ltm4619 Linear Technology Corporation, ltm4619 Datasheet - Page 11

• Manufacturer Part Number: ltm4619
• Description: Dual, 26vin, 4a Dc/dc ?module Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
Soft-Start and Tracking
The LTM4619 has the ability to either soft-start by itself
with a capacitor or track the output of another channel or
external supply. When one particular channel is confi gured
to soft-start by itself, a capacitor should be connected to
its TK/SS pin. This channel is in the shutdown state if its
RUN pin voltage is below 1.2V. Its TK/SS pin is actively
pulled to ground in this shutdown state.
Once the RUN pin voltage is above 1.2V, the channel pow-
ers up. A soft-start current of 1.3μA then starts to charge
its soft-start capacitor. Note that soft-start or tracking is
achieved not by limiting the maximum output current of
the controller but by controlling the output ramp voltage
according to the ramp rate on the TK/SS pin. Current
foldback is disabled during this phase to ensure smooth
soft-start or tracking. The soft-start or tracking range is
defi ned to be the voltage range from 0V to 0.8V on the
TK/SS pin. The total soft-start time can be calculated as:
5.5V TO
V
OUT1
3.3V
28V
t
V
SOFT-START
IN
C
C
Figure 3. Example of Coincident Tracking
IN
OUT1
R3
19.1k
=
0.8V • C
C1
0.1μF
1.3µA
C2
22pF
SS
V
V
COMP1
V
TK/SS1
RUN1
PGOOD
IN
FB1
OUT1
MODE/PLLIN
SGND
LTM4619
FREQ/PLLFLTR
INTV
PGND
EXTV
COMP2
TK/SS2
V
CC
RUN2
OUT2
V
4619 F03
FB2
CC
C3
22pF
V
OUT1
R1
60.4k
R2
28k
C
OUT2
R4
28k
V
2.5V
OUT2
Output voltage tracking can be programmed externally
using the TK/SS pin. The master channel is divided down
with an external resistor divider that is the same as the
slave channel’s feedback divider to implement coincident
tracking. The LTM4619 uses an accurate 60.4k resistor
internally for the top feedback resistor. Figure 3 shows an
example of coincident tracking. Figure 4 shows the output
voltages with coincident tracking.
V
pin. V
master’s output is divided down with the same resistor
values used to set the slave’s output, then the slave will
coincident track with the master until it reaches its fi nal
value. The master will continue to its fi nal value from the
slave’s regulation point.
Ratiometric modes of tracking can be achieved by select-
ing different divider resistors values to change the output
tracking ratio. The master output must be greater than the
slave output for the tracking to work. Master and slave
data inputs can be used to implement the correct resistors
values for coincident or ratio tracking.
TRACK
V
SLAVE
TRACK
VOLTAGE
OUTPUT
is the track ramp applied to the slave’s TK/SS2
= 1+
has a control range of 0V to 0.8V. When the
⎛
⎝ ⎜
Figure 4. Coincident Tracking
R2
R1
⎞
⎠ ⎟
• V
TRACK
TIME
SLAVE OUTPUT
MASTER OUTPUT
LTM4619
4619 F04
11
4619f