Datasheet

LTC2923
12
2923fa
In this example, all supplies remain low while the ON pin
is held below 1.23V. When the ON pin rises above 1.23V,
10µA pulls up C
GATE
and the gate of the FET at 1000V/s. As
the gate of the FET rises, the source follows and pulls up
the output to 3.3V at 1000V/s. This output serves as the
master signal and is buffered from the RAMP pin to the
RAMPBUF pin. As this output and the RAMPBUF pin rise,
the current from the TRACK pins is reduced. Conse-
quently, the voltage at the slave supply’s outputs in-
creases, and the slave supplies track the master supply.
When the ON pin is again pulled below 1.23V, 10µA will
pull down C
GATE
and the gate of the FET at 1000V/s. If the
loads on the outputs are sufficient, all outputs will track
down coincidently at 1000V/s.
APPLICATIO S I FOR ATIO
WUUU
Coincident Tracking Example
1ms/DIV 2923 F10b
1V/DIV
1ms/DIV
2923 F10a
1V/DIV
Figure 10. Coincident Tracking (from Figure 11)
MASTER
SLAVE2
SLAVE1
Figure 11. Coincident Tracking Example
A typical three supply application is shown in Figure 11. The
master signal is a 3.3V module. 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 coinci-
dently with the 3.3V supply that is controlled with an ex-
ternal FET. The ramp rate of the supplies is 1000V/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
A
Vs
nF
GATE
=
µ
=
10
1000
10
/
2. Solve for the pair of resistors that provide the desired
slave supply behavior, assuming no delay.
From Equation 2:
Rk
Vs
Vs
k
TB
= Ω = Ω16 5
1000
1000
16 5.•
/
/
.
From Equation 3:
R
V
V
k
V
k
V
k
k
TA
′
=
Ω
+
ΩΩ
≈Ω
08
1 235
16 5
1 235
35 7
08
16 5
13
.
.
.
.
.
–
.
.
3. Choose R
TA
to obtain the desired delay.
Since no delay is desired, R
TA
= R
TA
′
Q1
C
GATE
10nF
V
CC
R
ONB
138k
3.3V
R
TB1
16.5k
R
TB2
887k
R
FA1
35.7k
1.8V
SLAVE1
3.3V
MASTER
R
FB1
16.5k
R
TA2
412k
R
TA1
13k
R
ONA
100k
ON FB1
GATE
LTC2923
GND
2923 F11
RAMP
R
FA2
412k
2.5V
SLAVE2
R
FB2
887k
RAMPBUF
TRACK1
TRACK2
FB2
DC/DC
IN
3.3V
3.3V
FB = 1.235V OUT
DC/DC
IN
FB = 0.8V OUT
0.1µF
10Ω