CS1680 datasheet(10/16 Pages) CIRRUS

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CS1680

DS1055F1

boost output voltage continues to fall and reaches the

lowest threshold set by constant KDEC(on), the LED

output current is decreased at an accelerated rate.

If the LED output current IOUT is at a maximum and

boost output voltage VBST is still above the regulation

high threshold, the controller begins scaling down the

boost inductor current instead of increasing the LED

output current. If this happens, when the boost output

voltage falls below the regulation low threshold, the

boost input current is gradually increased back to

nominal before the LED output current begins to reduce

off from the maximum.

5.3.2 Boost Stage Control

The boost stage uses continuous conduction mode

operation

for

high

compatibility

with electronic

transformers. For current regulation, the controller

varies the peak current IBSTPK as necessary for dimmer

and transformer compatibility. When the dimmer is

paired with an electronic transformer, period T2BST is

modulated to maintain a constant ripple current on the

boost inductor. When the dimmer is paired with a

magnetic transformer, the demagnetization period has

a constant T2BST(fixed) time.

Maximum Peak Current

The maximum boost inductor peak current IBSTPK(max)

is set using the current sense resistor RBST(Sense) on

pin BSTSENSE, which is sampled by a comparator

referenced to an internal DAC. Boost peak current

IBSTPK(max) is calculated using Equation 1:

Boost overcurrent protection (BOCP) is provided using

a higher threshold to detect the event of inductor

saturation. If the voltage on the BSTSENSE pin

exceeds a threshold voltage VBSTOCP(th) of 1.05V, the

controller enters a BOCP fault. 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 boost overcurrent protection

current IBSTPK(OCP) is calculated using Equation 2:

Output BSTOUT Sense and Input VAC Sense

A current proportional to boost output voltage VBST is

supplied to the IC on pin BSTOUT and is used as a

control signal (see Figure 11). The ADC is used to

measure the magnitude of current IBSTOUT through

resistor RBST. The magnitude of current IBSTOUT is then

compared to an internal full scale reference current Iref

of 64

A.

Resistor RBST sets the system full-scale voltage and

determines boost output voltage VBST regulation, boost

overvoltage protection, and clamp behavior. Full-scale

voltage VBST(full) is calculated using Equation 3:

The CS1680-00 is designed for a resistor RBST equal to

604k

, creating a full-scale voltage of 40V. The

CS1680-02 is designed for a resistor RBST equal to

560k

, creating a full-scale voltage of 37V.

A current proportional to the AC/DC input voltage is

supplied to the IC on pin VAC and is used by the boost

control algorithm. Dimmer detection and dim level

calculations are dependent on specific levels of the line

voltage. Resistor Rrec is required to be set equal to

604k



For optimal performance, resistors Rrect and RBST

should use 1% or better resistors for best voltage

accuracy.

Boost Overvoltage Protection

The CS1680 supports boost overvoltage protection

(BOP) to protect bulk capacitor CBST (see Figure 13 on

T1BSTi

T2BSTi

BSTGD

IPKBST

IL = 0

Figure 10. Continuous Conduction Mode Operation

IBSTPK max



VBSTPK th



RBST Sense



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

[Eq.1]

IBSTPK OCP



VBSTOCP th



RBST Sense



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

[Eq.2]

DBST

Vrect

VBST

Rrect

CS1680

IBSTOUT

BSTOUT

VAC

ADC

Iref

20k

MUX

IVAC

RBST

LBST

Figure 11. VAC and BSTOUT Input Pin Model

VBST full



Iref

RBST 20k





[Eq.3]

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