CS1630 Datasheet, PDF (20/56 Pages) Cirrus Logic – 2-Channel TRIAC Dimmable LED Driver IC

CS1630/31
illustrates the functional block diagram when connecting an
optional external NTC temperature sensor to the eOTP circuit.
CS1630/31 V_DD
eOTP
Control _{Comp_Out}
ICONNECT
+
eOTP ₁₀
- _{VCONNECT (th)}
RS
NTC
CNTC
Figure 21. eOTP Functional Diagram
Current I_CONNECT is generated from an 8-bit controlled current
source with a full-scale current of 80A. See Equation 8:
I_CONNECT = -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 V_CONNECT(th). The digital ‘CODE’ output by
the ADC is used to generate I_CONNECT. In normal operating
mode, the I_CONNECT 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----^C_R---^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--V-_C---^C-_T---^O-_---^N_---R-^N---_NE----_TC---_C-^T---^-₊-^t-^h---R^----_S----_
= -_--₈---₀----_----_A2----⁵_---⁶-_---_-^-_R--¹---_N^.--²-_T--⁵--_C--^V---+----_R-----_S----_
[Eq. 10]
= _---_R----_N----⁴_T---^M_C-----+-^----_R-----_S---_-
The tracking range of this ADC is approximately 15.5k to
4M. The series resistor R_S 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
an external capacitance of a maximum of 470 pF. 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. This filter
is the faster low-pass filter with a programmable time constant
configured using bits EOTP_FLP[2:0] in register Config55
(see "Configuration 55 (Config55) – Address 87" on page 47)
and compared against a programmable code value that
corresponds to the desired shutoff temperature set point.
Shutoff temperature Temp_Shutdown is set using bits
SHUTDWN[3:0] in register Config58 (see "Configuration 58
(Config58) – Address 90" on page 48). If the temperature
exceeds this threshold, the chip enters an external
overtemperature state and shuts down. The external
overtemperature state 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 a
temperature code corresponding to Temp_Wakeup programmed
using bits WAKEUP[3:0] in register Config46 (see
"Configuration 46 (Config46) – Address 78" on page 40). 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 so that the programmed
Temp_Wakeup and Temp_Shutdown codes are greater than the
measured 8-bit code corresponding to the total resistance on
the pin.
When exiting reset, the chip enters startup and the ADC
quickly (<5ms) tracks the external temperature to check if it is
below the Temp_Wakeup reference code (CODE_Wakeup) 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 programmable time constant of 2 minutes is
configured using bits EOTP_SLP[2:0] in register Config55
(see "Configuration 55 (Config55) – Address 87" on page 47).
The filter is applied to the ADC output and uses it to scale
down the internal dim level of the system (and hence I_LED) if
the temperature exceeds a programmable 8-bit threshold that
corresponds to Temp_eOTP (see Figure 22). The large time
constant for this filter ensures that the dim scaling does not
happen spontaneously and is not noticeable (suppress
spurious glitches). Temperature threshold must be set such
that Temp_eOTP<Temp_Wakeup<Temp_Shutdown. Register
Config59 sets Temp_eOTP (see "Configuration 59 (Config59) –
Address 91" on page 48). Register Config46 sets Temp_Wakeup
(see "Configuration 46 (Config46) – Address 78" on page 40).
Register Config58 sets Temp_Shutdown (see "Configuration 58
(Config58) – Address 90" on page 48).
For example, the system can be set up such that I_LED starts
reducing when R_NTC ~6.3k (assuming a 14k 1%
tolerance, series resistor R_S), which corresponds to a
temperature of 95°C (Temp_eOTP code is 196) for a 100k
NTC with a Beta of 4334 (100 kW at 25°C). The I_LED current
is scaled based on the programmed slope using bits
RATE[1:0] in register Config44 (see "Configuration 44
(Config44) – Address 76" on page 39) until it reaches
Temp_Shutdown. The CS1630/31 uses this calculated value to
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