Cirrus Logic CS1613 - Manual (Page 11)

CS1610/11/12/13
DS929F6 11
A quasi-resonant buck stage is illustrated in Figure 13. The
buck stage is controlled by measuring current in the buck
inductor and voltage on the auxiliary winding.
The digital buck algorithm ensures monotonic dimming from
2% to 100% of the dimming range with a linear relationship
between the dimming signal and the LED current.
Quasi-resonant operation is achieved by detecting second
stage inductor demagnetization via an auxiliary winding. The
digital control algorithm rejects line-frequency ripple created
on the second stage input by the front-end boost stage,
resulting in the highest possible LED efficiency and long LED
life.
5.7.1 Auxiliary Winding Configuration
The auxiliary winding is also used for zero-current
detection (ZCD) and overvoltage protection (OVP). The
auxiliary winding is sensed through the FBAUX pin of the IC.
5.7.2 Control Parameters
The second stage control parameters assure the following:
• 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 second stage requires three inputs and generates one
key output. The FBSENSE pin is used to sense the current in
the second stage inductor.
When the current reaches a certain
threshold, the gate drive turns ‘OFF’ (output on pin GD). The
sensed current and the FB
Gain
input are used to determine the
total switching period TT. The zero-current detect input on pin
FBAUX is used to determine the demagnetization period T2.
The controller then uses the total switching period TT to
determine gate turn-on time.
The FB
Gain
input is set using resistor R
FBGAIN
. Resistor
R
FBGAIN
must be selected to ensure that the switching
period TT is greater than the resonant switching period T
critical
at maximum output power. See Equation 5:
where,
T
critical
= resonant switching period at maximum power
T1 = gate turn-on time
T2 = demagnetization time
The total switching period TT is computed for flyback topology
using Equation 6:
where,

= dimming factor, proportional to the duty cycle of the
dimmer, between 0 and 1
I
PK(FB)
= transformer primary winding current
FB
Gain
= constant TT/T2; computed at full load
For buck topology, the total switching period TT is computed
using Equation 7:
where,

= dimming factor, proportional to the duty cycle of the
dimmer, between 0 and 1
I
PK(FB)
= transformer primary winding current
FB
Gain
= constant TT/(T1 + T2); computed at full load
An appropriate value for resistor R
FBGAIN
needs to be
selected to provide the correct gain constant FB
Gain
. Resistor
R
FBGAIN
is calculated using Equation 8:
The value of gain constant FB
Gain
also has a bearing on the
linearity of the dimming factor versus the LED current curve
and must be selected using Application Note AN364: Design
Guide for a CS1610 and CS1611 Dimmer-compatible SSL
Circuit
and AN372: Design Guide for a CS1612 and CS1613
Dimmer-compatible SSL Circuit .
5.7.3 Output Open Circuit Protection
Output open circuit protection and output overvoltage
protection (OVP) is implemented by monitoring the output
voltage through the transformer auxiliary winding. If the
voltage on the FBAUX pin exceeds the threshold
voltage V
OVP(th)
of 1.25V, a fault condition occurs. The IC
output is disabled, and the controller attempts to restart after
one second.
13
11
R
FB GA IN
FBGAIN
FBAUX
GND
GD
FBSENSE
15
9 12
CS1612/13
R12
R11
R13
Q4
LED +
LED -
V B S T
C8
D8 C9
L3
Figure 13. Buck Model
TT T
critical
T1 T2 + =
   [Eq.5]
TT I
PK FB  
T2 
FB
Gain

-------------------


[Eq.6]
TT I
PK FB  
T1 T2 +
  
FB
Gain

------------------- 

[Eq.7]
R
FBGAIN
62.5k 
FB
Gain
2    1
–
-------------------------------------------
= [Eq.8]