Datasheet

LT3477
9
3477fc
APPLICATIONS INFORMATION
Setting Positive Output Voltages
To set a positive output voltage, select the values of R1 and
R2 (see Figure 2) according to the following equation:
V
OUT
=1.235V 1+
R1
R2






For designs needing an adjustable current level, the
I
ADJ1
and I
ADJ2
pins are provided for the fi rst and the
second current sense amplifi ers, respectively. With the
I
ADJ1
and I
ADJ2
pins tied higher than 650mV, the nominal
current sense voltage is 100mV (appearing between the
I
SP1
and I
SN2
or I
SP2
and I
SN2
pins). Applying a positive
DC voltage less than 600mV to the I
ADJ1
and I
ADJ2
pins
will decrease the current sense voltage according to the
following formula:
I
mV
R
V
mV
SENSE
SENSE
IADJ
=
100
618
•
For example, if 309mV is applied to the I
ADJ1
pin and
R
SENSE
is 0.5Ω, the current sense will be reduced from
200mA to 100mA. The adjustability allows the regulated
current to be reduced without changing the current sense
resistor (e.g., to adjust brightness in an LED driver or to
reduce the charge current in a battery charger).
Considerations When Sensing Input Current
In addition to regulating the DC output current for cur-
rent-source applications, the constant-current loop of
the LT3477 can also be used to provide an accurate input
current limit. Boost converters cannot provide output
short-circuit protection, but the surge turn-on current can
be drastically reduced using the LT3477 current sense at
the input. SEPICs, however, have an output that is DC-
isolated from the input, so an input current limit not only
helps soft-start the output but also provides excellent
short-circuit protection.
When sensing input current, the sense resistor should be
placed in front of the inductor (between the decoupling
capacitor and the inductor). This will regulate the average
inductor current and maintain a consistent inductor ripple
current, which will, in turn, maintain a well regulated input
current. Do not place the sense resistor between the input
source and the input decoupling capacitor, as this may allow
the inductor ripple current to vary widely (even though the
average input current and the average inductor current will
still be regulated). Since the inductor current is a triangular
waveform (not a DC waveform like the output current)
Figure 2. Positive Output Voltage Feedback Connections
R4
R3
3477 F03
FBP
V
REF
–V
OUT
LT3477
FBN
Figure 3. Negative Output Voltage Feedback Connections
R2
R1
3477 F02
FBN
V
REF
V
OUT
LT3477
FBP
Setting Negative Output Voltages
To s e t a n e g a t i v e o u t p u t v o l t a g e , s e l e c t t h e v a l u e s o f R 3 a n d
R4 (see Figure 3) according to the following equation:
V
OUT
= –1.235V
R3
R4






Selecting R
SENSE
/Current Sense Adjustment
Using the following formula to choose the correct current
sense resistor value (for constant current or fail-safe
operation).
R
mV
I
SENSE
SENSE
=
100