LTC2927 Datasheet, PDF (12/16 Pages) Linear Technology – Single Power Supply Tracking Controller

LTC2927
APPLICATIO S I FOR ATIO
Supply Sequencing Example
1V/DIV
MASTER
SLAVE2
SLAVE1
1V/DIV
10ms/DIV
10ms/DIV
2927 F15
Figure 15. Supply Sequencing (from Figure 16)
In Figure 15, the slave 1 supply and the slave 2 supply are
sequenced instead of tracking. The 3.3V master ramps up
at 100V/s. The 1.8V slave 1 supply ramps up at 1000V/s
beginning 10ms after the master signal starts to ramp up.
The 2.5V slave 2 supply ramps up at 1000V/s beginning
25ms after the master signal begins to ramp up. Note
that not every combination of ramp rates and delays is
possible. Small delays and large ratios of slave ramp rate
to master ramp rate may result in solutions that require
negative resistors. In such cases, either the delay must
be increased or the ratio of slave ramp rate to master
ramp rate must be reduced. In this example, solving for
the slave supply yields:
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.
R_TB
=
16.5kΩ
•
100V / s
1000V / s
=
1.65kΩ
From Equation 3:
R_TA^′
=
1.235V
+
0.8V
1.235V
−
0.8V
= −2.13kΩ
16.5kΩ 35.7kΩ 1.65kΩ
12
3. Choose R_TA to obtain desired delay.
From Equation 4:
R_TA^″
=
0.8V • 1.65kΩ
10ms • 100V / s
=
1.32kΩ
From Equation 5:
R_TA = −2.13kΩ 1.32kΩ = 3.48kΩ
EARLY V_IN
3.3V
R_ONB
138k
R_ONA
100k
R_TB1
1.65k
R_TA1
3.48k
V_CC
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
R_FA1
35.7k
R_FB1
16.5k
SLAVE1
1.8V
EARLY
3.3V
R_TB2
88.7k
R_TA2
36.5k
V_CC
ON
RAMP
LTC2927
SDO
RAMPBUF
FB
TRACK
GND
2927 F16
0.1μF
3.3V
RUN/SS IN
DC/DC
FB = 0.8V
OUT
R_FA2
412k
R_FB2
887k
Figure 16. Supply Sequencing Example
SLAVE2
2.5V
2927fb