lt3845ife-trpbf Linear Technology Corporation, lt3845ife-trpbf Datasheet - Page 16

• Manufacturer Part Number: lt3845ife-trpbf
• Description: High Voltage Synchronous Current Mode Step-down Controller With Adjustable Operating Frequency
• Manufacturer: Linear Technology Corporation

LT3845
The maximum ESR required to meet a ΔV
requirement can be calculated by:
Worst-case ΔV
paralleled multiple capacitors to meet the ESR require-
ments. Increasing the inductance is an option to lower the
ESR requirements. For extremely low ΔV
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:
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
APPLICATIONS INFORMATION
16
R2 = 10kΩ
ESR(MAX) =
R2 = R1
Figure 3. Output Voltage Feedback Divider
⎛
⎝ ⎜
OUT
1.231V
⎛
⎝ ⎜
V
V
1.231V
FB
OUT
OUT
= 12V; R1 = 10kΩ
12V
PIN
V
OUT
(ΔV
occurs at highest input voltage. Use
– 1
• 1–
− 1
OUT
⎞
⎠ ⎟
⎛
⎜
⎝
L1
⎞
⎠ ⎟
)(L)(f
= 87.48kΩ − use 86.6kΩ 1%
V
IN(MAX)
V
R2
R1
3845 F03
OUT
SW
)
⎞
⎟
⎠
C
OUT
OUT
V
OUT
, an additional
OUT
design
The V
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:
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
V
lockout is disabled and the supply turns on.
Example: Select R
on a 15V minimum input voltage)
SUPPLY(ON)
ΔV
R
R
A
A
OUT(BIAS)
FB
= R
= 49.9kΩ •
= 486.1kΩ (499kΩ resistor is selected)
pin input bias current is typically 25nA, so use
B
Figure 4. Undervoltage Feedback Divider
•
is the input voltage at which the undervoltage
⎛
⎜
⎝
V
= 25nA • R2
SUPPLY(ON)
SHDN PIN
1.35V
B
⎛
⎝ ⎜
V
= 49.9kΩ, V
SUPPLY
14.5V
1.35V
– 1
– 1
⎞
⎠ ⎟
⎞
⎟
⎠
SUPPLY(ON)
R
R
3845 F04
A
B
B
. Then
= 14.5V (based
3845fc