LT3756 datasheet(14/31 Pages) AD

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LT8355-1

Rev. A

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Introduction

LT8355-1 provides a variety of features to enable a wide

range of application options. Due to the small package

size and pin count, many pins perform more than one

function. The simplest LT8355-1 application is realizable

in only a few off-chip components and with very few

design choices on the part of the user. To fully utilize the

capability of the part, however, requires a good under-

standing of how the individual features play together. The

detailed explanations of each feature below along with

several example application circuits will help with devel-

oping a good understanding of the part.

Programming the LED Current

Full-scale current through the LED string is easily pro-

grammed using a single resistor (RILED) connected in

series with the LED string. For most applications, the LED

current sense resistor should be placed on the high side

of the LED string; and Kelvin connected to LED current

sense pins ISP and ISN for each channel. The loop will

regulate a drop of 250mV across the LED current sense

resistor, scaled by the voltage at the CTRL1,2 and IADJ2

pins, as discussed later. Choose a resistor with sufficient

power dissipation capability for the programmed LED

current. For LED currents less than 1.5A, a 0.5W resistor

will usually suffice. Since the loop regulates the voltage

across RILED, typical full-scale load current (IFS) is defined

in Equation 1.

IFS =

4RILED

(1)

For the best performance and protection, sense at the

highest potential in the LED string, and tie the source of

the external PWM dimming PMOS to ISN. More details

on the PWM dimming PMOS will be discussed later. Note

that the loop can regulate LED current when the voltage

at ISP is 0V, but a ground sensing configuration is not

desirable for many applications. For more information see

the Low Voltage (SEPIC) Start-Up section.

Adjusting LED Current with CTRL1,2 and IADJ2 Pins

The CTRL1,2 and IADJ2 pins provide an analog alterna-

tive to PWM dimming. The value of the voltage regulated

APPLICATIONS INFORMATION

Figure 1. VISP – VISN vs CTRL1,2 and VIADJ2 Pin Voltage

across the LED current sense resistor can be adjusted

with the CTRL1 pin for channel 1, and CTRL2 and/or

IADJ2 pins for channel 2. As the voltage present at either

pin is independently varied, the regulated voltage drop

across the LED current sense resistor varies from 0V to

250mV. The system will supply no current if the CTRL1,2

pins voltage falls below 500mV or the IADJ2 pin voltage

falls below 500mV, and will continue to supply full-scale

current if the voltage at the CTRL1,2 pins exceed 1.5V and

the IADJ2 pin exceeds 1.5V. The total LED current, ILED,

taking the effects of the CTRL pin into account is shown

in Equation 2a and Equation 2b:

ILED1 =

VCTRL1– 0.5

4RILED1

(2a)

ILED2 =

(VCTRL2 – 0.48)(VIADJ2 – 0.5)– 0.02

4RILED2

0.5V

< VCTRL1-2,VADJ2 < 1.5V

(2b)

The IADJ2 feature intentionally adds a small offset to

ensure that true zero can be reached for LED current

regulation. As a result, some combinations of CTRL2

and IADJ2 near the bottom of the range will result in no

current flowing in the load. Note that the CTRL1,2 and

IADJ2 pins can also be used for PWM by driving either

with a digital signal whose low value is below 300mV for

CTRL1,2 and 500mV for IADJ2; and whose high value is

above 1.6V. Figure 1 shows how CTRL pin voltage affects

LED current for varying values of IADJ2 for channel 2.

IADJ2 = 1.5V

IADJ2 = 1.25V

IADJ2 = 1.0V

IADJ2 = 0.75V

IADJ2 = 0.6V

CH2 ONLY

CTRL PIN VOLTAGE (V)

0.3 0.5 0.7 0.9 1.1 1.3 1.5 1.7 1.9

100

125

150

175

200

225

250

275

8355-1 F01

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