Cirrus Logic CS1631 User Manual
Page 20
CS1630/31
20
DS954F3
Figure 21 illustrates the functional block diagram when
connecting an optional external NTC temperature sensor to
the eOTP circuit.
Current I
CONNECT
is generated from an 8-bit controlled current
source with a full-scale current of 80
A. See Equation 8:
When the loop is in equilibrium, the voltage on the eOTP pin
fluctuates around threshold voltage V
CONNECT(th)
. The 8-bit
digital ‘CODE’ output by the ADC is used to generate
current I
CONNECT
. In normal operating mode,
current I
CONNECT
is updated once every seventh half
line-cycle by a single ± LSB step. See Equation 9:
Use Equation 9 to solve for the 8-bit digital CODE output.
See Equation 10:
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 thermistor R
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 using a 100k
NTC thermistor with a
Beta of 4334. 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 temperature Temp
Wakeup
programmed using bits WAKEUP[3:0] in register Config46
(see "Configuration 46 (Config46) – Address 78" on page 40).
When exiting reset, the chip enters startup and the ADC
quickly (<5ms) tracks the external temperature to check if it is
below the temperature 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
current I
LED
) if the temperature exceeds a programmable 8-bit
threshold that corresponds to temperature Temp
eOTP
(see
Figure 22. on page 21). The large time constant for this filter
ensures that the dim scaling does not happen spontaneously
(suppress spurious glitches) and is not noticeable.
Temperature thresholds must be set such that
Temp
eOTP
<Temp
Wakeup
<Temp
Shutdown
. Register Config59
sets temperature Temp
eOTP
(Config59) – Address 91" on page 48). Register Config46 sets
temperature Temp
Wakeup
(see "Configuration 46 (Config46) –
Address 78" on page 40). Register Config58 sets temperature
Temp
Shutdown
(see "Configuration 58 (Config58) – Address
90" on page 48).
For example, the system can be set up such that current I
LED
starts reducing when thermistor R
NTC
is approximately 6.3k
(assuming a 14k
1% tolerance, series resistor R
S
), which
corresponds to a temperature of 95°C (temperature
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 temperature Temp
Shutdown
. The CS1630/31
uses this calculated value to scale output LED current I
LED
, as
shown in Figure 22. on page 21.
CS1630/31
+
-
I
CONNECT
V
CONNECT(th)
Comp_Out
eOTP
Control
eOTP
R
S
C
NTC
NTC
V
DD
10
Figure 21. eOTP Functional Diagram
I
CONNECT
V
CONNECT th
R
-------------------------------------
=
[Eq.8]
CODE
I
CONNECT
2
N
---------------------------
V
CONNECT th
R
NTC
R
S
+
-------------------------------------
=
[Eq.9]
CODE
2
N
V
CONNECT th
I
CONNECT
R
NTC
R
S
+
-----------------------------------------------------------------
=
[Eq.10]
4M
R
NTC
R
S
+
---------------------------------
=
256 1.25 V
80
A
R
NTC
R
S
+
----------------------------------------------------------
=