Cirrus-logic AN374 Manual de usuario descargar pdf (Pagina 10)

AN374

10 AN374REV2

Step 1) LED string configuration

Determine if a series configuration or a parallel configuration is a viable solution for the identified light engine.

Figure 2a illustrates a series light configuration. The two LED strings are arranged in series so that current

passes through either one or both LED strings. A MOSFET is used to shunt current around one string on

alternating switching cycles. In this configuration, one string is required to have a larger current than the other

string. When considering a series design, it is recommended that the current flowing through one of the LED

channels be 80% or lower than that of the other LED channel at all times. The LED string that has current

flowing continuously is referred to as channel 1 LED (I

CH1

), while the string with the bypass FET is referred to

as channel 2 LED (I

CH2

); I

CH2

 0.8I

CH1

Figure 2b illustrates a parallel light configuration. The two LED strings are networked in parallel so that current

flows to either the channel 1 LED string or the channel 2 LED string at any given T2 time. The CS1630/31

controller uses the LED forward voltage to detect which LED string is being driven. One LED string must

always have a larger forward voltage compared to the other LED string. The LED string with the higher voltage

is referred to as channel 1 LED with forward voltage V

CH1

and the LED string with the lower voltage is referred

to as channel 2 LED with a forward voltage of V

CH2

A good rule of thumb is that channel 2 LED must always have a forward voltage of 85% or lower than the

channel 1 LED to consider a parallel design. Table 2 defines the selection process based on the requirements

of the series and parallel output configuration.

If there are problems converging on a target design using an existing light engine, the current and voltage

profile can be modified by adding, removing, or moving LEDs between the two LED channels. Figure 3

illustrates the parallel and series scenarios that can be configured using the CS163X Customer Application

tool.

CH2

< V

CH1

and V

CH2

Cross

CH2

< I

CH1

Series or Parallel Configuration Series Configuration

CH1

and I

CH2

Cross Parallel Configuration Not Functional

Table 2. Series vs. Parallel

Figure 2a. Flyback Series Output Model Figure 2b. Flyback Parallel Output Model

R22

R21

R23

CS1630 /31

FBAUX

GND

FB SEN SE

TX1

BST

C10

C15

GND

VCC

D15

R12 D10

C16

Channel 1 LED

(White)

Channel 2 LED

(Red)

GND

IGND

MOD Ex

PRI

MOD Ex

R22

R21

R23

CS1630 /31

FBAUX

GND

FB SEN SE

TX1

BST

C10

C15

GND

VCC

D15

R12 D10

C16

Channel 1 LED

(W hite )

Channel 2 LED

(Red)

GND

IGND

1 2 ... 5 6 7 8 9 10 11 12 13 14 15 ... 34 35

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