LT3598EUF(RevA) データシートの表示（PDF） - Linear Technology

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LT3598EUF
(Rev.:RevA)

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, al-

lowing 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+

R2 ⎞

R1 ⎠⎟

LT3598

VOUT

VO_SW

R2

FB

R1

3598 F02

Figure 2. Overvoltage Protection

Voltage Feedback Connections

The output voltage should be set 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 volt-

age 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 resis-

tor 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 follow-

ing equation:

ILED

≈

294V

RISET

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

sponding 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

3598fa

11

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