LT3932 datasheet(21/28 Pages) AD

21 / 28 page

LT8386

Rev. 0

For more information www.analog.com

This network forms part of a voltage regulation loop when

FB is near 1.2V. In this case, the LT8386 will override the

programmed LED current and adjust the inductor current

to lower the output voltage and limit FB to 1.2V. This

resistor configuration therefore determines the maximum

output voltage.

In this way, the LT8386 can also be configured as a volt-

age regulator instead of an LED driver. It will regulate the

output voltage near the programmed maximum as long

as the load current is less than the current programmed

by CTRL.

Note that this voltage limit may be reached inadvertently

if it is set too close to the typical output voltage and the

output capacitor is too small. To avoid interference with

the current regulation, the feedback resistors should be

chosen such that FB is below 1.14V when the LEDs are

conducting.

Understanding FB Overvoltage Lockout

Despite the voltage regulation loop, the FB voltage can

temporarily exceed the 1.2V 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 induc-

tor current and avoid overcharging the output. To quickly

respond to the overvoltage conditions, the LT8386 will

immediately stop switching and disconnect the LED

string by shutting the external PMOS off when the FB

pin exceeds the 1.266V FB overvoltage lockout threshold.

The FB overvoltage lockout threshold may be routinely

exceeded when the LT8386 is being operated as a volt-

age regulator if 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 RFB1 and

RFB2 values to ensure the output voltage is not greater

than 60V when the FB voltage is 1.266V.

APPLICATIONS INFORMATION

Figure 10. FAULT Resistor Configuration

LT8386

INTVCC

8386 F10

FAULT

RFAULT

Open LED Fault Detection and Response

The resistor network formed by RFB1 and RFB2 also defines

the criteria for the open-LED fault condition. An open-LED

fault is detected when the FB pin voltage is greater than

1.14V and simultaneously the difference between ISP and

ISN pins is less than 10mV. 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.

A fault is reported by an internal device pulling the voltage

at the FAULT pin low. There is nothing internal that pulls

this voltage high, so an external resistor between INTVCC

and FAULT is necessary as shown in Figure 10. This con-

figuration allows multiple FAULT pins and similar pins on

other parts to be connected and share a single resistor.

Shorted LED Fault Detection and Responses

The LT8386 prevents excessive currents that could dam-

age the LED and the driver by two detection schemes as

follows:

1) (VISP – VISN) > 150mV for more than 300µs, or

2) (VISP – VISN) > 700mV (typical)

If the LT8386 detects any one of these events, it imme-

diately stops switching, turns off the external PMOS

PWM switch, pulls down FAULT pin, and initiates a fault

response routine using the SS pin. Note that FAULT pin is

held low until the part successfully restarts.

LT3932 Datasheet(PDF) 21 Page - Analog Devices

LT3932 Datasheet(HTML) 21 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Datasheet Download

Link URL