Datasheet

 2013 Microchip Technology Inc. DS22331A-page 17
MCP19111
The value of R
S
and C
S
can be found by using
Equation 3-3. When the current sense filter time
constant is set equal to the inductor time constant, the
voltage appearing across C
S
approximates the current
flowing in the inductor, multiplied by the inductor
resistance.
EQUATION 3-3: CALCULATING FILTER
VALUES
Both AC gain and DC gain can be added to the current
sense signal. Refer to Section 6.3 “Current Sense
AC Gain” and Section 6.4 “Current Sense DC Gain”
for more information.
3.7.1 PLACEMENT OF THE CURRENT
SENSE FILTER COMPONENTS
The amplitude of the current sense signal is typically
less than 100 mV peak-to-peak. Therefore, the small
signal current sense traces are very susceptible to
circuit noise. When designing the printed circuit board,
placement of R
S
and C
S
is very important. The +I
SEN
and -I
SEN
traces should be routed parallel to each other
with minimum spacing. This Kelvin sense routing
technique helps minimize noise sensitivity. The filter
capacitor (C
S
), should be placed as close to the
MCP19111 as possible. This will help filter any noise
that is injected onto the current sense lines. The trace
connecting C
S
to the inductor should occur directly at
the inductor and not at any other +V
SEN
trace. The filter
resistor (R
S
), should be placed close to the inductor.
See Figure 3-3 for component placement. Care should
also be taken to avoid routing the +I
SEN
and -I
SEN
traces near the high current switching nodes of the
HDRV, LDRV, PHASE, or BOOST traces. It is
recommended that a ground layer be placed between
these high current traces and the small signal current
sense traces.
FIGURE 3-3: CURRENT SENSE FILTER COMPONENT PLACEMENT
L
R
L
------ R
S
C
S
=
Where:
- L is the inductance value of the output
inductor
-R
L
is the series resistance of the output
inductor
-R
S
is the current sense filter resistor
-C
S
is the current sense filter capacitor
+I
SEN
-I
SEN
INDUCTOR
To
Load
To
PHASE
R
S
C
S