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CS1612

TRIAC Dimmable LED Driver IC

Apex Microtechnology 

CS1610/11/12/13

5.8 Overtemperature Protection

The CS1610/11/12/13 incorporates both internal overtemper-

ature protection (iOTP) and the ability to connect an external

overtemperature sense circuit for IC protection. Typically, a

NTC thermistor is used.

5.8.1 Internal Overtemperature Protection

Internal overtemperature protection (iOTP) is activated, and

switching is disabled when the die temperature of the devices

exceeds 135°C. There is a hysteresis of about 14°C before

resuming normal operation.

5.8.2 External Overtemperature Protection

The external overtemperature protection (eOTP) pin is used to

implement overtemperature protection using an external

negative temperature coefficient (NTC) thermistor. The total

resistance on the eOTP pin is converted to an 8-bit digital

‘CODE’ (which gives an indication of the temperature) using a

digital feedback loop, which adjusts the current (ICONNECT)

into the NTC and series resistor (RS) to maintain a constant

reference voltage of 1.25V (VCONNECT(th)). Figure 14

illustrates the functional block diagram when connecting an

optional external NTC temperature sensor to the eOTP circuit.

CS1610/11/12/13

VDD

eOTP

Control Comp_Out

ICONNE CT

+

eOTP

10

V -

CONNE CT(th)

RS

NTC

CNTC

(Optional )

Figure 14. eOTP Functional Diagram

Current ICONNECT is generated from an 8-bit controlled current

source with a full-scale current of 80A. See Equation 8:

ICONNECT = V-----C----O---N----N-R---E---C----T-----t--h---

[Eq. 8]

When the loop is in equilibrium, the voltage on the eOTP pin

fluctuates around VCONNECT(th). The digital ‘CODE’ output by

the ADC is used to generate ICONNECT. In normal operating

mode, the ICONNECT current is updated once every seventh

half line-cycle by a single ± LSB step. See Equation 9:

CODE  -I-C----O----N-2---NN----E---C----T- = -V----CR---O--N--N-T---NC----E-+--C---R-T----S-t--h---

[Eq. 9]

Solving Equation 9 for CODE:

CODE = -I-C----O---2-N--N--N----E---C-V---T-C----O----N--R-N----NE----TC---CT-----+t--h---R----S----

= ---8---0--------A2----5---6--------R--1--N-.--2-T--5-C---V--+-----R-----S----

= ---R----N-4---T---C---1--+--0---R6-----S----

[Eq. 10]

The tracking range of this resistance ADC is approximately

15.5k to 4M. The series resistor RS is used to adjust the

resistance of the NTC to fall within this ADC tracking range so

that the entire 8-bit dynamic range of the ADC is well used. A

14k (±1% tolerance) series resistor is required to allow

measurements of up to 130°C to be within the eOTP tracking

range when a 100k NTC with a Beta of 4334 is used. The

eOTP tracking circuit is designed to function accurately with

external capacitance up to 470pF. A higher 8-bit code output

reflects a lower resistance and hence a higher external

temperature.

The ADC output code is filtered to suppress noise and

compared against a reference code that corresponds to

125/130°C. If the temperature exceeds this threshold, the

chip enters an external overtemperature state and shuts

down. This is not a latched protection state, and the ADC

keeps tracking the temperature in this state in order to clear

the fault state once the temperature drops below 110°C. If an

external overtemperature protection thermistor is not used,

connect the eOTP pin to GND using a 50k to 500k resistor

to disable the eOTP feature.

When exiting reset, the chip enters startup and the ADC

quickly (<5ms) tracks the external temperature to check if it is

below the 110°C reference code before the boost and second

stages are powered up. If this check fails, the chip will wait

until this condition becomes true before initializing the rest of

the system.

For external overtemperature protection, a second low-pass

filter with a time constant of two seconds filters the ADC output

and uses it to scale down the internal dim level of the system

(and hence the LED current, ILED) if the temperature exceeds

95 °C (see Figure 15). The large time constant for this filter

ensures that the dim scaling does not happen spontaneously

and is not noticeable (suppress spurious glitches). The ILED

starts reducing when RNTC ~ 6.3k (assuming a 14k1%

tolerance, series resistor), which corresponds to a

temperature of 95°C for a 100k NTC (100k at 25°C). The

ILED current is scaled until the NTC value reaches 2.5k

(125°C). The CS1610/11/12/13 uses this calculated value to

scale the output LED current, ILED, as shown in Figure 15.

100%

50%

0

25

95

125

Temperature (°C)

Figure 15. LED Current vs. eOTP Temperature

Beyond this temperature, the IC shuts down using the

mechanism discussed above. If the external overtemperature

protection feature is not required, connect the eOTP pin to

GND using a 50k-to-500k resistor to disable the eOTP

feature.

12

CS1610/11/12/13

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