Datasheet

ADP1828
Rev. C | Page 26 of 36
COMP
FB
TRK
SS
MASTER
VOLTAGE
R
TRKT
R
TRKB
0.6V
DETAIL VIEW OF
ADP1828
ERROR
AMPLIFIER
V
OUT
R
TOP
R
BOT
06865-043
Figure 42. Voltage Tracking
COINCIDENT TRACKING
The most common application is coincident tracking, used
in core vs. I/O voltage sequencing and similar applications.
Coincident tracking limits the slave output voltage to be the
same as the master voltage until it reaches regulation. Connect
the slave TRK input to a resistor divider from the master voltage
that is the same as the divider used on the slave FB pin. This
forces the slave voltage to be the same as the master voltage.
For coincident tracking, use the following equation:
R
TRKT
= R
TOP
and R
TRKB
= R
BOT
where:
R
TOP
and R
BOT
are the values chosen in the Compensating the
Voltage Mode Buck Regu lator section
.
See Figure 43 for an example of a coincident tracking circuit.
FB
TRK
SS
R
TRKB
10kΩ
R
TRKT
20kΩ
1.1V
ADP1828
OR
ADP1829
FB
POWER
COMPONENTS
POWER
COMPONENTS
ADP1828
EN
EN
SS
R
TOP
20kΩ
R
BOT
10kΩ
C
SS
1µF
C
SS
150nF
3.3V
V
OUT_MASTER
1.8V
V
OUT_SLAVE
0
6865-044
Figure 43. Example of a Coincident Tracking Circuit
TIME
SLAVE VOLTAGE
MASTER VOLTAGE
VOLTAGE
06865-045
Figure 44. Coincident Tracking
CH1 5.00V
CH3 1.00V
CH2 1.00V M 100ms A CH1 2.60V
1
4
CH4 1.00V
V
OUT_MASTER
EN FOR BOTH ADP1828
V
OUT_SLAVE
TRK_SLAVE
B
W
B
W
B
W
06865-046
Figure 45. Coincident Tracking of Figure 43
As the master voltage rises, the slave voltage also rises in the
same pattern. Eventually, the slave voltage reaches its regulation
voltage, where the internal reference takes over the regulation
while the TRK input continues to increase and thus removes
itself from influencing the output voltage. To ensure that the
output voltage accuracy is not compromised by the TRK pin
being too close in voltage to the 0.6 V reference, make sure
that the final value of the master voltage is greater than the
slave regulation voltage by at least 10%, or 60 mV as seen at the
FB node (the higher, the better). A difference of 60 mV between
TRK and the 0.6 V reference produces about 3 mV of offset in
the error amplifier, or 0.5%, at room temperature, while 100 mV
between them produces only 0.6 mV or 0.1% offset. For accurate
tracking, set the final voltage at TRK to less than or equal to
0.5 V. However, this condition would trip the PGOOD signal.
RATIOMETRIC TRACKING
Ratiometric tracking limits the output voltage to a fraction of
the master voltage. For example, the termination voltage for
DDR memories (VTT) is set to half the VDDQ voltage.
TIME
SLAVE VOLTAGE
MASTER VOLTAGE
VOLTAGE
0
6865-047
Figure 46. Ratiometric Tracking
For ratiometric tracking, the simplest configuration is to tie the
TRK pin of the slave channel to the FB pin of the master channel.
The advantage of this is having the fewest components, but the
accuracy suffers as the TRK pin voltage becomes equal to the
internal reference voltage and an offset is imposed on the error
amplifier of about −18 mV at room temperature.