參數(shù)資料
型號(hào)： LTC3869IUFD#TRPBF
廠商： LINEAR TECHNOLOGY CORP
元件分類： 穩(wěn)壓器
英文描述： SWITCHING CONTROLLER, PQCC28
封裝： 5 X 4 MM, LEAD FREE, PLASTIC, MO-220WXXX-X, QFN-28
文件頁(yè)數(shù)： 6/40頁(yè)
文件大小： 2705K
代理商： LTC3869IUFD#TRPBF
LTC3869/LTC3869-2
14
3869f
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
common. Under these conditions the voltage drop across
the sense resistor’s parasitic inductance is no longer neg-
ligible. A typical sensing circuit using a discrete resistor is
shown in Figure 2a. In previous generations of controllers,
a small RC filter placed near the IC was commonly used to
reducetheeffectsofcapacitiveandinductivenoisecoupled
inthe 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
scope probes and waveform math to obtain a differential
measurement. Based on additional measurements of the
inductor ripple current and the on-time and off-time of
the top switch, the value of the parasitic inductance was
determined to be 0.5nH using the equation:
ESL =
VESL(STEP)
ΔIL
tON tOFF
tON + tOFF
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
LTC3869
VOUT
3869 F02a
CF 2RF ≤ ESL/RS
POLE-ZERO
CANCELLATION
SENSE RESISTOR
PLUS PARASITIC
INDUCTANCE
RS
ESL
CF
RF
VIN
INTVCC
BOOST
TG
SW
BG
PGND
*PLACE C1 NEAR SENSE+,
SENSE– PINS
**PLACE R1 NEXT TO
INDUCTOR
DCR
L
SENSE+
SENSE–
SGND
LTC3869
VOUT
3869 F02b
R1**
R2
C1*
R1||R2 × C1 =
L
DCR
RSENSE(EQ) = DCR
R2
R1 + R2
相關(guān)PDF資料
PDF描述
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