LTC2972 datasheet(100/112 Pages) LINER

100 / 112 page

LTC2972

100

2972fa

For more information www.linear.com/LTC2972

APPLICATIONS INFORMATION

Measuring Output with a Sense Resistor

A circuit for measuring current with a sense resistor is

shown in Figure 36. The balanced filter rejects both com-

mon mode and differential mode noise from the output of

the DC/DC converter. The filter is placed directly across the

sense resistor in series with the DC/DC converter’s induc-

tor. Note that the current sense inputs must be limited to

less than 6V with respect to ground. Select RCM and CCM

such that the filter’s corner frequency is < 1/10 the DC/

DC converter’s switching frequency. This will result in a

current sense waveform that offers a good compromise

between the voltage ripple and the delay through the filter.

A value 1kΩ for RCMissuggestedinordertominimizegain

errors due to the current sense inputs’ internal resistance.

Measuring Output with Inductor DCR

Figure 37 shows the circuit for applications that require

DCR current sense. A second order R-C filter is required

in these applications in order to minimize the ripple volt-

age seen at the current sense inputs. A value of 1kΩ

is suggested for RCM1 and RCM2 in order to minimize

gain errors due the current sense inputs’ internal resis-

tance. CCM1 should be selected to provide cancellation

of the zero created by the DCR and inductance, i.e.

CCM1=L/(DCR •RCM1).CCM2shouldbeselectedtoprovide

a second stage corner frequency at < 1/10 of the DC/DC

converter’sswitchingfrequency.Inaddition,CCM2needsto

be much smaller than CCM1 in order to prevent significant

loading of the filter’s first stage.

Single Phase Design Example

As a design example for a DCR current sense application,

assume L = 2.2μH, DCR = 10mΩ, and FSW = 500kHz.

Let RCM1 = 1kΩ and solve for CCM1:

CCM1 ≥

2.2µH

10mΩ• 1kΩ

= 220nF

Let RCM2 = 1kΩ. In order to get a second pole at

FSW/10 = 50kHz:

CCM2 ≅

2π • 50kHz • 1kΩ

= 3.18nF

Figure 35. Application Circuit for DC/DC Converters with Trim Pin

Figure 36. Sense Resistor Current Sensing Circuits

Figure 37. DCR Current Sensing Circuits

VPWR

VDD33

VDD25

VIN_SNS

VDAC0

VSENSEP0

VSENSEM0

VOUT_EN0

LTC2972*

0.1µF

4.5V < VIBUS < 15V

GND

* SOME DETAILS OMITTED FOR CLARITY

ONLY ONE OF TWO CHANNELS AND NO

INPUT CURRENT SENSE SHOWN

VIN

VO+

R30

2972 F35

ON/OFFB

VSENSE–

VSENSE+

TRIM

VO–

DC/DC

CONVERTER

LOAD

RCM

RSNS

2972 F36

LOAD CURRENT

CCM

LTC2972

ISENSEP

ISENSEM

2972 F37

RCM2

RCM1

DCR

CCM2

LTC2972

ISENSEP

ISENSEM

CCM1

SWX0

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