CS1680 datasheet(12/16 Pages) CIRRUS

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CS1680

DS1055F1

period T2BUCK. The controller then uses the

time TTBUCK to determine gate turn-on time.

5.4.2 Current Sense Resistor Model

The digital algorithm ensures monotonic dimming from

5% to 100% of the dimming range with a linear

relationship between the dimming signal and the LED

current. The buck stage is regulated by peak current

control with a 1% external sense resistor RBUCK(Sense)

connected to the BUCKSENSE pin. Buck peak current

IBUCKPK(max) is calculated using Equation 7:

Overcurrent protection (OCP) is implemented by

monitoring

the

voltage

across

buck

sense

resistor RBUCK(Sense). If this voltage exceeds a

threshold voltage VBUCKOCP(th) of 1.05 V, a fault

condition occurs. The IC output is disabled, the gate

drive output pins BSTGD and BUCKGD turn off, and the

controller attempts to restart after one second. The buck

overcurrent

protection

current

IBUCKPK(OCP) is

calculated using Equation 8:

5.4.3 Zero-current Detection

Zero-current switching is achieved by detecting the

buck inductor current zero-crossing using a capacitive

coupling network. The digital control algorithm rejects

line-frequency ripple created on the second-stage input

by the front-end boost stage, resulting in the highest

possible LED efficiency and long LED life.

5.5 Overtemperature Protection

The CS1680 incorporates both internal overtemperature

protection (iOTP) and the ability to connect an external

overtemperature sense circuit for IC protection. Typical-

ly, a negative temperature coefficient (NTC) thermistor is

used.

5.5.1 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.

5.5.2 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 14). The CS1680 cyclically samples the resistance

connected to pin eOTP.

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 thermistor 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

A. When the loop is in equilibrium, the voltage on pin

eOTP fluctuates around voltage VCONNECT(th). A

resistance ADC is used to generate 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 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.

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 thermistor to fall within the ADC

tracking range, allowing the entire dynamic range of the

ADC to be well used. The CS1680 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 enters a fault

state and shuts down. This fault state is a latched

protection state, and the fault state is not cleared until the

power to the IC is recycled.

IBUCKPK max



VBUCKPK th



RBUCK Sense



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

[Eq.7]

IBUCKPK OCP



VBUCKOCP th



RBUCK Sense



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

[Eq.8]

CS1680

CONNE CT

(th)

Comp_Out

eOTP

Control

eOTP

CNTC

NTC

(Optional )

Figure 14. eOTP Functional Diagram

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