LT3795
applicaTions inForMaTion
V
When the lower CTRL pin voltage is between 1V and 1.±V,
the LED current varies with CTRL, but departs from the
previous equation by an increasing amount as the CTRL
voltage increases. Ultimately above 1.±V, the LED current
OUT
R5
LT3795
FB
R6
nolongervarieswithCTRL.ThetypicalV
vs CTRL is listed in Table 1.
threshold
(ISP-ISN)
3795 F03
Figure 3. Feedback Resistor Connections for Boost and SEPIC
Applications
Table 1. V(ISP-ISN) Threshold vs CTRL
V
(V)
V
(mV)
CTRL
(ISP-ISN)
1
±±5
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 4. The
output can be expressed as:
1.05
1.1
±36
±44.5
±48.5
±50
1.15
1.±
R7
R8
When both the CTRL pins are higher than 1.±V, the LED
current is regulated to:
VOUT =1.25 •
+ VBE(Q1)
250mV
RLED
+
ILED
=
R7
R
SENSE
V
OUT
LED
ARRAY
The CTRL pins should not be left open (tie to V
if not
LT3795
REF
–
Q1
used). Either CTRL pin can also be used in conjunction
with a thermistor to provide overtemperature protection
FB
for the LED load, or with a resistor divider to V to reduce
R8
3795 F04
IN
output power and switching current when V is low. The
IN
presenceofatimevaryingdifferentialvoltagesignal(ripple)
acrossISPandISNattheswitchingfrequencyisexpected.
The amplitude of this signal is increased by high LED load
current, low switching frequency and/or a smaller value
output filter capacitor. For best accuracy, the amplitude
of this ripple should be less than ±5mV.
Figure 4. Feedback Resistor Connection for Buck Mode or
Buck-Boost Mode LED Driver
If the open LED clamp voltage is programmed correctly
using the resistor divider, then the FB pin should never
exceed 1.±V when LEDs are connected.
To detect both open-circuit and short-circuit conditions
at the output, the LT3795 monitors both output voltage
Programming Output Voltage (Constant-Voltage
Regulation) and Output Voltage Open LED and
Shorted LED Thresholds
and current. When FB exceeds V - 5±mV, OPENLED is
FB
asserted if V
is less than ±5mV. OPENLED is de-
(ISP-ISN)
asserted when V
is higher than 70mV (typical) or
The LT3795 has a voltage feedback pin FB that can be
used to program a constant-voltage output. In addition,
FB programming determines the output voltage that will
causeOPENLEDandSHORTLEDtoassert.ForaboostLED
driver, the output voltage can be programmed by selecting
the values of R5 and R6 (see Figure 3) according to the
following equation:
(ISP-ISN)
FB drops below V - 6±mV (typical).
FB
The SHORTLED pin is asserted if V
> 375mV or
(ISP-ISN)
the FB pin falls below 300mV (typical) after initial start-up
and SS reaches 1.7V. The ratio between the FB OPENLED
threshold of 1.±V and the SHORTLED threshold of 0.3V
can limit the range of V . The range of V
using the
OUT
OUT
maximum SHORTLED threshold of 0.35V is 3.5:1. The
R5+R6
VOUT =1.25•
range of V
can be made wider using the circuits shown
OUT
R6
3795f
14
For more information www.linear.com/LT3795
LINEAR_DIMENSIONS [ Linear Dimensions ]
