LT3796 데이터 시트보기 (PDF) - Linear Technology

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LT3796

100V Constant-Current and Constant-Voltage Controller with Dual Current Sense

Linear Technology 

LT3796

APPLICATIONS INFORMATION

Inductor Selection

The inductor used with the LT3796 should have a saturation

current rating appropriate to the maximum switch current

selected with the RSENSE resistor. Choose an inductor value

based on operating frequency, input and output voltage to

provide a current mode signal on SENSE of approximately

20mV magnitude. The following equations are useful to

estimate the inductor value (TSW = 1/fOSC):

LBUCK

=

TSW

•

RSENSE • VLED(VIN

VIN • 0.02V

–

VLED

)

LBUCK

,

BOOST

=

TSW •RSENSE •

(VLED +VIN)

VLED • V

• 0.02V

IN

LBOOST

=

TSW

•

RSENSE • VIN (VLED

VLED • 0.02V

–

VIN)

Table 6 provides some recommended inductor vendors.

Table 6. Inductor Manufacturers

VENDOR

WEB

Sumida

www.sumida.com

Würth Elektronik

www.we-online.com

Coiltronics

www.cooperet.com

Vishay

www.vishay.com

Coilcraft

www.coilcraft.com

Loop Compensation

The LT3796 uses an internal transconductance error

amplifier whose VC output compensates the control loop.

The external inductor, output capacitor and the compen-

sation resistor and capacitor determine the loop stability.

The inductor and output capacitor are chosen based on

performance, size and cost. The compensation resistor

and capacitor at VC are selected to optimize control loop

response and stability. For typical LED applications, a 22nF

compensation capacitor at VC is adequate, and a series

resistor should always be used to increase the slew rate

on the VC pin to maintain tighter regulation of LED current

during fast transients on the input supply to the converter.

Soft-Start Capacitor Selection

For many applications, it is important to minimize the

inrush current at start-up. The built-in soft-start circuit

significantly reduces the start-up current spike and output

voltage overshoot. The soft-start interval is set by the

soft-start capacitor selection according to the equation:

TSS

= CSS •

2V

28µA

A typical value for the soft-start capacitor is 0.1µF. The

soft-start pin reduces the oscillator frequency and the

maximum current in the switch. Soft-start also operates

as fault protection, which forces the converter into hiccup

or latchoff mode. Detailed information is provided in the

Fault Protection: Hiccup Mode and Latchoff Mode section.

Fault Protection: Hiccup Mode and Latchoff Mode

If an LED overcurrent condition, INTVCC undervoltage, or

thermal limit happens, an open-drain pull-down on FAULT

asserts. The TG pin is pulled high to disconnect the LED

array from the power path, and the GATE pin is driven low.

If the soft-start pin is charging and still below 1.7V, then

it will continue to do so with a 28µA source. Once above

1.7V, the pull-up source is disabled and a 2.8µA pull-down

is activated. While the SS pin is discharging, the GATE is

forced low. When SS pin is discharged below 0.2V, a new

cycle is initiated. This is referred as hiccup mode operation.

If the fault still exists when SS crosses below 0.2V, then

a full SS charge/discharge cycle has to complete before

switching is enabled and the FAULT flag is deasserted.

If a resistor is placed between VREF pin and SS pin to hold

SS pin higher than 0.2V during a fault, then the LT3796

will enter latchoff mode with GATE pin low, TG pin high

and FAULT pin low. To exit latchoff mode, the EN/UVLO

pin must be toggled low to high.

3796f

21

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