LT3795
applicaTions inForMaTion
1.7
in Figure 5 and Figure 6. For a V
range that is greater
OUT
R11=1000
=12.64,
than 8:1, consult factory applications.
(1.65)91.4–(0.8)18.3–1.7
UseR11=12.7kΩ
V
OUT
1.7
R10
R12=1000
=169kΩ
LT3795
R12
(0.35)91.4–(1.2)18.3
V
FB
REF
R11
The resistor values for R14 and R15 in Figure 6 can be
calculated as shown below. See the example that follows
for a suggested R13 value.
3795 F05
Figure 5. Feedback Resistor Connection for Wide Range
Output in Boost and SEPIC Applications
1.7
R14=R13•
1.65•VHOUT –0.8•VLOUT –0.85•VBE(Q1)
+
R13
R
SENSE
1.7
V
OUT
LED
ARRAY
R15=R13•
0.35•VHOUT –1.2•VLOUT +0.85•VBE(Q1)
–
Q1
LT3795
R15
V
FB
REF
Example: Calculate the resistor values required to in-
R14
crease the V
range of a buck-boost mode LED driver
3795 F06
OUT
to 7.5:1 and have OPENLED occur when V
Use V (Q1) = 0.7V:
is 43.5V.
OUT
Figure 6. Feedback Resistor Connection for Wide Range
Output in Buck Mode and Buck-Boost Mode Applications
BE
Step 1: Choose R13 = 357k
L
Step ±: V
Step 3:
= 43.5/7.5 = 5.8
OUT
TheequationstowidentherangeofV
a SHORTLED threshold of 0.35V, an OPENLED threshold
of 1.±V and a reference voltage V of ±V. The resistor
values for R11 and R1± in Figure 5 can be calculated as
shown below. See the example that follows for a sug-
gested R10 value.
arederivedusing
OUT
REF
1.7
(1.65) 43.5–(0.8)5.8–(0.85)0.7
R14= 357
= 9.12,
= 68.5,
UseR14= 9.09kΩ
1.7
1.7
R15= 357
R11=R10•
(0.35) 43.5–(1.2)5.8+(0.85)0.7
1.65•VHOUT –0.8•VLOUT –1.7
UseR15= 68.1kΩ
1.7
R12=R10•
0.35•VHOUT –1.2•VL
LED Overcurrent Protection Feature
OUT
TheISPandISNpinshaveashort-circuitprotectionfeature
independent of the LEDcurrent sense feature. This feature
preventsthedevelopmentofexcessiveswitchingcurrents
and protects the power components. The short-circuit
protection threshold (375mV, typ) is designed to be 502
higher than the default LED current sense threshold. Once
the LED overcurrent is detected, the GATE pin is driven to
GND to stop switching, TG pin is pulled high to disconnect
the LED array from the power path, and fault protection
is initiated via the SS pin.
Example:Calculatetheresistorvaluesrequiredtoincrease
the V
range of a boost LED driver to 5:1 and have
OUT
OPENLED occur when V
is 91.4V:
OUT
Step 1: Choose R10 = 1M
L
Step ±: V
Step 3:
= 91.4/5 = 18.3
OUT
3795f
15
For more information www.linear.com/LT3795
LINEAR_DIMENSIONS [ Linear Dimensions ]
