CS1610 Datasheet, PDF (11/17 Pages) Cirrus Logic – TRIAC Dimmable LED Driver IC

CS1610/11/12/13
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.
VB S T
LED +
C8
D8
C9
LED -
L3
CS1612/13
GD ¹³
FBAUX ¹⁵
FBSENSE ¹¹
GND
12
FBGAIN
9
R_{FB GA IN}
Q4
R12
R13
R11
Figure 13. Buck Model
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:
• 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 input 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 FBGAIN input are used to
determine the switching period T_total. The zero-current detect
input on pin FBAUX is used to determine the demagnetization
period T₂. The controller then uses the time T_total to determine
gate turn-on time.
The FBGAIN input is set using an external resistor, R_FBGAIN.
Resistor R_FBGAIN must be selected to ensure that the
switching period T_total is greater than the resonant switching
period T_critical at maximum output power. See Equation 5:
T_total  T_critical = T₁ + T₂
[Eq. 5]
where,
T_critical = resonant switching period at max power
T₁ = gate turn-on time
T₂ = demagnetization time
The switching period T_total is computed for flyback topology
using Equation 6:
T_total

I_{PKFB}

T₂

-^F----^B----^g---^a---ⁱ-ⁿ-

[Eq. 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 = T_total/T₂; a constant, computed at full load
DS929F5
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