LTM4603-1_15 Datasheet, PDF (14/26 Pages) Linear Technology – 20V, 6A DC/DC Module Regulator with PLL, Output Tracking and Margining

LTM4603/LTM4603-1
APPLICATIONS INFORMATION
control the ramp of the internal reference and the output
voltage. The total soft-start time can be calculated as:
( ) ^t_SOFTSTART ^{= 0.8 •}
0.6V ± V_OUT(MARGIN)
• ^CSS
1.5µA
When the RUN pin falls below 1.5V, then the TRACK/SS
pin is reset to allow for proper soft-start control when the
regulator is enabled again. Current foldback and forced
continuous mode are disabled during the soft-start pro-
cess. The soft-start function can also be used to control
the output ramp up time, so that another regulator can
be easily tracked to it.
Output Voltage Tracking
Output voltage tracking can be programmed externally
using the TRACK/SS pin. The output can be tracked up
and down with another regulator. Figure 5 shows an ex-
ample of coincident tracking where the master regulator’s
output is divided down with an external resistor divider
that is the same as the slave regulator’s feedback divider.
The master output must be greater than the slave output
for the tracking to work. Figure 6 shows the coincident
output tracking characteristics.
Ratiometric tracking can be achieved by a few simple
calculations and the slew rate value applied to the master’s
TRACK/SS pin. The TRACK/SS pin has a control range
from 0V to 0.6V. The master’s TRACK/SS pin slew rate
is directly equal to the master’s output slew rate in volts/
time. The equation:
MR
SR
•
60.4k
=
R_T
where MR is the master’s output slew rate and SR is the
slave’s output slew rate in volts/time. When coincident
tracking is desired, then MR and SR are equal, thus R_TB
is equal to 60.4k. R_B is derived from equation:
R_B
=
V_FB
60.4k
0.6V
+
V_FB
R_FB(Slave)
–
V _TRACK
R_TB
where V_FB is the feedback voltage reference of the regula-
tor, and V_TRACK is 0.6V.
14
MASTER
OUTPUT
R_T
TRACK CONTROL
60.4k
V_IN
R_B
40.2k
100k
V_IN PLLIN TRACK/SS
PGOOD
V_OUT
SLAVE OUTPUT
MPGM
RUN
V_FB
MARG0
C_OUT
C_IN
COMP
INTV_CC
LTM4603 ^MARG1
V_{OUT_LCL}
DRV_CC
DIFFV_OUT
V_OSNS+
V_OSNS–
SGND PGND ^fSET
R_SET
40.2k
4603 F05
Figure 5. Coincident Tracking Schematic
OUTPUT
VOLTAGE
MASTER OUTPUT
SLAVE OUTPUT
TIME
4603 F06
Figure 6. Coincident Output Tracking Characteristics
In ratiometric tracking, a different slew rate maybe desired
for the slave regulator. R_T can be solved for when SR is
slower than MR. Make sure that the slave supply slew
rate is chosen to be fast enough so that the slave output
voltage will reach its final value before the master output.
For example, MR = 1.5V/ms, and SR = 1.2V/ms. Then R_T
= 75k. Solve for R_B to equal 51.1k.
For applications that do not require tracking or sequencing,
simply tie the TRACK/SS pin to INTV_CC to let RUN control
the turn on/off. When the RUN pin is below its threshold
or V_IN is below the undervoltage lockout threshold, then
TRACK/SS is pulled low.
4603fb