Datasheet

ADP1877 Data Sheet
Rev. D | Page 24 of 32
The ratio of the slave output voltage to the master voltage is a
function of the two dividers.
RATIOMETRIC TRACKING
Ratiometric tracking limits the output voltage to a fraction of
the master voltage. For ratiometric tracking, the simplest
configuration is to tie the TRK pin of the slave channel to the
FBx pin of the master channel. However, because of the large
internal offset between TRKx and FBx, this ratiometric tracking
configuration is not recommended. A tracking configuration
that requires the TRKx voltage of the slave channel below 0.6 V
is not recommended because of the large internal TRKx to FBx
offset voltage.
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+
=
TRKB
TRKT
BOT
TOP
MASTER
OUT
SLAVE
OUT
R
R
R
R
V
V
1
1
_
_
As the master voltage rises, the slave voltage rises identically.
Eventually, the slave voltage reaches its regulation voltage,
where the internal reference takes over the regulation while the
TRKx input continues to increase and thus removes itself from
influencing the output voltage.
Another ratiometric tracking configuration is having the slave
channel rise more quickly than the master channel, as shown in
Figure 38 and Figure 39. The tracking circuits in Figure 37 and
Figure 38 are virtually identical with the exception that RTRKB
> RTRKT, as shown in Figure 38.
To ensure that the output voltage accuracy is not compromised
by the TRKx pin being too close in voltage to the 0.6 V
reference, make sure that the final value of the TRKx voltage of
the slave channel is at least 0.7 V.
C
SS1
100nF
C
SS2
20nF
3.3
V
V
OUT1_MASTER
FB1
EN
R
BOT
10kΩ
FB2
SS1
ADP1877
EN1 EN2
SS2
VCCO
TRK1
TRK2
R
TOP
20kΩ
1.8V
V
OUT2_SLAVE
R
TRKB
10kΩ
R
TRKT
5kΩ
2.2V
10kΩ
45.3kΩ
0
8299-015
1MΩ
Figure 38. A Simple Ratiometric Tracking Circuit (Slave Channel Has
a Faster Ramp Rate)
TIME
SLAVE VOLTAGE
MASTER
V
OLTA
G
E
VOLTAGE (V)
08299-016
Figure 39. Ratiometric Tracking (Slave Channel Has a Faster Ramp Rate)