LTM4657 datasheet(14/28 Pages) AD

14 / 28 page

LTM4657

Rev. A

For more information www.analog.com

APPLICATIONS INFORMATION

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 0.5V, the slave

output voltage and the master output voltage should sat-

isfy the following equation during start-up:

VOUT(SL) •

RFB(SL)

RFB(SL)+60.4k

VOUT(MA) •

RTR(BOT)

RTR(TOP)+RTR(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 6.

Following the previous equation, the ratio of the master’s

output slew rate (MR) to the slave’s output slew rate (SR)

is determined by:

RFB(SL)

60.4k+RFB(SL)

RTR(BOT)

RTR(TOP)+RTR(BOT)

For example, VOUT(MA)=1.5V, MR = 1.5V/1ms and VOUT(SL)

= 1.0V, SR = 1.0V/1ms. From the equation, we could solve

that RTR(TOP) = 60.4k and RTR(BOT) = 30.1k are a good

combination for the ratiometric tracking.

The TRACK/SS pin will have the 2µA current source on

when a resistive divider is used to implement tracking

on the slave regulator. This will impose an offset on the

TRACK/SS 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/SS

pin offset to a negligible value.

Coincident output tracking can be recognized as a special

ratiometric output tracking in which the master’s output

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

(SR), waveform as shown in Figure 7.

Figure 7. Output Coincident Tracking Waveform

From the equation, we could easily find that, in coinci-

dent tracking, the slave regulator’s TRACK/SS pin resistor

divider is always the same as its feedback divider:

RFB(SL)

RFB(SL)+60.4k

RTR(BOT)

RTR(TOP)+RTR(BOT)

For example, RTR(TOP) = 60.4k and RTR(BOT) = 60.4k is a

good combination for coincident tracking for a VOUT(MA)

= 1.5V and VOUT(SL) = 1.0V application.

Power Good

The PGOOD pin is an open-drain pin that can be used to

monitor valid output voltage regulation. This pin is pulled

low when the output voltage exceeds a ±8% window

around the regulation point. To prevent unwanted PGOOD

glitches during transients or dynamic VOUT changes, the

LTM4657’s PGOOD falling edge includes a blanking delay

of approximately 25 switching cycles.

RUN Enable

Pulling the RUN pin to ground forces the LTM4657 into

its shutdown state, turning off both power MOSFETs and

most of its internal control circuitry. Bringing the RUN pin

above 0.6V turns on the internal reference only, while still

keeping the power MOSFETs off. Increasing the RUN pin

voltage above 1.2V will turn on the entire chip.

TIME

MASTER OUTPUT

SLAVE OUTPUT

4657 F07

LTM4657 Datasheet(PDF) 14 Page - Analog Devices

LTM4657 Datasheet(HTML) 14 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

Datasheet Download

Link URL