CS1610 Datasheet, PDF (12/17 Pages) Cirrus Logic – TRIAC Dimmable LED Driver IC

CS1610/11/12/13
For buck topology, the switching period T_total is computed
using Equation 7:
T_total

I_{PKFB}

T₁
+
T₂

-^F----^B----^g---^a---ⁱ-ⁿ-

[Eq. 7]
where,
 = dimming factor, proportional to the duty cycle of the
dimmer, between 0 and 1
I_PK(FB) = transformer primary winding current
FB_gain = T_total/(T₁ + T₂); a constant, computed at full load
An appropriate value for R_FBGAIN needs to be selected to
provide the correct FB_gain. Resistor R_FBGAIN is calculated
using the formula shown in Equation 8:
R_FBGAIN = _---F----B----_g---_a-6--_i-2-_n--5--_--0--2----_----_–----1--
[Eq. 8]
The value of FB_gain also has a bearing on the linearity of the
dimming factor versus the LED current curve and must be
chosen using Application Note AN364: CS1610/11 Design
Guide.
5.7.3 Output Open Circuit Protection
Output open circuit protection and output overvoltage
protection (OVP) is implemented by monitoring the output
voltage through the transformer auxiliary winding. If the
voltage on the FBAUX pin exceeds a threshold (V_OVP(th)) of
1.25V, a fault condition occurs. The IC output is disabled and
the controller attempts to restart after one second.
5.7.4 Overcurrent Protection (OCP)
Overcurrent protection is implemented by monitoring the
voltage across the second stage sense resistor. If this voltage
exceeds a threshold (V_OCP(th)) of 1.69V, a fault condition
occurs. The IC output is disabled and the controller attempts
to restart after one second.
5.7.5 Open Loop Protection (OLP)
Both open loop protection and protection against a short of the
second stage sense resistor are implemented by monitoring
the voltage across the resistor. If the voltage on pin FBSENSE
does not reach the protection threshold (V_OLP(th)) of 200mV,
the IC output is disabled and the controller attempts to restart
after one second.
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 (I_CONNECT)
into the NTC and series resistor (R_S) to maintain a constant
reference voltage of 1.25V (V_CONNECT(th)). Figure 14
illustrates the functional block diagram when connecting an
optional external NTC temperature sensor to the eOTP circuit.
CS1610/11/12/13
V_DD
eOTP
Control _{Comp_Out}
ICONNE CT
+
eOTP
10
V -
CONNE CT(th)
RS
NTC
CNTC
(Optional )
Figure 14. 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 9:
I_CONNECT = -V----^C---^O----^N----^N-R---^E---^C----^T---^--^t--^h---^
[Eq. 9]
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 10:
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. 10]
Solving Equation 10 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----_
[Eq. 11]
= _---_R----_N-⁴---_T--^-_C---¹--+--⁰---_R⁶-----_S---_-
The tracking range of this resistance 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
external capacitance up to 470pF. A higher 8-bit code output
reflects a lower resistance and hence a higher external
temperature.
12
DS929F5