Datasheet

LT3845
17
3845fd
The maximum ESR required to meet a ΔV
OUT
design
requirement can be calculated by:
ESR(MAX)=
(
V
OUT
)(L)(f
SW
)
V
OUT
•1–
V
OUT
V
IN(MAX)
Worst-case ΔV
OUT
occurs at highest input voltage. Use
paralleled multiple capacitors to meet the ESR require-
ments. Increasing the inductance is an option to lower the
ESR requirements. For extremely low ΔV
OUT
, an additional
LC fi lter stage can be added to the output of the supply.
Application Note 44 has some good tips on sizing an ad-
ditional output fi lter.
Output Voltage Programming
A resistive divider sets the DC output voltage according
to the following formula:
R2 = R1
V
OUT
1.231V
–1
The external resistor divider is connected to the output
of the converter as shown in Figure 3. Tolerance of the
feedback resistors will add additional error to the output
voltage.
Example: V
OUT
= 12V; R1 = 10kΩ
R2 = 10k
12V
1.231V
1 = 87.48k use 86.6k 1%
The V
FB
pin input bias current is typically 25nA, so use
of extremely high value feedback resistors could cause a
converter output that is slightly higher than expected. Bias
current error at the output can be estimated as:
ΔV
OUT(BIAS)
= 25nA • R2
Supply UVLO and Shutdown
The SHDN pin has a precision voltage threshold with
hysteresis which can be used as an undervoltage lockout
threshold (UVLO) for the power supply. Undervoltage
lockout keeps the LT3845 in shutdown until the supply
input voltage is above a certain voltage programmed by
the user. The hysteresis voltage prevents noise from falsely
tripping UVLO.
Resistors are chosen by fi rst selecting R
B
. Then
R
A
= R
B
•
V
SUPPLY(ON)
1.35V
–1
V
SUPPLY(ON)
is the input voltage at which the undervoltage
lockout is disabled and the supply turns on.
Example: Select R
B
= 49.9kΩ, V
SUPPLY(ON)
= 14.5V (based
on a 15V minimum input voltage)
R
A
= 49.9k •
14.5V
1.35V
–1
= 486.1k (499k resistor is selected)
APPLICATIONS INFORMATION
L1
V
FB
PIN
R2
R1
V
OUT
C
OUT
3845 F03
SHDN PIN
R
A
R
B
V
SUPPLY
3845 F04
Figure 3. Output Voltage Feedback Divider Figure 4. Undervoltage Feedback Divider