LTM4650A datasheet(19/36 Pages) AD

19 / 36 page

LTM4671

Rev. B

For more information www.analog.com

Since the slave regulator’s TRACK/SS is connected to

the master’s output through a RTR(TOP)/RTR(BOT) resistor

divider and its voltage used to regulate the slave output

voltage when TRACK/SS voltage is below 0.6V, the slave

outputvoltageandthemasteroutputvoltageshouldsatisfy

the following equation during the start-up.

VOUT(SL) •

RFB(SL)

RFB(SL) + 60.4k

= VOUT(MA) •

R TR(BOT)

R TR(TOP) + R TR(BOT)

The RFB(SL) is the feedback resistor and the RTR(TOP)/

RTR(BOT) is the resistor divider on the TRACK/SS pin of

the slave regulator, as shown in Figure 5.

Following the upper equation, the master’s output slew

rate (MR) and the slave’s output slew rate (SR) in Volts/

Time is determined by:

RFB(SL)

RFB(SL) + 60.4k

RTR(BOT)

RTR(TOP) + RTR(BOT)

Forexample,VOUT(MA)=3.3V,MR=3.3V/msandVOUT(SL)=

1.0V, SR = 1.0V/ms as VOUT1andVOUT0fromtheequation,

we could solve out that RTR(TOP)0 = 60.4k and RTR(BOT)0 =

13.3k is a good combination. Follow the same equation,

we can get the same RTR(TOP)/RTR(BOT) resistor divider

value for VOUT2 and VOUT3.

The TRACK pins will have the 1.5µA current source on

when a resistive divider is used to implement tracking on

that specific channel. This will impose an offset on the

TRACK pin input. Smaller value resistors with the same

ratios as the resistor values calculated from the above

equation can be used. For example, where the 60.4k is

used then a 6.04k can be used to reduce the TRACK pin

offset to a negligible value.

The coincident output tracking can be recognized as a

special ratiometric output tracking which the master’s

output slew rate (MR) is the same as the slave’s output

slew rate (SR), see Figure 6.

APPLICATIONS INFORMATION

Figure 5. Output Ratiometric Tracking Schematic

4671 F05

CH1

CSS

0.1µF

RFB1

13.3k

VIN

4V TO 20V

RTR(TOP)2

60.4k

CH2

RFB(SL)2

19.1k

CH3

RFB(SL)3

60.4k

CH0

RFB(SL)0

90.6k

RTR(BOT)2

13.3k

RTR(TOP)3

60.4k

RTR(BOT)3

13.3k

RTR(TOP)0

60.4k

RTR(BOT)0

13.3k

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