CS1613 datasheet(12/17 Pages) APEX

12 / 17 page

CS1610/11/12/13

DS929F5

For buck topology, the switching period Ttotal is computed

using Equation 7:

where,

 = dimming factor, proportional to the duty cycle of the

dimmer, between 0 and 1

IPK(FB) = transformer primary winding current

FBgain =Ttotal/(T1 + T2); a constant, computed at full load

An appropriate value for RFBGAIN needs to be selected to

provide the correct FBgain. Resistor RFBGAIN is calculated

using the formula shown in Equation 8:

The value of FBgain 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 (VOVP(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 (VOCP(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 (VOLP(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 (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.

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 9:

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 10:

Solving Equation 10 for CODE:

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.

Ttotal

IPK FB



T1 T2





FBgain



------------------





[Eq.7]

RFBGAIN

6250

FBgain 2



 1

–

------------------------------------------

[Eq.8]

CS1610/11/12/13

CONNE CT

(th)

Comp_Out

eOTP

Control

eOTP

CNTC

NTC

(Optional )

ICONNECT

VCONNECT th



-------------------------------------

[Eq.9]

CODE

ICONNECT

---------------------------



VCONNECT th



RNTC RS

-------------------------------------

[Eq.10]

CODE

VCONNECT th





ICONNECT

RNTC RS





-------------------------------------------------------------------

256 1.25 V



A



RNTC RS





-----------------------------------------------------------

410



RNTC RS



---------------------------------

[Eq.11]

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