LT3598 Просмотр технического описания (PDF) - Linear Technology

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LT3598

6-String 30mA LED Driver with ±1.5% Current Matching

Linear Technology 

LT3598

APPLICATIONS INFORMATION

Overvoltage Protection

The LT3598 uses the FB pin to provide regulated overvoltage

protection when all LED strings are open. A resistor divider

is connected between the VO_SW pin and ground (Figure 2).

There is an internal PMOS switch between VOUT and

VO_SW, which is controlled by the PWM signal. The PMOS

switch addition prevents the feedback resistor divider from

draining the output capacitor during PWM off-period,

allowing for a higher dimming range without falsely tripping

the OPENLED flag. It also reduces the system current in

shutdown. This PMOS has about 1k resistance, so select

FB resistors taking this resistance into account.

To set the maximum output voltage, select the values

of R1 and R2 (see Figure 2) according to the following

equation:

VOUT(MAX) = 1.230V ⎛⎝⎜1+ RR21⎞⎠⎟

LT3598

VOUT

VO_SW

R2

FB

R1

3598 F02

Figure 2. Overvoltage Protection

Voltage Feedback Connections

The output voltage should be set 15% higher than the

normal LED string operating voltage. Under normal

operation, LED1 to LED6 pin voltages are monitored and

provide feedback information to the converter for output

voltage regulation given the programmed LED current.

The maximum output regulation loop is activated only

when all LEDs are open.

Programming Maximum LED Current

Maximum LED current is programmed by placing a resistor

between the ISET pin and ground (RISET). The ISET pin

resistor can be selected from 10k to 100k.

The LED current is programmed according to the following

equation:

ILED

≈

294V

RISET

See Table 4 and Figure 3 for resistor values and corr-

esponding programmed LED current.

LED current can also be adjusted by programming the

CTRL pin voltage.

Table 4. RISET Value Selection for LED Current

LED CURRENT (mA)

RESISTOR ON ISET PIN (k)

3mA

97.6

10mA

29.4

20mA

14.7

30mA

9.76

30

25

20

15

10

5

0

0

20

40

60

80 100

RISET (k)

3598 F03

Figure 3. RISET Value Selection for LED Current

LED Current Dimming

Two different types of dimming control can be used with

the LT3598. The LED current can be set by modulating

the CTRL pin or the PWM pin.

For some applications, a variable DC voltage that adjusts

the LED current is the preferred method of brightness

control. The CTRL pin voltage can be modulated to set

the dimming of the LED string (see Figures 4 and 5). As

the voltage on the CTRL pin increases from 0V to 1.0V,

the LED current increases from 0 to the programmed LED

current level. As the CTRL pin voltage increases beyond

1V, it has no effect on the LED current.

3598fb

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