CS1610/11/12/13

DS929F3

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,

shown in Equation 6:

where,

dimmer, between 0 and 1

using the formula shown in Equation 7:

dimming factor versus the LED current curve and must be

chosen using Application Note AN364: CS1610/11 Design

Guide.

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

1.25V, a fault condition occurs. The IC output is disabled and

the controller attempts to restart after one second.

5.7.4

Overcurrent Protection (OCP)

Overcurrent protection is implemented by monitoring the

voltage across the second stage sense resistor. If this voltage

occurs. The IC output is disabled and the controller attempts

to restart after one second.

5.7.5

Open Loop Protection (OLP)

Both open loop protection and protection against a short of the

second stage sense resistor are implemented by monitoring

the voltage across the resistor. If the voltage on pin FBSENSE

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 -

C8

D8

C9

L3

+

=





[Eq.5]







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





[Eq.6]

4000000



 64

–

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

=

[Eq.7]