CS1630-FSZ datasheet(18/56 Pages) APEX

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CS1630/31

DS954F2

Figure 20 illustrates the tapped buck stage configured for

series output mode.

Figure 20. Tapped Buck Series Output Model

To maintain constant output current with minimum line-

frequency ripple, the following are required:

• For parallel configurations, a minimum voltage potential

difference between two strings

• For series configurations, a minimum current amplitude

difference between two strings

5.8.2 Primary-side Current Control for

Two-Channel Output

The CS1630/31 regulates two-channel output current

independently using primary-side control, which eliminates

the need for opto-coupler feedback. The control loop operates

in peak current control mode, with the peak current set cycle-

by-cycle by the two independent current regulation loops.

Demagnetization time of the second stage inductor is sensed

by the FBAUX pin using an auxiliary winding on the second

stage inductor. The FBAUX pin supplies an input to the digital

control loop.

The power conversion for two-channel output is carried out by

interleaving the PWM. The two-channel control system

consists of two components:

• A toggle device (phase synchronizer circuit) on the

secondary side that alternatively activates each output

channel for each switching event

• A digital sequencer on the primary side determines which

output channel is active for any given switching event

As the output is toggled between each channel, a sequencer

on the primary side identifies the current control phase and

regulates the current in each output channel. To ensure

proper operation for a parallel configuration, the two output

channels should target a voltage differential that is greater

than 20%. For a series configuration, the two output channels

should target a current differential that is greater than 20%.

5.8.3 Auxiliary Winding Configuration

The second-stage inductor auxiliary winding is used for zero-

current detection (ZCD) and overvoltage protection (OVP). The

auxiliary winding is sensed through the FBAUX pin of the IC.

5.8.4 Control Parameters

The second-stage control parameters are set to assure:

• Line Regulation — The LED current remains constant

despite a ±10% AC line voltage variation.

• Effect of Variation in Transformer Magnetizing

Inductance — The LED current remains constant over

a ±20% variation in magnetizing inductance.

The FBSENSE input is used to sense the current in the

second stage inductor. When this current reaches a certain

threshold, the gate drive turns ‘OFF’ (output on pin GD).

Two OTP values are required to set the second-stage output

currents, CH1CUR for channel 1 and CH2CUR for channel 2

(see "Channel 1 Output Current (CH1CUR) – Address 41" on

page 35 and "Channel 2 Output Current (CH2CUR) – Address

43" on page 35). Equations 6 and 7 are used to calculate the

values to be programmed into registers CH1CUR and

CH2CUR.

where,

RSense = Resistance of current sense resistor

VSense = Full scale voltage across sense resistor (~1.4V)

ICH1 = Target current in channel 1 LED string

ICH2 = Target current in channel 2 LED string

RSense is determined by the input voltage, switching

frequency, auxiliary transformer turns ratio, target output

current and output voltage for each channel.

The zero-current detect input on pin FBAUX is used to

determine the demagnetization cycle T2. The controller then

uses these inputs to control the gate drive output, GD.

5.8.5 Frequency Dithering

The peak amplitude of switching harmonics can be reduced by

spreading the energy into wider spectrums. The frequency

dithering level can be managed using bits DITLEVEL[1:0] in

93" on page 49). Additionally, the CS1630/31 has an option to

enable dithering only in No-dimmer Mode by setting bit

DITNODIM to ‘1’. If output currents differ, the CS1630/31 also

has an option to allow for less dither on one of the two

channels by selecting the channel using bit DITCHAN. The

channel selected for less dither attenuates the dither level by

the percentage configured by bits DITATT[1:0].

R13

R11

R14

CS1630 /31

FBAUX

GND

FBSENSE

VBST

R12

R15

D10

R16

C10

GND

VCC

IGND

C12

LED1+

LED 1-

C11

LED 2+

LED 2-

D11

CH1CUR

5112RSense ICH1





NVSense



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

[Eq.6]

CH2CUR

5112RSense ICH2





NVSense



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

[Eq.7]

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