LT3757 datasheet(17/22 Pages) AD

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LT3935

Rev. 0

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APPLICATIONS INFORMATION

Understanding FB Overvoltage Lockout

It is possible that the FB voltage can exceed the 1V limit.

If the output voltage is near the maximum when the LED

string opens, it may take too long for the feedback loop

to adjust the inductor current and avoid overcharging the

output. However, if the FB voltage exceeds the 1.05V Over-

voltage Lockout Threshold, the LT3935 will immediately

stop switching and resume only when FB decreases to 1V.

This threshold may be routinely exceeded when the

LT3935 is being operated as a voltage regulator and the

load current decreases rapidly. In this case, the pause in

switching limits the output overshoot and ensures that the

voltage is back in regulation as quickly as possible. For

safe operation, choose RFB2 and RFB1 values to ensure

the output voltage is not greater than VIN when the FB

voltage is 1.05V.

Open and Shorted LED Fault Detection and Response

The resistor network formed by RFB1andRFB2alsodefines

the criteria for two fault conditions with respect to the LED

string: short and open-circuits. For the LT3935, a short-

circuit is when FB is less than 200mV. An open-circuit

is when FB is greater than 950mV and simultaneously

the difference between ISP and ISN is less than 10mV

(the C/10 threshold). The latter condition ensures that the

output current is low (as it should be in an open-circuit)

not just that output voltage is high as it may be when the

LEDs are conducting a large current.

In both cases, a fault is indicated by an internal device

pulling the voltage at the FAULT pin low. There is nothing

internal that pulls this voltage high, so an external resis-

tor between INTVCC and FAULT is necessary as shown in

Figure 9. This configuration allows multiple FAULT pins

and similar pins on other parts to be connected and share

a single resistor.

Soft-Start and Fault Modes

The SS pin has two functions. First, it allows the user to

program the output voltage startup ramp rate. An internal

20μA current pulls up the SS pin to INTVCC. Connecting

an external capacitor CSS from the SS pin to GND, as

shown in Figure 10, will generate a linear ramp voltage.

The LT3935 regulates the FB pin voltage to track the SS

pin voltage until VOUT is high enough to drive the LED at

the commanded current level.

The SS pin is also used as a fault timer. After a fault is

detected, an internal 1.25μA current sink will begin to

discharge the soft-start capacitor and lower the voltage

at the SS pin. When the voltage falls from 3.3V to 1.7V,

all switching will cease, but the SS pin will continue to

discharge. Switching will not resume until SS reaches

200mV. At this point, the 20μA current will recharge the

soft-start capacitor, and the LT3935 will try to switch

again. If the fault persists when SS returns to 1.7V, the

process will repeat as shown in Figure 11.

Figure 9. FAULT Resistor Configuration

LT3935

3935 F09

FAULTB

INTVCC

RFAULT

Figure 10. SS Capacitor and Resistor Configuration

LT3935

3935 F10

INTVCC

RSS

CSS

Figure 11. Hiccup Response to Fault

FAULT DETECTED

FAULT CLEARED

10ms/DIV

1V/DIV

3935 F11

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