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LTC2927CDDB 查看數據表(PDF) - Linear Technology

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LTC2927CDDB
Linear
Linear Technology 
LTC2927CDDB Datasheet PDF : 16 Pages
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APPLICATIO S I FOR ATIO
Coincident Tracking Example
1V/DIV
MASTER
SLAVE2
SLAVE1
LTC2927
1V/DIV
10ms/DIV
10ms/DIV
2927 F09
Figure 9. Coincident Tracking (from Figure 10)
A typical application is shown in Figure 10. The master
signal is a 3.3V ramp generated by the LTC2927. The
slave 1 supply is a 1.8V switching power supply and the
slave 2 supply is a 2.5V switching power supply. Both
slave supplies track coincidently with the 3.3V ramping
master signal. The ramp rate of the supplies is 100V/s.
The 3-step design procedure detailed previously can be
used to determine component values. Only the slave 1
supply is considered here as the procedure is the same
for the slave 2 supply.
1. Set the ramp rate of the master signal.
From Equation 1:
C RAMP
=
10μA
100V /s
=
0.1μF
2. Solve for the pair of resistors that provide the desired
slave supply behavior, assuming no delay.
From Equation 2:
RTB
=
16.5kΩ •
100V
100V
/s
/s
=
16.5kΩ
From Equation 3:
RTA
=
1.235V
+
0.8V
1.235V
0.8V
13kΩ
16.5kΩ 35.7kΩ 16.5kΩ
3. Choose RTA to obtain desired delay.
Since no delay is desired, RTA = RTA
In this example, the supply remains low while the ON pin
is held below 1.23V. When the ON pin rises above 1.23V,
10μA pulls up the master signal on CRAMP at 100V/s.
The master signal is buffered from the RAMP pin to the
RAMPBUF pin. As this output and the RAMPBUF pin rise,
the current from the TRACK pin is reduced. Consequently,
the voltage at the slave supply’s output is increased, and
the slave supply tracks the master signal. When the ON
pin is again pulled below 1.23V, 10μA will pull down CRAMP
at 100V/s. If the loads on the outputs are sufficient, all
outputs will track down coincidently at 100V/s.
EARLY VIN
3.3V
RONB
138k
RONA
100k
RTB1
16.5k
RTA1
13k
VCC
ON
RAMP
LTC2927
SDO
RAMPBUF
FB
TRACK
GND
0.1μF
0.1μF 3.3V
MASTER
3.3V
RUN/SS IN
DC/DC
FB = 1.235V
OUT
RFA1
35.7k
RFB1
16.5k
SLAVE1
1.8V
EARLY
3.3V
RTB2
887k
RTA2
412k
VCC
ON
RAMP
LTC2927
SDO
RAMPBUF
FB
TRACK
GND
2927 F10
0.1μF
3.3V
RUN/SS IN
DC/DC
FB = 0.8V
OUT
RFA2
412k
RFB2
887k
Figure 10. Coincident Tracking Example
SLAVE2
2.5V
2927fb
9

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