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LTM4646 数据表(PDF) 17 Page - Linear Technology

部件名 LTM4646
功能描述  Dual 10A or Single 20A 關Module Regulator
Download  34 Pages
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制造商  LINER [Linear Technology]
网页  http://www.linear.com
标志 LINER - Linear Technology

LTM4646 数据表(HTML) 17 Page - Linear Technology

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LTM4646
17
4646f
For more information www.linear.com/LTM4646
APPLICATIONS INFORMATION
Output Voltage Soft Starting and Tracking
Output voltage tracking can be programmed externally
using the TRACK/SS pins. The output can be tracked up
and down with another regulator. The master regulator’s
output is divided down with an external resistor divider
that is the same as the slave regulator’s feedback divider
for to implement coincident tracking. The LTM4646 uses
an accurate 60.4k resistor internally for the top feedback
resistor for each channel. Figure 5 shows an example of
coincident tracking.
SLAVE
1
60.4k
RFB
• VTRACK
VTRACK is the track ramp applied to the slave’s track pin.
VTRACK has a control range of 0V to 0.6V, or the internal
reference voltage. When the 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 final value. The master will continue to
its final value from the slave’s regulation point. Voltage
tracking is disabled when VTRACK/SS is more than 0.6V.
RTB in Figure 5 will be equal to the RFB for coincident track-
ing. Figure 6 shows the coincident tracking waveforms.
The TRACK/SS pins can be controlled by a capacitor
placed on the regulator TRACK/SS pin to ground. A 1.0A
current source will charge the TRACK/SS pin up to the
voltage reference and then proceed up to INTVCC. After the
0.6V ramp, the TRACK/SS pin will no longer be in control,
and the internal voltage reference will control output regu-
lation from the feedback divider. Foldback current limit is
disabled during this sequence of turn-on during tracking
or soft-starting. The TRACK/SS pins are pulled low when
the RUN pin is below 1.2V. The total soft-start time can
be calculated as:
tSOFT-START
CSS
1.0µA
• 0.6V
Ratiometric tracking can be achieved by a few simple
calculations and the slew rate value applied to the mas-
ter’s TRACK/SS pin. As mentioned above, the TRACK/SS
pin has a control range from 0 to 0.6V. The master’s
TRACK/SS pin slew rate is directly equal to the master’s
output slew rate in volts per second. The equation:
MR
SR
• 60.4k RTA
where MR is the master’s output slew rate and SR is
the slave’s output slew rate in volts per second. When
coincident tracking is desired, then MR and SR are equal,
thus RTA is equal to 60.4k. RTB is derived from equation:
RTB
0.6V
VFB
60.4k
VFB
RFB
VTRACK
RTA
where VFB is the feedback voltage reference of the regula-
tor, and VTRACK/SS is 0.6V. Since RTA is equal to the 60.4k
top feedback resistor of the slave regulator in equal slew
rate or coincident tracking, then RTB is equal to RFB with
VFB = VTRACK/SS. In ratiometric tracking, a different slew
rate may be desired for the slave regulator. RTB 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 it final value before the
master output. For example, MR = 20V/s, and SR = 15V/s.
Then RTA = 80.6k. Solve for RTB to equal to 80.6k.
Each of the TRACK/SS pins will have the 1.0A current
source on when a resistive divider is used to implement
tracking on that specific channel. This will impose an off-
set on the TRACK/SS 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/SS pin offset to a negligible value.
Power Good
Each PGOOD pin is connected to an internal open-drain
N-channel MOSFET. An external resistor or current source
can be used to pull this pin up to 6V (e.g., VOUT1,2 or
DRVCC). Overvoltage or undervoltage comparators (OV,
UV) turn on the MOSFET and pull the PGOOD pin low
when the feedback voltage is outside the ±7.5% window
of the reference voltage. The PGOOD pin is also pulled low
when the channel’s RUN pin is below the 1.2V threshold


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