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

longer varies with CTRL. The typical V(ISP-ISN) threshold

vs CTRL is listed in the Table 1.

Table 1. V(ISP-ISN) Threshold vs CTRL

VCTRL (V)

1

1.05

1.1

1.15

1.2

V(ISP-ISN) (mV)

225

236

244.5

248.5

250

When CTRL is higher than 1.2V, the LED current is regu-

lated to:

have this extra comparator. The output voltage can be set

by selecting the values of R3 and R4 (see Figure 2) ac-

cording to the following equation:

VOUT

=

1.25

•

R3+R4

R4

VOUT

LT3796

R3

FB1/FB2

R4

3796 F02

ILED

=

250mV

RLED

The CTRL pin should not be left open (tie to VREF if not

used). The CTRL pin can also be used in conjunction with

a thermistor to provide overtemperature protection for the

LED load, or with a resistor divider to VIN to reduce output

power and switching current when VIN is low. The presence

of a time varying differential voltage signal (ripple) across

ISP and ISN at the switching frequency is expected. The

amplitude of this signal is increased by high LED load

current, low switching frequency and/or a smaller value

output filter capacitor.

Figure 2. Feedback Resistor Connections for Boost and SEPIC

Applications

For a boost type LED driver, set the resistor from the output

to the FB1 pin such that the expected VFB1 during normal

operation does not exceed 1.15V. For an LED driver of buck

mode or a buck-boost mode configuration, the FB voltage

is typically level shifted to a signal with respect to GND as

illustrated in Figure 3. The output can be expressed as:

VOUT

=

1.25

•

R5

R8

•

R6 + R7

R6

Programming Output Voltage (Constant-Voltage

Regulation) or Open LED/Overvoltage Threshold

The LT3796 has two voltage feedback pins, FB1 and FB2.

Either one can be used for a boost or SEPIC application. The

difference between these two pins is FB1 has a compara-

tor that senses when FB1 exceeds VFB – 60mV (VMODE

threshold) and asserts the VMODE output if V(ISP-ISN) is

less than 25mV. This indicates that the output is in voltage

regulation mode and not current regulation. FB2 does not

VS

LT3796

CSP

CSN

CSOUT

FB1

3796 F03

+

R5 VOUT

–

R6

RLED

LED

STRING

R7

R8

Figure 3. Feedback Resistor Connection for Buck Mode or

Buck-Boost Mode LED Driver

3796f

15

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