LT3756 datasheet(15/31 Pages) AD

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LT8355-1

Rev. A

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Figure 2. Voltage Feedback Network

Figure 3. Voltage Feedback Network with Level Shifter

APPLICATIONS INFORMATION

Setting the Output Regulation Voltage

In addition to output current regulation, LT8355-1 can

also provide output voltage regulation. The loop will go

into voltage regulation mode when the voltage at the FB

pin of each channel approaches 1.2V, regulating output

voltage will reduce LED current below its programmed

value. Voltage regulation mode is primarily included as a

protection feature, allowing the part to gracefully manage

some faults such as an open LED string. To set the output

regulation voltage, connect a resistive voltage divider from

the output voltage to FB as shown in Figure 2. Choose RFB1

and RFB2 to set the output voltage according to Equation 3.

VOUT = 1+

RFB1

RFB2

⎛

⎝⎜

⎞

⎠⎟

• 1.2V

(3)

It is important that the divider should be Kelvin connected

to the output capacitor, as shown in Figure 2. The resistors

RFB1 and RFB2 should be chosen such that their total value

is between 1k and 1M. Be sure to keep resistor power dis-

sipation capabilities in mind with higher output voltages.

In some configurations, such as buck mode, where the

load voltage is not directly referred to ground, a level

shifter may be needed to use constant-voltage mode con-

trol. Figure 3 shows an example circuit to accomplish out-

put voltage control for buck and buck-boost topologies.

Choose RFB1, RFB2, RFB3, and RBUCK to set the output

voltage according to Equation 4. Note the use of RBUCK in

buck mode operation to keep PNP properly biased when

the input supply gets within 2V of the LED voltage. When

not needed, set RBUCK in Equation 4 to infinity.

VOUT = 1+

RFB3

RBUCK

⎛

⎝⎜

⎞

⎠⎟

VEB +

RFB1

RFB2

• 1.2V

⎛

⎝⎜

⎞

⎠⎟

(4)

If the open LED clamp voltage is programmed correctly

using the resistor divider, then the FB pin should never

exceed 1.12V when LEDs are connected.

Setting Switching and PWM Frequency

To program LT8355-1’s switching frequency, connect a

single resistor to ground from the RT pin. Use Equation 5

to approximate a starting value for the RT pin resistor, or

Table 1 to use suggested values. It is important to connect

only a resistor to GND on the RT pin.

RT =

4.4 • 1010

(fSW1)1.06

(5)

Table 1. RT Values for Selected Switching Frequencies

CH1 SWITCHING FREQUENCY

RT VALUE

102kHz

215kΩ

200kHz

105kΩ

250kHz

84.5kΩ

300kHz

69.8kΩ

350kHz

59kΩ

400kHz

51.1kΩ

1MHz

19.1kΩ

1.95MHz

9.09kΩ

Albeit switching frequency of CH2 is also determined by

the RT value; to achieve high PWM dimming ratios, each

channel has an independent switching frequency. For this

reason, CH2 switches at a minimum of 5.5% faster than

CH1’s switching frequency to reduce audible noise.

During start-up, the switching frequency is reduced to

allow sufficient switch OFF time, especially for 2MHz

operation, so that inductor current can ramp below cur-

rent limit when there is minimal voltage across the induc-

tor during the OFF time. This condition is encountered

RFB1

COUT

VOUT

RFB2

LT8355-1

8355-1 F02

RFB1

COUT

VOUT

VISP

RFB2

LT8355-1

8355-1 F03

RFB3

100k

RBUCK

100k

(BUCK ONLY)

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