Datasheet

ManualsBrandsLINEAR TECHNOLOGY ManualsDev KitsPCB design board

LTC2977

2977fa

For more information www.linear.com/LTC2977

APPLICATIONS INFORMATION

LTC2977 bit b[9] = 1 in order to enable high res mode.

The V

OUT_EN

pin will assert low in this mode and cannot

be used to control a DC/DC converter. The V

DACP

output

pin is also unavailable.

Measuring Current with a Sense Resistor

A circuit for measuring current with a sense resistor is

shown in Figure 25. 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 R

and C

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 R

is suggested in order to minimize gain er-

rors due to the current sense inputs’ internal resistance.

Measuring Current with Inductor DCR

Figure

26 shows the circuit for applications that require

DCR current sense. A second order

RC filter is required

in these applications in order to minimize the ripple volt-

age seen

at the current sense inputs. A value of 1kΩ

suggested for R

CM1

and R

CM2

in order to minimize

gain errors due the current sense inputs’ internal resis-

tance. C

CM1

should be selected to provide cancellation

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

CM1

= L/(DCR • R

CM1

). C

CM2

should be selected to

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

DC/DC converter’s switching frequency. In addition, C

CM2

needs to be much smaller than C

CM1

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 F

= 500kHz.

Let R

CM1

= 1kΩ and solve for C

CM1

≥

2.2µH

10mΩ • 1kΩ

= 220nF

Let R

CM2

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

/10 = 50kHz:

CM2

≅

2π• 50kHz • 1kΩ

= 3.18n

Let C

CM2

= 3.3nF. Note that since C

CM2

is much less than

CM1

the loading effects of the second stage filter on the

matched first stage are not significant. Consequently, the

delay time constant through the filter for the current sense

waveform will be approximately 3μs.

Measuring Multiphase Currents

For current sense applications with more than one phase,

RC averaging may be employed. Figure 27 shows an

example of this approach for a 3-phase system with DCR

current sensing. The current sense waveforms are averaged

together prior to being applied to the second stage of the

filter consisting of R

CM2

and C

CM2

. Because the R

CM1

resistors for the three phases are in parallel, the value of

CM1

must be multiplied by the number of phases. Also

note that since the DCRs are effectively in parallel, the

value for IOUT_CAL_GAIN will be equal to the inductor’s

Figure 25. Sense Resistor Current Sensing Circuits

SNS

2977 F25

LOAD CURRENT

LTC2977

SENSEP1

SENSEM1

Figure 26. Inductor DCR Current Sensing Circuits

CM2

CM1

DCR

2977 F26

SWX0

CM2

CM1

CM2

LTC2977

SENSEP1

SENSEM1