LTM4622A
13
Rev B
APPLICATIONS INFORMATION
Output voltage tracking can also be programmed externally
using the TRACK/SS pin. The output can be tracked up and
down with another regulator. Figure 4 and Figure 5 show
an example waveform and schematic of a Ratiometric
tracking where the slave regulator’s output slew rate is
proportional to the master’s.
The R
FB(SL)
is the feedback resistor and the R
TR(TOP)
/
R
TR(BOT)
is the resistor divider on the TRACK/SS pin of
the slave regulator, as shown in Figure 5.
Following the upper equation, the master’s output slew
rate (MR) and the slave’s output slew rate (SR) in Volts/
Time is determined by:
MR
SR
=
R
FB(SL)
R
FB(SL)
+ 60.4k
R
TR(TOP)
R
TR(TOP)
+R
TR(BOT)
For example, V
OUT(MA)
= 1.5V, MR = 1.5V/ms and V
OUT(SL)
= 3.3V, SR = 3.3V/ms. From the equation, we could solve
out that R
TR(TOP)
= 60.4k and R
TR(BOT)
= 40.2k is a good
combination for the Ratiometric tracking.
The TRACK pins will have the 1.5µA current source on
when a resistive divider is used to implement tracking on
that specific channel. This will impose an offset on the
TRACK pin input. Smaller values resistors with the same
ratios as the resistor values calculated from the above
equation can be used. For example, where the 60.4k is
used then a 6.04k can be used to reduce the TRACK pin
offset to a negligible value.
The Coincident output tracking can be recognized as a
special Ratiometric output tracking which the master’s
output slew rate (MR) is the same as the slave’s output
slew rate (SR), as waveform shown in Figure 6.
Figure 5. Example Schematic of Ratiometric
Output Voltage Tracking
Figure 6. Output Coincident Tracking Waveform
TIME
MASTER OUTPUT
SLAVE OUTPUT
OUTPUT VOL
TAGE
4622A F06
Since the slave regulator’s TRACK/SS is connected to
the master’s output through a R
TR(TOP)
/R
TR(BOT)
resistor
divider and its voltage used to regulate the slave output
voltage when TRACK/SS voltage is below 0.6V, the slave
output voltage and the master output voltage should satisfy
the following equation during the start-up.
V
OUT(SL)
•
R
FB(SL)
R
FB(SL)
+ 60.4k
=
V
OUT(MA)
•
R
TR(TOP)
R
TR(TOP)
+R
TR(BOT)
Figure 4. Output Ratiometric Tracking Waveform
TIME
SLAVE OUTPUT
MASTER OUTPUT
OUTPUT VOL
TAGE
4622A F04
40.2k
4622A F05
10µF
25V
V
IN
4V TO 20V
V
OUT1
1.5V, 2A
47µF
6.3V
0.1µF
V
OUT1
V
OUT2
3.3V, 2A
47µF
6.3V
V
OUT2
FB1
COMP1
COMP2
FREQ
GND
LTM4622A
PGOOD1 PGOOD2
V
IN2
V
IN1
RUN1
RUN2
INTV
CC
SYNC/MODE
TRACK/SS1
TRACK/SS2
13.3k
40.2k
60.4k
FB2
V
OUT1
