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LT3954IUHE

40VIN LED Converter with Internal PWM Generator

Linear Technology 

LT3954

APPLICATIONS INFORMATION

the LED driver efficiency. Therefore, the sufficient low ESR

ceramic capacitors should be selected. The X5R or X7R

ceramic capacitor is recommended for CDC.

Short-Circuit Protection for a Boosted Output

The LT3954 has two features that provide protection from

a shorted circuit load on a boost. The first of these is the

ISP/ISN based overcurrent response. The second is the FB

overvoltage response. The primary mode of action for both

features is to drive the PWMOUT pin low, which turns off the

switch connecting the output to the load. The ISP/ISN short-

circuit protection also drives the PWM and DIM/SS pins

low for a brief period of time. For best protection, a PMOS

disconnect switch M1 is placed as shown in Figure 10.

During an overcurrent event caused by a short across

the LED string, the current in Rs increases until PNP Q1

turns on and pulls up the gate of M1, throttling back the

current. In approximately 1µs, the ISP/ISN overcurrent

response will cause the PWMOUT pin to drive low, which

will turn off M1 altogether. If an external PWM signal is

used, then the circuit including Q3, the 1N4148 diode and

two resistors must be used to ensure the switch remains

off while the output is in a faulted state. This sub-circuit

drives the FB pin into the overvoltage state

If the PWM pin is configured (with a capacitor load) as

shown in the application titled Boost LED Driver with

Output Short Protection, then the small circuit driving

FB may be omitted. In this case, the boost converter will

demonstrate a hiccup mode response, turning on M1 at an

interval determined by the PWM capacitor, then turning off

after ~1µs due to excessive current, until the fault clears.

Board Layout

The high speed operation of the LT3954 demands careful

attention to board layout and component placement. The

exposed pads of the package are important for thermal

management of the IC. It is crucial to achieve a good electri-

cal and thermal contact between the GND exposed pad and

the ground plane of the board. To reduce electromagnetic

interference (EMI), it is important to minimize the area

of the high dV/dt switching node between the inductor,

D1

RS

0.5Ω

PGND

LT3954

ISP

ISN

PWMOUT

FB

INTVCC

GND

COUT

1k

2k

1M

Q1

M1

0.15nF

24.9k

INTVCC

1µF

Q3

27k

2.2k

20k

Q2

1k

1N4148

D2

3954 F10

Figure 10. Protection Circuit for Fault to Ground on LED Load. Includes Fast Level Shift for PWM Switch M1

For more information www.linear.com/3954

3954fa

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