LT3757 datasheet(18/22 Pages) AD

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LT3935

Rev. 0

For more information www.analog.com

Figure 12. Latch-Off Response to a Fault

FAULT DETECTED

CONTINUOUS

RSS = 1MEG

LATCH–OFF

RSS = 2MEG

10ms/DIV

1V/DIV

3935 F12

APPLICATIONS INFORMATION

The charging rate of the soft-start capacitor is much faster

than the discharging rate, so while the fault persists, the

LT3935 will only attempt switching for a relatively short

part of the cycle before being interrupted. Although the

LT3935cansafelyendureashort-circuitwhilecontinuously

switching, this hiccup action saves power. The frequency

of the hiccups is inversely proportional to CSS and 100nF

yields about 8Hz.

The operating point of a voltage regulator supplying light

loads will frequently satisfy the criteria for an open-circuit,

andthehiccupbehaviorwouldthereforebeverydisruptive.

So when the LT3935 is configured as a voltage regulator,

a resistor RSS should be connected between INTVCC and

SS as shown in Figure 10.

The current that pulls down the SS pin during a fault is

so weak that if RSS is 1MΩ, the voltage at the SS pin

will never reach 1.7V. Therefore, the LT3935 will not

stop switching or start to hiccup. With this resistor, the

LT3935 will continue switching and rely on overvoltage

and overcurrent protection to guarantee safe operation in

the event of open-circuits and short-circuits.

If the resistor is changed to 2MΩ, then the SS pin may be

discharged to less than 1.7V, but not less than 200mV as

shown in Figure 12. The LT3935 will consequently cease

switching permanently until being reset by the EN/UVLO

pin or by powering off. Some applications may demand

this behavior so that short and open-circuits can be in-

vestigated manually before resuming normal operation.

Programming the EN/UVLO Threshold

An external voltage source can be used to set the voltage

at the EN/UVLO pin to enable or disable the LT3935. The

LT3935 will stop switching, and reset the SS pin when

the voltage at EN/UVLO drops below 1.16V, but internal

circuitry will continue drawing current. Full shutdown is

guaranteed when EN/UVLO is below 300mV, and in full

shutdown the LT3935 will draw less than 16μA. For ap-

plications in which the level of the source driving EN/UVLO

changes slowly, 20mV of hysteresis has been added to

the 1.16V enable threshold.

Alternatively, a resistor network can be placed between

VIN and EN/UVLO as shown in Figure 13. In this case,

EN/UVLO automatically falls below 1.16V and disables

switching when VIN falls below a certain level, called the

Undervoltage Lockout (UVLO) threshold, which is defined

by resistors REN1 and REN2. Additionally, a 4μA current

is designed to flow into EN/UVLO when the pin voltage

is below the threshold. This current provides additional

hysteresis. To define the hysteresis (VHYST) and the UVLO

threshold (VUVLO) select REN1 and REN2 according to the

following equations:

REN2 =

VHYST

4µA

–

VUVLO

480µA

REN1 =

1.16 •REN2

VUVLO – 1.16

Forexample,toprograma10Vthresholdwith1Vofhyster-

esis, use 226k and 29.4k for REN2 and REN1, respectively.

This latch-off behavior is the third of three ways that the

LT3935 can be programmed to respond to faults—the

other two being continuous operation and the default

hiccup behavior.

Figure 13. EN/UVLO Resistor Configuration

LT3935

3935 F13

EN/UVLO

VIN

REN2

REN1

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