LT3955

21

3955fb

For more information www.linear.com/LT3955

applicaTions inFormaTion

1µF

0.15nF

LT3955

GND

2.2k

1N4148

3955 F10

0.5

2k

1k

FB

ISP

ISN

PGND

PWMOUT

27k

D2

20k

1k

Q2

Q3

1M

24.9k

D1

Q1

M1

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

Short-Circuit Protection for a Boosted Output

The LT3955 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

overvoltageresponse. Theprimarymodeofactionforboth

featuresistodrivethePWMOUTpinlow,whichturnsoffthe

switchconnectingtheoutputtotheload.TheISP/ISNshort-

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

intervaldeterminedbythePWMcapacitor, thenturningoff

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