參數(shù)資料
型號(hào)： LT3845EFE#TR
廠商： LINEAR TECHNOLOGY CORP
元件分類(lèi)： 穩(wěn)壓器
英文描述： SWITCHING CONTROLLER, 600 kHz SWITCHING FREQ-MAX, PDSO16
封裝： 4.40 MM, PLASTIC, TSSOP-16
文件頁(yè)數(shù)： 8/24頁(yè)
文件大?。?/td> 263K
代理商： LT3845EFE#TR
LT3845
16
3845fb
The maximum ESR required to meet a
ΔVOUT design
requirement can be calculated by:
ESR MAX
VL f
V
OUT
SW
OUT
IN MAX
()
()( )(
)
–
()
=
Δ
1
Worst-case
ΔVOUT 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
ΔVOUT,anadditional
LC lter stage can be added to the output of the supply.
Application Note 44 has some good tips on sizing an ad-
ditional output lter.
Output Voltage Programming
A resistive divider sets the DC output voltage according
to the following formula:
RR
V
OUT
21
1 231
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: VOUT = 12V; R1 = 10kΩ
Rk
V
kuse
k
210
12
1 231
1
8748
866
1
=Ω
=Ω
Ω
.
..
%
The VFB 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:
ΔVOUT(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 rst selecting RB. Then
RR
V
AB
SUPPLY ON
=
.
–
()
135
1
VSUPPLY(ON) is the input voltage at which the undervoltage
lockout is disabled and the supply turns on.
Example: Select RB = 49.9kΩ, VSUPPLY(ON) = 14.5V (based
on a 15V minimum input voltage)
Rk
V
A =Ω
=
49 9
14 5
135
1
.
–
486.1k
(499k
ΩΩ
Ω resistor is selected)
APPLICATIONS INFORMATION
L1
VFB PIN
R2
R1
VOUT
COUT
3845 F03
SHDN PIN
RA
RB
VSUPPLY
3845 F04
Figure 3. Output Voltage Feedback Divider
Figure 4. Undervoltage Feedback Divider
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