LTM4645 datasheet(17/34 Pages) LINER

17 / 34 page

LTM4646

4646f

For more information www.linear.com/LTM4646

APPLICATIONS INFORMATION

Output Voltage Soft Starting and Tracking

Output voltage tracking can be programmed externally

using the TRACK/SS pins. The output can be tracked up

and down with another regulator. The master regulator’s

output is divided down with an external resistor divider

that is the same as the slave regulator’s feedback divider

for to implement coincident tracking. The LTM4646 uses

an accurate 60.4k resistor internally for the top feedback

resistor for each channel. Figure 5 shows an example of

coincident tracking.

SLAVE

60.4k

RFB

• VTRACK

VTRACK is the track ramp applied to the slave’s track pin.

VTRACK has a control range of 0V to 0.6V, or the internal

reference voltage. When the master’s output is divided

down with the same resistor values used to set the slave’s

output, then the slave will coincident track with the master

until it reaches its final value. The master will continue to

its final value from the slave’s regulation point. Voltage

tracking is disabled when VTRACK/SS is more than 0.6V.

RTB in Figure 5 will be equal to the RFB for coincident track-

ing. Figure 6 shows the coincident tracking waveforms.

The TRACK/SS pins can be controlled by a capacitor

placed on the regulator TRACK/SS pin to ground. A 1.0A

current source will charge the TRACK/SS pin up to the

voltage reference and then proceed up to INTVCC. After the

0.6V ramp, the TRACK/SS pin will no longer be in control,

and the internal voltage reference will control output regu-

lation from the feedback divider. Foldback current limit is

disabled during this sequence of turn-on during tracking

or soft-starting. The TRACK/SS pins are pulled low when

the RUN pin is below 1.2V. The total soft-start time can

be calculated as:

tSOFT-START

CSS

1.0µA

• 0.6V

Ratiometric tracking can be achieved by a few simple

calculations and the slew rate value applied to the mas-

ter’s TRACK/SS pin. As mentioned above, the TRACK/SS

pin has a control range from 0 to 0.6V. The master’s

TRACK/SS pin slew rate is directly equal to the master’s

output slew rate in volts per second. The equation:

• 60.4k RTA

where MR is the master’s output slew rate and SR is

the slave’s output slew rate in volts per second. When

coincident tracking is desired, then MR and SR are equal,

thus RTA is equal to 60.4k. RTB is derived from equation:

RTB

0.6V

VFB

60.4k

VFB

RFB

VTRACK

RTA

where VFB is the feedback voltage reference of the regula-

tor, and VTRACK/SS is 0.6V. Since RTA is equal to the 60.4k

top feedback resistor of the slave regulator in equal slew

rate or coincident tracking, then RTB is equal to RFB with

VFB = VTRACK/SS. In ratiometric tracking, a different slew

rate may be desired for the slave regulator. RTB can be

solved for when SR is slower than MR. Make sure that the

slave supply slew rate is chosen to be fast enough so that

the slave output voltage will reach it final value before the

master output. For example, MR = 20V/s, and SR = 15V/s.

Then RTA = 80.6k. Solve for RTB to equal to 80.6k.

Each of the TRACK/SS pins will have the 1.0A current

source on when a resistive divider is used to implement

tracking on that specific channel. This will impose an off-

set on the TRACK/SS pin input. Smaller values 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/SS pin offset to a negligible value.

Power Good

Each PGOOD pin is connected to an internal open-drain

N-channel MOSFET. An external resistor or current source

can be used to pull this pin up to 6V (e.g., VOUT1,2 or

DRVCC). Overvoltage or undervoltage comparators (OV,

UV) turn on the MOSFET and pull the PGOOD pin low

when the feedback voltage is outside the ±7.5% window

of the reference voltage. The PGOOD pin is also pulled low

when the channel’s RUN pin is below the 1.2V threshold

LTM4645 Datasheet(PDF) 17 Page - Linear Technology

LTM4645 Datasheet(HTML) 17 Page - Linear Technology

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

Datasheet Download

Link URL