains off until the beginning of the next switching cycle. This protects the device and external component under overload conditions.

Under severe overload or short-circuit conditions, if the OVP pin voltage is detected below V_{OVP_SC} ramp-down threshold (70 mV typical), The TPS61176 shuts down, and the MODE/FAULT pin is pulled to VIN by an internal switch immediately. As a result, the external isolation MOSFET can be turned off at once, cutting off the power path from input to output. The device restarts after a POR toggling or PWM toggling.

7.4.5 Thermal Protection

An internal thermal shutdown turns off the device when the typical junction temperature of 160°C is exceeded. The device is released from shutdown automatically when the junction temperature decreases by 15°C.