Cirrus Logic CS1616 - Manual (Page 10)

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.
The values obtained from resistors R
CTRL1
and R
CTRL2
are the
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
current target is set using resistor R
CTRL2
, which is connected on
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
I
OUT
= current through LED at maximum output
R
Sense
= resistor attached to pin FBSENSE
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
hold current level is sensed using resistor R
CTRL1
on pin CTRL1,
which is sampled periodically by an ADC. The value of this
resistor can be determined using the formula shown in
Equation 2.
where,
I
IN(CC)
= constant input current used when designing circuit
R
Sense
= resistor attached to pin FBSENSE
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
Maximum primary-side switching current I
PK(max)
is set using
resistor R
Sense
connected to pin FBSENSE of the CS1615/16.
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
exceeds a threshold V
OVP(th)
of 1.25V, a fault condition occurs.
The IC output is disabled and the controller attempts to restart
after approximately one second.
R
CTRL2
CTRL2
FBAUX
GND
GD
9 12
CS1615/16
16
13 Q3
R
S ense
V
rect
FBSENSE
11
LED+
LED-
L2
R7
R8
D4
D5
C8
V
AUX
C7
Figure 14. Buck-boost Model
R
CTRL2
1.4V N 4M   
1.25 511  R
Sense
I
OUT
 
------------------------------------------------------------------------ = [Eq.1]
R
CTRL1
1.4V 4M  
511 I
IN CC  
R
Sense
 
------------------------------------------------------------ = [Eq.2]
I
PK max  
1.4
R
Sense
------------------- = [Eq.3]