AN191 - How to Adjust the MPQ7200’s LED Current Derating with an NTC

Application Note

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Abstract

The MPQ7200 is a 42V, 1.2A buck-boost or 3A buck, synchronous LED driver, which is AEC-Q100 qualified. The MPQ7200 supports applications including automotive front lamps, daytime running lights (DRLs), turn indicator lights, and rear lights. A front lamp typically has 10W to 15W of electrical LED power. In some designs, the LEDs and MPQ7200 share a common PCB, which is cost-effective. However, the LED power warms up the PCB, which increases the MPQ7200’s die temperature.

The goals of this application note are to increase the MPQ7200’s maximum operating temperature at full light intensity and prevent a thermal shutdown due to high die temperature. If the MPQ7200 exceeds a certain ambient temperature (T_AMBIENT), the LED current and visible light power are automatically reduced. The MPQ7200 uses an external negative temperature coefficient (NTC) resistor to measure the PCB temperature.

This application note discusses how to use an automotive front lamp to reduce the LED current when T_AMBIENT exceeds 50°C.

Introduction

The MPQ7200 is a high-frequency, constant-current, buck-boost LED driver with integrated power MOSFETs. It offers a very compact solution to achieve up to 1.2A of continuous output current (I_OUT), with excellent load and line regulation across a wide input supply range. The MPQ7200 can also be configured for buck mode to provide up to 3A of constant load current.

Constant frequency hysteretic control provides extremely fast transient response without loop compensation. The switching frequency (f_SW) can be fixed up to 2.3MHz in buck mode to reduce the current ripple and improve EMI. It can also be configured to as low as 1.15MHz for optimized efficiency and thermal performance in buck-boost mode.

Full protection features include over-current protection (OCP), output over-voltage protection (OVP), output under-voltage protection (UVP), thermal derating (TD), and thermal shutdown (TSD). The fault indicator outputs an active logic low signal if a fault condition occurs.

The MPQ7200 requires a minimal number of readily available, standard external components, and is available in a space-saving QFN-19 (3mmx4mm) package.

Applications include automotive front lamps, turns indicator lights, fog lights, rear lights, daytime running lights (DRLs), battery-powered flashlights, and vehicle lamps.

Evaluation Boards

The EVQ7200-L-00A and EVQ7200-L-00B are evaluation boards designed to demonstrate the capabilities of the MPQ7200 for buck mode and buck-boost mode, respectively.

This application note discusses the MPQ7200’s negative temperature coefficient (NTC) thermal derating. The MPQ7200 operates in buck-boost mode with a 1.15MHz f_SW. The MPQ7200 also has a variant (MPQ7200A) with lower f_SW values (410kHz for buck-boost mode and boost mode) and different NTC thermal derating levels. Refer to the MPQ7200 or MPQ7200A datasheets for more details on selecting the NTC thermal derating.

Figure 1 shows the EVQ7200-L-00A evaluation board. Figure 1 and Figure 2 on page 5 show the layout differences between buck mode and buck-boost mode. In particular, buck mode requires fewer passive components.

Figure 1: EVQ7200-L-00A Evaluation Board (Buck Mode)

Board Number	MPS IC Number
EVQ7200-L-00A	MPQ7200GLE-AEC1

Figure 2 shows the EVQ7200-L-00B evaluation board.

Figure 2: EVQ7200-L-00B Evaluation Board (Buck-Boost Mode)

Board Number	MPS IC Number
EVQ7200-L-00B	MPQ7200GLE-AEC1

Measurement Set-Up

Figure 3 shows the measurement set-up to adjust the NTC thermal dimming derating.

Figure 3: MPQ7200 Measurement Set-Up for NTC Thermal Dimming Derating

The test equipment in Figure 3 on page 5 is described in further detail below:

MPS efficiency meter (A): The efficiency meter was created by MPS to provide accurate power loss and efficiency measurements for the MPQ7200 using four-wire voltage and current measurements for the IC input voltage (V_{IN_IC}), PCB input current (I_IN), LED voltage (V_LED), and LED current (I_LED).
Voltmeter (B): The voltmeter measures the input voltage on the PCB input terminals (V_{IN_PCB}). In this instance, V_{IN_PCB} = 13.5V.
Thermocouple thermometer for the PCB (C): The PCB’s on-board NTC resistor (R_NTC) measures the PCB temperature and is the temperature signal of the MPQ7200’s LED current derating. An external NTC resistor glued on top of R_NTC measures the RNTC temperature. The external NTC resistor’s temperature (T_NTC) is read out by the multimeter.
Thermocouple thermometer for the IC (D): The IC’s thermocouple measures the temperature on top of the MPQ7200 package (T_IC). A silicon die temperature cannot be measured with full accuracy using an external thermocouple. However, the measurement error is negligible when using small-sized thermocouples. The package’s top side consists of a thin plastic layer, which makes the temperature difference between the silicon and the thermocouple only a few Kelvin.

Figures 4 shows two different Type K thermocouples. The thermocouple shown on the left is a Keysight TCK-401301-SE with thin wires and is recommended for gluing on small objects onto a PCB using a thermally conductive glue. The thermocouple shown on the right is a general-purpose type with thick wires and is not recommended for gluing on small objects.

The thermocouple wires are made of metal to transfer heat from the point of interest to the internal and external environments of the climate chamber. This heat transfer results in a measurement error, which specifically results in a measured temperature that is below the object’s real temperature. To reduce the measurement error, use thin-wired thermocouples, and place long, thermally insulated wires inside the heated chamber.

Figure 4: Type K Thermocouples

Power supply (E): The power supply supplies the power for V_{IN_PCB}. The device under test (DUT) PCB has an on-board EMC filter, followed by a Schottky diode for reverse polarity protection. V_{IN_PCB} is measured on the EMC filter’s input, and V_{IN_IC} is measured on the Schottky diode’s output.
External trimmer potentiometer (R_PARALLEL) (F): When placed outside of the climate chamber, R_PARALLEL replaces the PCB surface mount technology (SMT) resistor and sets the NTC thermal dimming derating start point to the desired ambient temperature (T_AMBIENT, about 50°C).

Figure 5 on page 7 shows the experimental set-up for R_PARALLEL. Through experimentation, it is possible to find the correct R_PARALLEL resistor to place in parallel with R_NTC.

Figure 5: R_PARALLEL Adjusts the Starting NTC Thermal Derating to T_AMBIENT = 50°C

Oscilloscope (G) (I_LED and pin 19’s V_NTC (V_NTC2)): G precisely measures the top square-wave amplitude for V_NTC2. The top amplitude provides temperature-related information for the analog NTC thermal derating. The oscilloscope measures I_LED with a current probe and displays the calculated RMS value. The I_LED waveform characteristics are monitored on the oscilloscope. It should be noted that the efficiency meter can measure the RMS value with higher accuracy.
Climate chamber for the DUT (H): The climate chamber controls T_AMBIENT. It also controls the air temperature using a sensor mounted a few centimeters above the cardboard box.

Figure 6 shows the cardboard box enclosing the tested PCB, which prevents the airflow caused by the chamber. The entire PCB operates within these measurements without air convention, similar to the serial product.

Figure 6: DUT PCB Under a Closed Carton in the Climate Chamber

How to Adjust the Thermal Derating

Set DUT to the desired T_AMBIENT (50°C) and adjust R_PARALLEL until the maximum I_LED (I_{LED_MAX}) begins decreasing from 100% of its typical value to a lower value. The NTC dimming derating can be expressed as a ratio (DIM_RATIO) or a percentage. See Figure 7 and Figure 8, and Figure 9 on page 9, for details on the schematic and components.

The percentage for DIM_RATIO can be calculated with Equation (1):

$$DIM_{RATIO} [\%] = 100 \times \frac {I_{LED MAX}}{I_{LED}}$$

Based on I_{LED_MAX}, the maximum RMS I_LED is below the derating start point at a 50°C T_AMBIENT, and I_LED is measured at T_AMBIENT.

Thermocouples and Resistors

This section will describe several resistors – including R_NTC and R_PARALLEL – and the thermocouples on the PCB. To detect T_NTC on the PCB, this application uses a 47kΩ R_NTC (at T_NTC= 25°C) and a negative temperature coefficient. Place R_NTC a few centimeters from the MPQ7200. R_NTC sets DIM_RATIO. This is an analog-based I_LED current derating, and it is not based on pulse-width modulation (PWM).

A Type K thermocouple is glued on top of R_NTC. The thermocouple signal is measured with a digital multimeter using a temperature scale. A second Type K thermocouple measures the MPQ7200’s top package temperature. Figure 7 shows the placement for the two Type K thermocouples on the IC package and R_NTC.

Figure 7: Placement of Thermocouples on IC Package and RNTC

Where R_P is defined as the parallel resistance of R_PARALLEL (20.89kΩ) and R_NTC (47kΩ) at T_NTC = 25°C. The aim of these measurements is to find the experimental R_PARALLEL. During the measurements, R_PARALLEL is set by an external trimmer. For the later series PCB, R_PARALLEL acts as a SMT resistor.

Figure 8 shows the relationship between resistance and T_NTC.

Figure 8: Curve Characteristics (R_NTC, R_NTC||20.89kΩ, and R_PARALLEL vs. T_NTC)

R_NTC covers a wide, non-linear resistance range. R_PARALLEL improves linearization when T_A ≥ 50°C and sets the desired dimming derating starting point at T_AMBIENT = 50°C. Note that T_NTC exceeds T_A because the PCB is heated by 11W of electrical LED power.

Schematic Thermal Derating

Figure 9 show the resistors that are required to adjust the MPQ7200’s analog NTC I_LED derating.

Figure 9: NTC Thermal Derating Adjustment

The key components for NTC thermal derating adjustments are described in the full PDF.

Conclusion

The MPQ7200 is a flexible I_LED driver designed for applications with a low external part count and low BOM cost. The devices offers numerous built-in features for full flexibility in controlling I_LED.

In this application note, NTC I_LED derating expands the usable T_A range even under the worst-case scenario. This scenario occurs when the electrical LED power heats up the PCB, which is comprised of four LEDs and the MPQ7200. The power loss for the four LEDs can be calculated with Equation (7):

$$Four-LED Power Loss = 12.15V \times 0.909A = 11W$$

This means that the IC and inductance power loss is 1.8W.

NTC thermal dimming derating allows for a cost-effective solution where the two heat sources can share the same PCB.