CS1615/16

10

DS961F1

Figure 14 illustrates the dual-mode buck-boost topology. The

CS1615/16 regulates the output current by controlling the peak

current to ensure that the target output charge is achieved every

half line-cycle. Demagnetization time of the inductor is sensed by

the FBAUX pin using an auxiliary winding and is used as an input

to the control loop.

5.6.1 Primary-Side Current Control

All input current shaping and output power transfer is attained

using a peak current control algorithm. Demagnetization time of

the primary inductor is sensed by the FBAUX pin using an

auxiliary winding and is used as an input to the control algorithm.

other inputs to the control algorithm that help shape the input

current and control the LED current, respectively.

5.6.2 Output Current Regulation

The CS1615/16 regulates output current by controlling the

charge transferred over a half line-cycle. The full-scale output

pin CTRL2. This pin is sampled periodically by an ADC. The

value of this resistor can be determined using Equation 1.

where,

N = turns ratio

When designing a buck-boost topology the turns ratio N is set to

one.

The CS1615/16 uses the value obtained from the resistor along

with the phase-cut and line-cycle period information to determine

the corresponding target full-scale output charge. The IC controls

the inductor switching frequency and peak current to ensure that

the target output charge is achieved every half line-cycle, thus

regulating the output current.

5.6.3 Input Current Shaping

The CS1615/16 shapes the input current by controlling the peak

primary current and the flyback/buck-boost switching frequency.

It shapes the currents differently when behind a dimmer

compared to when not behind a dimmer.

5.6.3.1 Operation Behind a Dimmer

Operating behind a dimmer, the CS1615/16 controls the

switching frequency to ensure that the average input current is

greater than the dimmer hold current requirement. The dimmer

which is sampled periodically by an ADC. The value of this

resistor can be determined using the formula shown in

Equation 2.

where,

5.6.3.2 Operation in No-dimmer Mode

Operating in No-dimmer Mode, the CS1615/16 controls the

switching frequency to ensure that the average input current

follows the line voltage to provide power factor correction. In No-

dimmer Mode the controller is designed to operate in quasi-

resonant mode to improve efficiency.

5.6.4 Max Primary-side Switching Current

The maximum primary-side switching current can be calculated

using Equation 3.

5.6.5 Auxiliary Winding Configuration

The auxiliary winding is used for zero-current detection (ZCD),

overvoltage protection (OVP), fast startup, and the steady-state

power supply. The voltage on the auxiliary winding is sensed

through pin FBAUX of the CS1615/16 for zero-current detection,

overvoltage protection, and fast startup. The auxiliary winding is

also used to provide the steady-state power supply to the

CS1615/16.

5.6.6 Output Open Circuit Protection

Output open circuit protection and output overvoltage protection

(OVP) are implemented by monitoring the output voltage through

the transformer auxiliary winding. If the voltage on the FBAUX pin

The IC output is disabled and the controller attempts to restart

after approximately one second.

CTRL2

FBAUX

GND

GD

9

12

CS1615/16

16

13

Q3

FBSENSE

11

LED+

LED-

L2

R7

R8

D4

D5

C8

C7

Figure 14. Buck-boost Model

1.4V N 4M







1.25 511







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

=

[Eq.1]

1.4V 4M











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

=

[Eq.2]



1.4

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

=

[Eq.3]