參數(shù)資料
型號(hào)： LTC3850EUF#TR
廠商： LINEAR TECHNOLOGY CORP
元件分類： 穩(wěn)壓器
英文描述： 0.1 A DUAL SWITCHING CONTROLLER, 860 kHz SWITCHING FREQ-MAX, PQCC28
封裝： 4 X 4 MM, PLASTIC, QFN-28
文件頁(yè)數(shù)： 5/32頁(yè)
文件大?。?/td> 474K
代理商： LTC3850EUF#TR
LTC3850
13
3850f
Low Value Resistors Current Sensing
A typical sensing circuit using a discrete resistor is
shown in Figure 2a. RSENSE is chosen based on the
required output current.
The current comparator has a maximum threshold
VSENSE(MAX) determined by the ILIM setting. The input
common mode range of the current comparator is 0V to
5V. The current comparator threshold sets the peak of
the inductor current, yielding a maximum average output
current IMAX equal to the peak value less half the peak-to-
peak ripple current, ΔIL. To calculate the sense resistor
value, use the equation:
R
V
I
SENSE
SENSE MAX
MAX
L
=
+
()
2
Inductor DCR Sensing
For applications requiring the highest possible efciency
at high load currents, the LTC3850 is capable of sensing
the voltage drop across the inductor DCR, as shown in
Figure 2b. The DCR of the inductor represents the small
amountofDCwindingresistanceofthecopper,whichcanbe
less than 1mΩ for today’s low value, high current inductors.
In a high current application requiring such an inductor,
conduction loss through a sense resistor would cost sev-
eral points of efciency compared to DCR sensing.
If the external R1 || R2 C1 time constant is chosen to be
exactly equal to the L/DCR time constant, the voltage drop
across the external capacitor is equal to the drop across
the inductor DCR multiplied by R2/(R1 + R2). R2 scales the
voltage across the sense terminals for applications where
the DCR is greater than the target sense resistor value.
To properly dimension the external lter components, the
DCR of the inductor must be known. It can be measured
using a good RLC meter, but the DCR tolerance is not
always the same and varies with temperature; consult the
manufacturers’ datasheets for detailed information.
Using the inductor ripple current value from the Inductor
Value Calculation section, the target sense resistor value is:
R
V
I
SENSE EQUIV
SENSE MAX
MAX
L
()
=
+
2
To ensure that the application will deliver full load cur-
rent over the full operating temperature range, choose
the minimum value for the Maximum Current Sense
Threshold (VSENSE(MAX)) in the Electrical Characteristics
table (20mV, 40mV, or 60mV, depending on the state of
the ILIM pin).
Next, determine the DCR of the inductor. Where provided,
use the manufacturer’s maximum value, usually given
at 20°C. Increase this value to account for the tempera-
ture coefcient of resistance, which is approximately
0.4%/°C. A conservative value for TL(MAX) is 100°C.
To scale the maximum inductor DCR to the desired sense
resistor value, use the divider ratio:
R
DCR
at T
D
SENSE EQUIV
MAX
L MAX
=
()
C1 is usually selected to be in the range of 0.1F to
0.47F. This forces R1 || R2 to around 2kΩ, reducing
error that might have been caused by the SENSE pins’
±1A current.
The equivalent resistance R1 || R2 is scaled to the room
temperature inductance and maximum DCR:
RR
L
DCR at
C
12
20
1
||
()
=
°
The sense resistor values are:
R
RR
R
RR
R
D
1
12
2
1
==
||
;
The maximum power loss in R1 is related to duty cycle,
and will occur in continuous mode at the maximum input
voltage:
PR
VV
V
R
LOSS
IN MAX
OUT
1
=
()
Ensure that R1 has a power rating higher than this value.
If high efciency is necessary at light loads, consider this
power loss when deciding whether to use DCR sensing or
sense resistors. Light load power loss can be modestly
higher with a DCR network than with a sense resistor,
due to the extra switching losses incurred through R1.
APPLICATIONS INFORMATION
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