CS1615/16
of the system (and hence LED current ILED) if the temperature
exceeds 95°C. The large time constant for this filter ensures that
the dim scaling does not happen spontaneously and is not
noticeable (suppress spurious glitches). The eOTP tracking
circuit is designed to function accurately with external
capacitance up to 470pF.
5.6.7 Overcurrent Protection
Overcurrent protection (OCP) is implemented by monitoring the
voltage across the sense resistor. If this voltage exceeds a
threshold VOCP(th) of 1.69V, a fault condition occurs. The IC
output is disabled and the controller attempts to restart after
approximately one second.
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 the ADC tracking range,
allowing the entire dynamic range of the ADC to be well used.
The CS1615/16 recognizes a resistance (RS+RNTC) equal to
20.3k which corresponds to a temperature of 95°C, as the
beginning of an overtemperature dimming event and starts
reducing the power dissipation. The output current is scaled until
the series resistance (RS+RNTC) value reaches 16.6k (125°C).
Beyond this temperature, the IC shuts down until the resistance
(RS+RNTC) rises above 19.23k. 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.
5.6.8 Open Loop Protection
Open loop protection (OLP) and sense resistor short protection
are implemented by monitoring the voltage across the resistor. If
the voltage on pin FBSENSE does not reach the protection
threshold VOLP(th) of 200mV, the IC output is disabled, and the
controller attempts to restart after approximately one second.
5.7 Overtemperature Protection
The CS1615/16 incorporates internal overtemperature
protection (iOTP) and the ability to connect an external
overtemperature sense circuit for IC protection. Typically, an
NTC thermistor is used.
5.7.1 Internal Overtemperature Protection
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 controller is powered up. If this
check fails, the chip will wait until this condition becomes true
before initializing the rest of the system.
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.
For example, 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 4275. If the
temperature exceeds 95°C, thermistor RNTC is approximately
6.3k and series resistor RS is 14k, so the eOTP pin has a total
resistance of 20.3k. The eOTP pin initiates protective dimming
action by reducing the power dissipation. At 125°C the thermistor
RNTC has 2.6k plus a series resistor RS equal to 14k present
a resistance of 16.6k at the eOTP pin reaching the point where
a thermal shutdown fault intervenes. The CS1615/16 will
continue to monitor pin eOTP and once the series resistor RS
plus the thermistor RNTC rises above 19.23k the device will
resume power conversion (see Figure 16).
5.7.2 External Overtemperature Protection
The external overtemperature protection (eOTP) pin is used to
implement overtemperature protection. A negative temperature
coefficient (NTC) thermistor resistive network is connected to pin
eOTP, usually in the form of a series combination of a resistor RS
and a thermistor RNTC (see Figure 15). The CS1615/16 cyclically
samples the resistance connected to pin eOTP.
CS1615/16
VDD
ICONNECT
eOTP
Control
eOTP
Comp_Out
+
-
10
VCONNECT
(th)
RS
CNTC
(Optional )
NTC
100%
50%
0
Figure 15. eOTP Functional Diagram
The total resistance on the eOTP pin gives an indication of the
temperature and is used in a digital feedback loop to adjust
current ICONNECT into the NTC and series resistor RS to maintain
a constant reference voltage VCONNECT(th) of 1.25V. Current
ICONNECT is generated from a controlled current source with a
full-scale current of 80A. When the loop is in equilibrium, the
voltage on the eOTP pin fluctuates around reference
voltage VCONNECT(th). A resistance ADC is used to generate
current ICONNECT. The ADC output is filtered to suppress noise
and compared against a reference that corresponds to 125°C. A
second low-pass filter with a time constant of two seconds filters
the ADC output and is used to scale down the internal dim level
125
25
95
Temperature (°C)
Figure 16. eOTP Temperature vs. Impedance
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.
DS961F1
11
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