LTC3829_15 Datasheet, PDF (15/40 Pages) Linear Technology – 3-Phase, Single Output Synchronous Step-Down DC/DC Controller with Diffamp

APPLICATIONS INFORMATION
V_IN
INTV_CC
BOOST
TG
SW
LTC3829
BG
PGND
SENSE⁺
SENSE^–
SGND
R_F
C_F
R_F
FILTER COMPONENTS
PLACED NEAR SENSE PINS
V_IN
SENSE RESISTOR
PLUS PARASITIC
INDUCTANCE
R_S ESL
C_F • 2 • R_F ≤ ESL/R_S
POLE-ZERO
CANCELLATION
V_OUT
3829 F02a
(2a) Using a Resistor to Sense Current
LTC3829
OPTIONAL
TEMP COMP
NETWORK
R_S
R_NTC RP
V_IN
INTV_CC
BOOST
TG
SW
LTC3829
ITEMP
BG
PGND
SENSE⁺
SENSE^–
SGND
R1**
C1* R2
V_IN
INDUCTOR
L DCR
V_OUT
*PLACE C1 NEAR SENSE⁺, R1||R2 × C1 =
SENSE^– PINS
L
DCR
R_SENSE(EQ) = DCR
R2
R1 + R2
**PLACE R1 NEXT TO INDUCTOR
(2b) Using the Inductor DCR to Sense Current
3829 F02b
Figure 2. Two Different Methods of Sensing Current
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 negligible. A typical sens-
ing 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 reduce the ef-
fects of capacitive and inductive noise coupled in the sense
traces on the PCB. A typical filter consists of two series
10Ω 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 resis-
tive component of the current sense signal in the presence
of parasitic inductance. For example, Figure 3 illustrates
the voltage waveform across a 2mΩ 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
3829fc
For more information www.linear.com/LTC3829
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