APPLICATION NOTE 7616

Guideline on implementing temperature foldback with MAX25510, MAX25511 and MAX25512 using an NTC

By: Martina Anichini

Abstract:

This application note explains how to correctly size the circuit through an NTC thermistor external to the MAX25510, MAX25511, and MAX25512 LED Backlight driver so that it can perform a temperature control on the LED current until it switches off if the temperature is above the desired threshold.


Introduction

The MAX25510, MAX25511, and MAX25512 are 4-channel, white-LED backlight drivers with an integrated current-mode boost converter that operate over a switching frequency range from 400kHz to 2.2MHz and incorporate a spread spectrum.

These devices can implement LED current reduction at high temperatures using an NTC temperature sensor to GND with resistors from the NTC to TEMP and to V18. The purpose of this application note is to provide information to assist in the selection of the external components for thermal foldback.

The temperature foldback current control system helps to prevent damage to the display system due to high temperature.

Note: If the function described is not necessary and therefore unused, connect TEMP to V18.

First, it's necessary to decide the temperature T1 from which the current reduction in the LEDs must be activated and a TOFF temperature from which the switch-off is required. At this point, choose an NTC suitable for the application, having a known characteristic and as linear as possible in the required temperature range.

In the MAX25512, when the temperature is approaching T1, the over temperature warning (OTW) bit in register DIAG_REG is asserted.

In all devices when the temperature reaches TOFF, the LED current is turned off, and the FLTB pin asserts low. In the MAX25512, the over temperature (OT) bit in register DIAG_REG is asserted.

Temperature Foldback CurveFigure 1. Temperature Foldback Curve.

How to choose an NTC thermistor

An NTC thermistor decreases its resistance when the temperature increases with a characteristic that, in the first order, follows this law:

The following NTC thermistor parameters can be found in the manufacturer's data sheet:

  • Resistance - This is the thermistor resistance at the temperature specified by the manufacturer, often 25°C. For this application, use 10kΩ.
  • Tolerance - Indicates how much the resistance can vary from the specified value. For this application, tolerance of 3% or less is recommended.
  • B (or β) constant - A value that represents the relationship between the resistance and temperature over a specified temperature range. For example, β25/85 = 3977 [K] indicates a beta constant of 3977 over a temperature range from 25°C to 85°C.
  • Tolerance on Beta constants - Beta constant tolerance in percent.
  • Others - Operating Temperature Range, Thermal Dissipation etc.

Sometimes, the manufacturer directly provides the standardized R/T characteristics of the minimum, typical and maximum values of the NTC considering both the effect of the resistance tolerance and the tolerance of the Beta constant. In this application note, only the nominal value is considered.

The MAX25510, MAX25511, and MAX25512 are designed to be used with the NTCLE100E3103*B0 or a similar NTC device.

Relevant Electrical Characteristics

The following values are indicated in the MAX25510, MAX25511, and MAX25512 data sheets:

Table 1. Electrical Characteristics

Parameter Symbol Condition MIN TYP MAX Units
V18 Voltage V18 - 1.75 1.8 1.85 V
ISET Output Voltage VISET - - 0.75 - V
TEMP Pin Voltage VTEMP - 180 200 220 mV
TEMP to ISET Gain TEMPGAIN VTEMP<250mV 13.7 14.3 14.9 V/mA
ISET Voltage Threshold for LED Current Disable VTEMPD - 125 150 175 mV

VIN = VEN = 12V, TA = TJ = -40°C to +125°C, unless otherwise noted. Typical values are at TA = +25°C, unless otherwise noted.

Typical Application Circuit

Typical Application CircuitFigure 2. Typical Application Circuit.

When an NTC temperature sensor is connected between GND and a resistor (RT1) connected to the V18 supply, with a further resistor (RT2) connected from the junction of the NTC and RT1 to the TEMP pin, a temperature foldback is implemented. The resistor R3 is optional.

When the voltage at the TEMP pin reaches 200mV, a current foldback begins. The value of RT1 together with the resistance of the NTC at temperature T1 set this threshold. Select RT1 according to:

Where, V18 is 1.8V, VTEMP is 200mV and RNTCT1 is the NTC resistor value at temperature T1.

The current in the LEDs is reduced according to the linear scheme shown in Figure 1. For temperatures greater than T1, ITEMP increases and is given by the equation:

up to the switch-off temperature TOFF, such that:

The slope of the current reduction is nominally set by RT2, such that:

Where, RNTCTOFF is the NTC resistor value at temperature TOFF.

Design example 1

As an example, calculate the resistor values to attain a T1 of 60°C and a TOFF of 100°C using the NTCLE100E3103*B0 NTC.

The value of the NTC resistance at 60°C is 2490Ω, and thus the value of the RT1 resistor is:

The nearest standard value is 20kΩ.

Using the equation for RT2, we can calculate its value knowing that the NTC resistance is 677Ω at 100°C. The equation becomes:

And the calculated value is 2707Ω, the nearest standard value being 2700Ω.

Design example 2

Table 2 shows some further examples of values of RT1 and RT2 to obtain certain values of T1 and TOFF with NTCLE100E3103*B0 as NTC.

Table 2. RT1 and RT2 examples of values

T1 TOFF thELTA RNTCT1 RNTCTOFF RT1 RT2
60°C 100°C 40°C 2490Ω* 677Ω* 20kΩ 2700Ω
70°C 100°C 30°C 1753Ω* 677Ω* 14kΩ 2100Ω
80°C 105°C 25°C 1256Ω* 585Ω* 10kΩ 1780Ω

*with NTCLE100E3103*B0 as NTC.

Temperature Foldback curves for different RT1 - RT2 valuesFigure 3. Temperature Foldback curves for different RT1 - RT2 values.

Using optional R3 resistor for adjusting the NTC R/T characteristic

If a linear curve is required for measurements over a temperature range, a series-connected or a paralleled resistors are quite useful.

If correctly sized, the paralleled resistor R3 (optional in Figure 2) can help to adjust the characteristic of the chosen NTC thermistor.

Replacing RNTCT1 with (RNTCT1|| R3) and RNTCTOFF with (RNTCTOFF|| R3) in the previous equations, a degree of freedom is introduced to tune the value of the equivalent resistance such that 8(RNTCT1|| R3) matches an available RT1 value.

References/Other Resources