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| 型号： | LTC3853IUJ#TR |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 0.15 A DUAL SWITCHING CONTROLLER, 830 kHz SWITCHING FREQ-MAX, PQCC40 |
| 封装： | 6 X 6 MM, PLASTIC, QFN-40 |
| 文件页数： | 6/36页 |
| 文件大小： | 1780K |
| 代理商： | LTC3853IUJ#TR |
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LTC3853
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3853fa
APPLICATIONS INFORMATION
Figure 2. Two Different Methods of Sensing Current
(2a) Using a Resistor to Sense Current
(2b) Using the Inductor DCR to Sense Current
VIN
INTVCC
BOOST
TG
SW
BG
PGND
FILTER COMPONENTS
PLACED NEAR SENSE PINS
SENSE+
SENSE–
SGND
LTC3853
RS
RF
CF
ESL
L1
3853 F02a
RSENSE
VIN
INTVCC
BOOST
TG
SW
BG
PGND
*PLACE C1 NEAR SENSE+,
SENSE– PINS
INDUCTOR
DCR
L
SENSE+
SENSE–
SGND
LTC3853
VOUT
3853 F02b
R1
R2
C1*
R1||R2 C1 =
L
DCR
RSENSE(EQ) = DCR
R2
R1 + R2
the information at the sense terminals and making the
programmed current limit unpredictable. If DCR sensing
is used (Figure 2b), sense resistor R1 should be placed
closetotheswitchingnode,topreventnoisefromcoupling
into sensitive small-signal nodes. The capacitor C1 should
be placed close to the IC pins.
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 current
comparatorthresholdsetsthepeakoftheinductorcurrent,
yielding a maximum average output current IMAX equal to
the peak value less half the peak-to-peak ripple current,
DIL.Tocalculatethesenseresistorvalue,usetheequation:
RSENSE =
VSENSE(MAX)
I(MAX) +
DIL
2
Because of possible PCB noise in the current sensing loop,
the AC current sensing ripple of
DVSENSE = DIL RSENSE
also needs to be checked in the design to get a good
signal-to-noise ratio. In general, for a reasonably good
PCB layout, a 15mV
DVSENSE voltage is recommended
as a conservative number to start with, either for RSENSE
or DCR sensing applications.
For previous generation current mode controllers, the
maximum sense voltage was high enough (e.g., 75mV for
theLTC1628/LTC3728family)thatthevoltagedropacross
the parasitic inductance of the sense resistor represented
a relatively small error. For today’s highest current density
solutions, however, the value of the sense resistor can be
less than 1m and the peak sense voltage can be as low
as 20mV. In addition, inductor ripple currents greater than
50% with operation up to 1MHz are becoming more com-
mon. Under these conditions the voltage drop across the
senseresistor’sparasiticinductanceisnolongernegligible.
A typical sensing circuit using a discrete resistor is shown
inFigure2a.Inpreviousgenerationsofcontrollers,asmall
RC filter placed near the IC was commonly used to reduce
the effects of capacitive and inductive noise coupled in
the sense traces on the PCB. A typical filter consists of
two series 10 resistors connected to a parallel 1000pF
capacitor, resulting in a time constant of 20ns.
This same RC filter, with minor modifications, can be used
to extract the resistive component of the current sense
signalinthepresenceofparasiticinductance.Forexample,
Figure 3 illustrates the voltage waveform across a 2m
sense resistor with a 2010 footprint for the 1.2V/15A
converter operating at 100% load. The waveform is the
superposition of a purely resistive component and a
purely inductive component. It was measured using two
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