CS1680 datasheet(8/16 Pages) CIRRUS

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CS1680

DS1055F1

5. GENERAL DESCRIPTION

5.1 Overview

The CS1680 is a cascade boost-buck dimmable LED

driver for the 12V halogen lamp-replacement market.

The CS1680 uses a Cirrus Logic proprietary intelligent

digital control that provides exceptional single-lamp and

multi-lamp transformer compatibility for non-dimmer

systems and dimmer systems paired with electronic and

magnetic low-voltage transformers.

The CS1680 integrates a continuous conduction mode

(CCM) boost converter that provides transformer

compatibility and dimmer compatibility. An adaptive

digital algorithm controls the boost stage and dimmer

compatibility operation mode to enable flicker-free

operation down to 5% output current with leading-edge

and trailing-edge dimmers.

5.2 IC Startup and Power Supply

The startup circuit is constructed of a linear regulator

and charge pump, and is used to supply a power-on

voltage to the CS1680. The device provides a GPIO pin

that is used to disable the startup circuit once the boost

output voltage reaches 50% of full scale.

The linear regulator circuit uses transistor Q1 to provide

a supply voltage to a Schmitt-trigger inverter which

enables the charge pump circuit. The GPIO pin is

tri-stated while the controller is held in reset due to low

supply voltage. The charge pump increases the voltage

until the device starts converting. Once the supply

voltage VDD exceeds threshold voltage VST(th), the

controller polls the boost output voltage for 50% of full

scale before driving the GPIO pin low to disable the

startup circuit.

5.3 Boost Stage

The boost stage in the CS1680 is a low-side

asynchronous boost converter. Once the IC reaches its

UVLO start threshold voltage VST(th) and begins

operating, the CS1680 executes a detection algorithm

to set the operating state of the IC (see Table 1 on

page 8). The boost stage utilizes a continuous current

mode (CCM) control algorithm.

5.3.1 Dimmer Compatibility

The CS1680 dimmer switch detection algorithm

determines if the solid-state lighting (SSL) system is

controlled: first, using a regular switch or a leading-edge

dimmer paired with a magnetic transformer, or a

12VAC/VDC source (Mode1); second, by a regular

switch or a trailing-edge dimmer paired with an electronic

transformer (Mode2); third, by a leading-edge dimmer

paired with an electronic transformer (Mode3).

Dimmer switch detection is implemented using a

process of elimination. The method of elimination

progresses through the detection algorithm to find the

best matching state of operation. In an attempt to find a

dimmer compatible mode, the detection algorithm starts

in Mode1, then tries Mode2, if Mode1 and Mode2 are

excluded the algorithm defaults to Mode3.

Mode1

In Mode1, the detectable inputs are a leading-edge

dimmer paired with a magnetic transformer, no dimmer

switch paired with a magnetic transformer, or a

12VDC/VAC source. Upon detection of a magnetic

transformer, the CS1680 operates in a PFC conduction

mode where the device provides a power factor that is

in excess of 0.9. The boost peak current IBSTPK is

modulated across the input voltage to follow a constant

resistance. The target resistance is modulated to

provide boost output regulation. The RMS input voltage

is used to determine the output LED current as a

fraction of full scale. If a DC input voltage is detected,

the controller will set the LED output at 100% of the

available RMS energy.

The boost output voltage VBST is measured at the

trough of the rectified voltage every half-line cycle and

compared against the regulation point, which is set by

resistor RBST (see Figure 11 on page 10). The voltage

difference, the setting of LED output current IOUT, and

the clamp activity are used in the control loop to scale

the boost inductor current allowing the boost output

Boost

Mode

Source

Line Switch

Digital Control Loop

Mode1

12 VAC/VDC

Non-dimming

Executes a boost peak-

current algorithm with

PFC based control.

Magnetic

Transformer

Leading-edge

Dimmer

Non-dimming

Mode2

Electronic

Transformer

Trailing-edge

Dimmer

Executes a constant

boost peak-current

algorithm during the turn-

on time of the electronic

transformer.

Non-dimming

Mode3

Electronic

Transformer

Leading-edge

Dimmer

Executes a constant

power control algorithm

where the boost inductor

current is controlled by

the instantaneous

rectified voltage signal.

Table 1. Operating State

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