LTC2927ITS8-PBF データシートの表示（PDF） - Linear Technology

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LTC2927ITS8-PBF

Single Power Supply Tracking Controller

Linear Technology 

LTC2927

APPLICATIO S I FOR ATIO

EARLY

VIN

3.3V

RONB

138k

RONA

100k

0.1μF

VCC

ON

RAMP

LTC2927

CRAMP

10pF

MASTER

VIN

RTB

16.5k

RTA

13k

SDO

RAMPBUF

FB

RUN/SS IN

DC/DC

FB = 1.235V

OUT

1.8V

TRACK

GND

2927 F07

RFA

35.7k

RFB

16.5k

Figure 7. SDO Shutdown Application

SDO pulls low again when the ON pin is pulled below 1.23V

and the RAMP pin is below about 200mV.

3-Step Design Procedure

The following 3-step procedure allows one to complete

a design for any of the tracking or sequencing profiles

shown in Figures 1 to 4. A basic single supply application

circuit is shown in Figure 8.

1. Set the ramp rate of the master signal.

Solve for the value of CRAMP, the capacitor on the RAMP

pin, based on the desired ramp rate (V/s) of the master

supply, SM.

C

RAMP

=

IRAMP

SM

where IRAMP ≈ 10μA

(1)

2. Solve for the pair of resistors that provide the desired

ramp rate of the slave supply, assuming no delay.

EARLY

VIN

RONB

RONA

RTB

RTA

0.1μF

VCC

ON

RAMP

LTC2927

RAMPBUF

TRACK

GND

FB

2927 F08

CRAMP

MASTER

VIN

IN

DC/DC

FB

OUT

RFA

RFB

SLAVE

Figure 8. Single Supply Application

8

Choose a ramp rate for the slave supply, SS. If the slave

supply ramps up coincident with the master signal or

with a fixed voltage offset, then the ramp rate equals

the master supply’s ramp rate. Be sure to use a fast

enough ramp rate for the slave supply so that it will finish

ramping before the master signal has reached its final

supply value. If not, the slave supply will be held below

the intended regulation value by the master signal. Use

the following formulas to determine the resistor values

for the desired ramp rate, where RFB and RFA are the

feedback resistors in the slave supply and VFB is the

feedback reference voltage of the slave supply:

RTB

=

RFB

•

SM

SS

(2)

RTA′

=

VFB

+

VTRACK

VFB − VTRACK

(3)

RFB RFA RTB

where VTRACK ≈ 0.8V.

Note that large ratios of slave ramp rate to master ramp

rate, SS/SM, may result in negative values for RTA’. If

sufficiently large delay is used in step 3, RTA will be

positive, otherwise SS/SM must be reduced.

3. Choose RTA to obtain the desired delay.

If no delay is required, such as in coincident and ratio-

metric tracking, then simply set RTA = RTA’. If a delay

is desired, as in offset tracking and supply sequencing,

calculate RTA” to determine the value of RTA where tD

is the desired delay in seconds.

RTA″

=

VTRACK • RTB

tD • SM

(4)

RTA = RTA′ ||RTA″

(5)

the parallel combination of RTA’ and RTA”.

As noted in step 2, small delays and large ratios of slave ramp

rate to master ramp rate (usually only seen in sequencing)

may result in solutions with negative values for RTA. In such

cases, either the delay must be increased or the ratio of

slave ramp rate to master ramp rate must be reduced.

2927fb

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