CS1610/11/12/13

DS929F5

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.

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

input on pin FBAUX is used to determine the demagnetization

gate turn-on time.

where,

using Equation 6:

where,

dimmer, between 0 and 1

13

11

R

FB GA IN

FBGAIN

FBAUX

GND

GD

FBSENSE

15

9

12

CS1612/13

R12

R11

R13

Q4

LED +

LED -

C8

D8

C9

L3

+

=





[Eq.5]







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





[Eq.6]