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LT3591

3591f

APPLICATIONS INFORMATION

SCHOTTKY DIODE

The LT3591 has a built-in Schottky diode. The internal

schottky saves board space in space constrained appli-

cations. In less space sensitive applications, an external

schottky diode connected between the SW node and the

CAP node increases efﬁciency one to two percent. It is

important to use a properly rated schottky diode that can

handle the peak switch current of the LT3591. In addition,

the schottky diode must have a breakdown voltage of at least

40V along with a low forward voltage in order to achieve

higher efﬁciency. One recommended external schottky

diode for the LT3591 is the Phillips PMEG4005AEA.

OVERVOLTAGE PROTECTION

The LT3591 has an internal open-circuit protection circuit.

In the cases of output open circuit, when the LEDs are

disconnected from the circuit or the LEDs fail open circuit,

VCAP is clamped at 42V (typ). The LT3591 will then switch

at a very low frequency to minimize input current. The VCAP

and input current during output open circuit are shown in

the Typical Performance Characteristics. Figure 4 shows

the transient response when the LEDs are disconnected.

For low DCR inductors, which is usually the case for this

application, the peak inrush current can be simpliﬁed as

follows:

⎛

⎝⎜

⎞

⎠⎟

–.

•

• exp –

•

–

4•

where L is the inductance, r is the DCR of the inductor

and C is the output capacitance.

Table 3 gives inrush peak currents for some component

selections.

Table 3. Inrush Peak Currents

VIN (V)

r (Ω)L (µH)

COUT (µF)

IP (A)

4.2

0.3

2.2

1.06

4.2

0.71

2.2

0.96

4.2

0.58

0.83

4.2

1.6

0.68

PROGRAMMING LED CURRENT

The feedback resistor (RSENSE) and the sense voltage

(VCAP – VLED) control the LED current.

The CTRL pin controls the sense reference voltage as

shown in the Typical Performance Characteristics. For

CTRL higher than 1.5V, the sense reference is 200mV,

which results in full LED current. In order to have accurate

LED current, precision resistors are preferred (1% is rec-

ommended). The formula and table for RSENSE selection

are shown below.

SENSE

LED

200

VCAP

20V/DIV

500mA/DIV

VIN = 3.6V

CIRCUIT OF

FRONT PAGE

APPLICATION

LEDs DISCONNECTED

AT THIS INSTANT

500

µs/DIV

3591 F04

Figure 4. Output Open-Circuit Waveform

INRUSH CURRENT

The LT3591 has a built-in Schottky diode. When supply

voltage is applied to the VIN pin, an inrush current ﬂows

through the inductor and the Schottky diode and charges

up the CAP voltage. The Schottky diode inside the LT3591

can sustain a maximum current of 1A.

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