LTC2972 datasheet(102/112 Pages) LINER

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LTC2972

102

2972fa

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Figure 40. Anti-Aliasing Filter on VSENSE Lines

APPLICATIONS INFORMATION

Multiphase Design Example

Using the same values for inductance and DCR from

the previous design example, the value for RCM1 will be

3kΩ for a three phase DC/DC converter if CCM1 is left at

220nF. Similarly, the value for IOUT_CAL_GAIN will be

DCR/3 = 3.33mΩ.

Anti-Aliasing Filter Considerations

Noisy environments require an anti-aliasing filter on

the input to the LTC2972’s ADC. The R-C circuit shown

in Figure 40 is adequate for most situations. Keep

R40 = R50 ≤ 200Ω to minimize ADC gain errors, and select

a value for capacitors C10 and C20 that doesn’t add too

much additional response time to the OV/UV supervisor,

e.g. τ = 10μs (R = 100Ω, C = 0.10μF).

Sensing Negative Voltages

Figure 41 shows the LTC2972 sensing a negative power

supply (VEE). The R1/R2 resistor divider translates the

negative supply voltage to the LTC2972’s VSENSEM1 input

while the VSENSEP1 input is tied to the REFP pin which

has a typical output voltage of 1.23V. The voltage divider

should be configured in order to present about 0.5V to the

voltage sense inputs when the negative supply reaches its

POWER_GOOD_ON threshold so that the current flowing

out of the VSENSEMn pin is minimized to ~1µA. The rela-

tionship between the POWER_GOOD_ON register value

and the corresponding negative supply value can be be

expressed as:

VEE = VREFP – READ _ VOUT

(

) •

+ 1

⎛

⎝

⎜

⎞

⎠

⎟

⎟ – 1µA • R2

where READ_VOUT returns VSENSEP – VSENSEM

Figure 41. Sensing Negative Voltages

VPWR

VDD33

VDD25

VIN_SNS

VDAC0

VSENSEP0

VSENSEM0

VOUT_EN0

LTC2972*

0.1µF

4.5V < VIBUS < 15V

*SOME DETAILS OMITTED FOR CLARITY

ONLY ONE OF TWO CHANNELS SHOWN

VIN

VOUT

R20

R30

R10

2972 F40

RUN/SS

SGND

VFB

GND

DC/DC

CONVERTER

LOAD

C10

C20

R50

R40

GND

LTC2972

4.5V < VIBUS < 15V

GND

ONLY ONE OF TWO CHANNELS SHOWN,

SOME DETAILS OMITTED FOR CLARITY

POWER_GOOD_ON = 0.5V FOR VEE POWER_GOOD = –11.414V

WHERE VEE POWER_GOOD =

VREFP – POWER_GOOD_ON (R2/R1 + 1) – 1µA • R2

0.1µF

1.23V TYP

2972 F41

VPWR

SDA

SCL

ALERTB

CONTROL

FAULTB

WDI/RESETB

SHARE_CLK

ASEL0

ASEL1

VIN_SNS

REFM

VSENSEP1

WDI/RESETB

VSENSEM1

REFP

PWRGD

0.1µF

R1 = 4.99k

1µA AT 0.5V

R2 = 120k

VEE = –12V

PMBus

INTERFACE

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