LTC3613 datasheet(29/44 Pages) AD

29 / 44 page

LTC7131-1

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

reference at FB until TRACK/SS charges above the REF

pin voltage. The external soft-start duration can be cal-

culated by Equation 15.

tSS =

VREF •CSS

5µA

(15)

3. The TRACK/SS pin can be used to track the output

voltage of another supply. Using the TRACK/SS pin for

tracking is only allowed when the hiccup mode is dis-

abled (see Fault Response Programming section) since

the TRACK/SS pin is used for fault response timing.

Regardless of either internal or external soft-start state,

the MODE pin is ignored and soft-start will always be in

discontinuous mode until SS voltage reaches the pro-

grammed VOUT reference voltage for the first time.

TRACKING

Using the TRACK/SS or the REF pin, two types of track-

ing/sequencing can be used for the LTC7131-1 (see

Figure 16). For ratiometric tracking, VOUT will always track

a ratio of the input tracking voltage. In coincident tracking,

VOUT will track a ratio of the input tracking voltage until

VOUT ≥ VREF, then VOUT is regulated to VREF. Also, with

ratiometric tracking, the VOUT can be adjusted with the

margin commands and MARGIN pin relative to the track-

ing voltage. With coincident tracking, VOUT can only be

adjusted relative to VREF when VMASTER ≥ VREF.

For coincident tracking, be aware that the PGOOD window

is always centered around the DAC adjusted REF pin volt-

age, not the TRACK/SS pin voltage.

Ratiometric Tracking

To implement ratiometric tracking (Figure 17a and

Figure 17b) the controlling voltage, VMASTER is con-

nected to the REF pin. This source must be able to sink

the 100µA IREF current. Using the REF pin allows VOUT to

be adjusted with the margining commands and MARGIN

pin. For VMASTER > VSLAVE connect VMASTER to the REF

pin thru a resistive divider. The relationship of VMASTER to

VSLAVE is given by Equation 16.

VMASTER = VSLAVE • 1+

⎛

⎝

⎜

⎞

⎠

⎟−R1• 100µA

(16)

Note that the 100µA IREF current requires VMASTER > R1

• 100µA before VSLAVE starts rising above 0V. Choose a

low value for R1 to minimize this offset.

For VMASTER < VSLAVE connect VMASTER directly to REF pin.

The ratio of VMASTER to VSLAVE is given by Equation 17.

VMASTER

VSLAVE

R1+R2

(17)

Coincident Tracking

To implement coincident tracking with VMASTER ≥ VSLAVE

(Figure 17c) connect VMASTER to the TRACK/SS pin

directly or with a resistive divider given by Equation 18.

VMASTER

VSLAVE

R1+R2

(18)

VOUT will follow VMASTER (or a ratio of it as set by R1/R2)

until VMASTER > VREF at which time VOUT is regulated to

VREF. Do not let TRACK/SS pin exceed VREF + 0.3V due

to internal clamp.

TIME

(b) Ratiometric Tracking

VOUT1

VOUT2

TIME

7131-1 F16

(a) Coincident Tracking

VOUT1

VOUT2

Figure 16. Two Different Modes of Output Voltage Tracking

LTC3613 Datasheet(PDF) 29 Page - Analog Devices

LTC3613 Datasheet(HTML) 29 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44

Datasheet Download

Link URL