ANSELA datasheet(17/222 Pages) MICROCHIP

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DS22331A-page 17

MCP19111

The value of RS and CS 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 CS 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 RS and CS is very important. The +ISEN

and -ISEN traces should be routed parallel to each other

with minimum spacing. This Kelvin sense routing

technique helps minimize noise sensitivity. The filter

capacitor (CS), 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 CS to the inductor should occur directly at

the inductor and not at any other +VSEN trace. The filter

resistor (RS), should be placed close to the inductor.

See Figure 3-3 for component placement. Care should

also be taken to avoid routing the +ISEN and -ISEN

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

------





Where:

- L is the inductance value of the output

inductor

-RL is the series resistance of the output

inductor

-RS is the current sense filter resistor

-CS is the current sense filter capacitor

+ISEN

-ISEN

INDUCTOR

Load

PHASE

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