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LT3797

3797f

For more information www.linear.com/LT3797

Figure 6. A Typical Inductor Waveform

APPLICATIONS INFORMATION

The LT3797 fault protection can be configured as the latch-

off mode by connecting a 470k resistor between the SS

pin and VREF pin. The FAULT2 conditions (see Figure 4)

cause the LT3797 latch off. The LT3797 does not retry a

soft-start even if the fault condition is cleared, since the SS

pin is not able to fall below 0.2V by the 2.5µA pull-down

current to reset the latch, due to the pulling up of the 470k

resistor. The latch-off can only be cleared by toggling the

EN/UVLO pin low to high.

The open-LED fault and the output overvoltage fault are

not included in FAULT2 in Figure 5. These two faults do

not affect the soft-start status. The open-LED fault in

channel 1 causes FLT1 low. The output overvoltage fault

in channel 1 causes FLT1 low and TG1 high to disconnect

the LED load from power path.

APPLICATION CIRCUIT DESIGN GUIDELINE

The LT3797 contains three independent switching regu-

lators. The following sections describe the LT3797 LED

driver design guideline for the key parameters calculation

and external components selection. The design guideline

applies to each of the switching regulators.

Switch Duty Cycle

The LT3797 can be configured with different topologies.

The boost LED driver is used for the applications where the

LED voltage is higher than the input voltage. The LT3797

can be configured as a buck mode LED driver for the ap-

plications where the LED voltage is lower than the input

voltage. The buck-boost mode and the SEPIC LED driver

allow for the input voltage to be higher, equal to or lower

than the LED voltage. The switch duty cycles of different

topologies in continuous conduction mode (CCM) are:

DBOOST =

VLED – VIN

VLED

DBUCK =

VLED

VIN

DBUCK-BOOST =

VLED

VLED + VIN

DSEPIC =

VLED

VLED + VIN

Themaximumdutycycle(DMAX)occurswhentheconverter

has the minimum input voltage (VIN(MIN)).

Inductor Selection

Figure 6 shows a typical inductor current waveform

when the LED driver has maximum output current at the

minimum input voltage.

∆IL and IL_AVG(MAX) denote the

inductorripplecurrentandthemaximumaverageinductor

current respectively.

∆IL

3739 F06

IL_AVG(MAX)

IL(PEAK)

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