TPS61193-Q1 Low-EMI Automotive LED Driver With Three 100-mA Channels

8.3.1 Integrated DC-DC Converter

The TPS61193-Q1 DC-DC converter generates supply voltage for the LEDs and can operate in boost mode or in SEPIC mode. The maximum output voltage V_{OUT_MAX} is defined by an external resistive divider (R1, R2).

V_{OUT_MAX} voltage should be chosen based on the maximum voltage required for LED strings. Recommended maximum voltage is about 30% higher than maximum LED string voltage. DC-DC output voltage is adjusted automatically based on LED current sink headroom voltage. Maximum, minimum, and initial boost voltages can be calculated with :

Equation 1.

where

• V_BG = 1.2 V
• R2 recommended value is 130 kΩ
• Resistor values are in kΩ
• K = 1 for maximum adaptive boost voltage (typical)
• K = 0 for minimum adaptive boost voltage (typical)
• K = 0.88 for initial boost voltage (typical)

Figure 9. Maximum Converter Output Voltage vs R1 Resistance

Alternatively, a T-divider can be used if resistance less than 100 kΩ is required for the external resistive divider. Refer to Using the TPS61193-Q1 Evaluation Module for details.

The converter is a current mode DC-DC converter, where the inductor current is measured and controlled with the feedback. Switching frequency is adjustable between 250 kHz and 2.2 MHz with R_FSET resistor as Equation 2:

In most cases lower frequency has higher system efficiency. DC-DC internal parameters are chosen automatically according to the selected switching frequency (see Table 2) to ensure stability. In boost mode a 15-pF capacitor C_FB must be placed across resistor R1 when operating in 300-kHz to 500-kHz range (see Typical Application for 3 LED Strings). When operating in the 1.8-MHz to 2.2-MHz range C_FB = 4.7 pF.

Figure 10. Boost Block Diagram

DC-DC can be driven by an external SYNC signal between 300 kHz and 2.2 MHz. If the external synchronization input disappears, DC-DC continues operation at the frequency defined by R_FSET resistor. When external frequency disappears and SYNC pin level is low, converter continues operation without spread spectrum immediately. If SYNC remains high, converter continues switching with spread spectrum enabled after 256 µs.

External SYNC frequency must be 1.2 to 1.5 times higher than the frequency defined by R_FSET resistor. Minimum frequency setting with R_FSET is 250 kHz to support 300-kHz switching with external clock.

The optional spread spectrum feature (±3% from central frequency, 1-kHz modulation frequency) reduces EMI noise at the switching frequency and its harmonic frequencies. When external synchronization is used, spread spectrum is not available.

The converter SW pin DC current is limited to 2 A (typical). To support warm-start transient conditions the current limit is automatically increased to 2.5 A for a short period of 1.5 seconds when a 2-A limit is reached.

To keep switching voltage within safe levels there is a 48-V limit comparator in the event that FB loop is broken.

8.3.2 Internal LDO

The internal LDO regulator converts the input voltage at VIN to a 4.3-V output voltage for internal use. Connect a minimum of 1-µF ceramic capacitor from LDO pin to ground, as close to the LDO pin as possible.

8.3.3 LED Current Sinks

8.3.3.2 Current Setting

Maximum current for the LED outputs is controlled with external R_ISET resistor. R_ISET value for target maximum current can be calculated using Equation 3:

Equation 3.

where

• R_ISET is current setting resistor, kΩ
• I_LED is output current per output, mA

8.3.4 Protection and Fault Detections

The TPS61193-Q1 has fault detection for LED open and short, VIN input overvoltage protection (VIN_OVP) , VIN undervoltage lockout (VIN_UVLO), and thermal shutdown (TSD).

8.3.4.1 Adaptive DC-DC Voltage Control and Functionality of LED Fault Comparators

Adaptive voltage control function adjusts the DC-DC output voltage to the minimum sufficient voltage for proper LED current sink operation. The current sink with highest V_F LED string is detected and DC-DC output voltage adjusted accordingly. DC-DC adaptive control voltage step size is defined by maximum voltage setting, V_STEP = (V_{OUT_MAX} – V_{OUT_MIN)} / 256. Periodic down pressure is applied to the target voltage to achieve better system efficiency.

Every LED current sink has 3 comparators for the adaptive DC-DC control and LED fault detections. Comparator outputs are filtered, filtering time is 1 µs.

Figure 11. Comparators for Adaptive Voltage Control and LED Fault Detection

Figure 12 shows different cases which cause DC-DC voltage increase, decrease, or generate faults. In normal operation voltage at all the OUT# pins is between LOW_COMP and MID_COMP levels, and boost voltage stays constant. LOW_COMP level is the minimum for proper LED current sink operation, 1.1 × V_SAT + 0.2 V (typical). MID_COMP level is 1.1 × V_SAT + 1.2 V (typical) so typical headroom window is 1 V.

When voltage at all the OUT# pins increases above MID_COMP level, DC-DC voltage adapts downwards.

When voltage at any of the OUT# pins falls below LOW_COMP threshold, DC-DC voltage adapts upwards. In the condition where DC-DC voltage reaches the maximum and there are one or more outputs still below LOW_COMP level, an open LED fault is detected.

HIGH_COMP level, 6 V typical, is the threshold for shorted LED detection. When the voltage of one or more of the OUT# pins increases above HIGH_COMP level and at least one of the other outputs is within the normal headroom window, shorted LED fault is detected.

Figure 12. Protection and DC-DC Voltage Adaptation Algorithms

Figure 13. V_IN Overvoltage Protection (DC-DC OVP)

Figure 14. V_IN Overvoltage Protection (V_IN OVP)

Figure 15. V_IN Undervoltage Lockout

Figure 16. LED Open Fault

Figure 17. LED Short Fault

8.4.1 Device States

The TPS61193-Q1 enters STANDBY mode when the internal LDO output rises above the power-on reset level, V_LDO > V_POR. In STANDBY mode the device is able to detect VDDIO/EN signal. When VDDIO/EN is pulled high, the device powers up. After start LED outputs are sensed to detect grounded outputs. Grounded outputs are disabled and excluded from the adaptive voltage control loop of the DC-DC.

If a fault condition is detected, the device enters FAULT_RECOVERY state. Faults that cause the device to enter FAULT_RECOVERY are listed in Table 3. When LED open or short is detected, the faulty string is disabled, but device stays in ACTIVE mode.

Figure 18. State Diagram

Figure 19. Timing Diagram for the Typical Start-Up and Shutdown

9.2.1 Typical Application for 3 LED Strings

Figure 20 shows the typical application for TPS61193-Q1 which supports 3 LED strings with maximum current 100 mA, with a boost switching frequency of 300 kHz.

Figure 20. Three Strings 100 mA/String Configuration

9.2.1.2 Detailed Design Procedure

9.2.1.2.1 Inductor Selection

There are two main considerations when choosing an inductor; the inductor must not saturate, and the inductor current ripple must be small enough to achieve the desired output voltage ripple. Different saturation current rating specifications are followed by different manufacturers so attention must be given to details. Saturation current ratings are typically specified at 25°C. However, ratings at the maximum ambient temperature of application should be requested from the manufacturer. Shielded inductors radiate less noise and are preferred. The saturation current must be greater than the sum of the maximum load current, and the worst case average-to-peak inductor current. Equation 4 shows the worst case conditions

Equation 4.

where

• I_RIPPLE - peak inductor current
• I_OUTMAX - maximum load current
• V_IN - minimum input voltage in application
• L - min inductor value including worst case tolerances
• f - minimum switching frequency
• V_OUT - output voltage
• D - Duty Cycle for CCM Operation

As a result, the inductor should be selected according to the I_SAT. A more conservative and recommended approach is to choose an inductor that has a saturation current rating greater than the maximum current limit. A saturation current rating of at least 2.5 A is recommended for most applications. See Table 2 for recommended inductance value for the different switching frequency ranges. The inductor’s resistance should be less than
300 mΩ for good efficiency.

See detailed information in Understanding Boost Power Stages in Switch Mode Power Supplies. Power Stage Designer™ Tool can be used for the boost calculation: http://www.ti.com/tool/powerstage-designer.

9.2.1.2.2 Output Capacitor Selection

A ceramic capacitor with 2 × V_{MAX BOOST} or more voltage rating is recommended for the output capacitor. The DC-bias effect can reduce the effective capacitance by up to 80%, which needs to be considered in capacitance value selection. Capacitance recommendations for different switching frequencies are shown in Table 2. To minimize audible noise of ceramic capacitors their physical size should typically be minimized.

9.2.1.2.3 Input Capacitor Selection

A ceramic capacitor with 2 × V_{IN MAX} or more voltage rating is recommended for the input capacitor. The DC-bias effect can reduce the effective capacitance by up to 80%, which needs to be considered in capacitance value selection. Capacitance recommendations for different boost switching frequencies are shown in Table 2.

9.2.1.2.4 LDO Output Capacitor

A ceramic capacitor with at least 10-V voltage rating is recommended for the output capacitor of the LDO. The DC-bias effect can reduce the effective capacitance by up to 80%, which needs to be considered in capacitance value selection. Typically a 1-µF capacitor is sufficient.

9.2.1.2.5 Diode

A Schottky diode should be used for the boost output diode. Do not use ordinary rectifier diodes because slow switching speeds and long recovery times degrade the efficiency and the load regulation. Diode rating for peak repetitive current should be greater than inductor peak current (up to 3 A) to ensure reliable operation in boost mode. Average current rating should be greater than the maximum output current. Schottky diodes with a low forward drop and fast switching speeds are ideal for increasing efficiency. Choose a reverse breakdown voltage of the Schottky diode significantly larger than the output voltage.

9.2.1.3 Application Curves

Load 3 strings, 8 LEDs per string		ƒsw = 300 kHz, 33 μH
100 mA/string for VIN = 12 V and VIN = 16 V
60 mA/string for VIN = 8 V
50 mA/string for VIN = 5 V

Figure 21. Boost Efficiency

Figure 23. Typical Start-Up

Load 3 strings, 8 LEDs per string		ƒsw = 300 kHz, 33 μH
100 mA/string for VIN = 12 V and VIN = 16 V
60 mA/string for VIN = 8 V
50 mA/string for VIN = 5 V

Figure 22. System Efficiency

Figure 24. Open LED Fault

9.2.2 SEPIC Mode Application

When LED string voltage can be above or below V_IN voltage, SEPIC configuration can be used. In this example, two separate coils are used for SEPIC. This can enable lower height external components to be used, compared to a coupled coil solution. On the other hand, coupled coil typically maximizes the efficiency. Also, in this example, an external clock is used to synchronize SEPIC switching frequency. External clock input can be modulated to spread switching frequency spectrum.

Figure 25. SEPIC Mode, 3 Strings, 100 mA/String Configuration

9.2.2.3 Application Curves

Load 100mA per string, 3 strings, 2 LEDs per string
ƒsw = 2.2 MHz
2 × 10 μH, IHLP2525BDER100M

Figure 26. SEPIC Efficiency

Load 100mA per string, 3 strings, 2 LEDs per string

ƒsw = 2.2 MHz

2 × 10 μH, IHLP2525BDER100M

Figure 27. System Efficiency