LTM4628(RevC) View Datasheet(PDF) - Linear Technology
Part Name
Description
Manufacturer
LTM4628 Datasheet PDF : 36 Pages
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LTM4628
Applications Information
The typical LTM4628 application circuit is shown in
Figure 28. External component selection is primarily
determined by the maximum load current and output
voltage. Refer to Table 4 for specific external capacitor
requirements for particular applications.
VIN to VOUT Step-Down Ratios
There are restrictions in the maximum VIN and VOUT step-
down ratio that can be achieved for a given input voltage.
Each output of the LTM4628 is capable of 98% duty cycle,
but the VIN to VOUT minimum dropout is still shown as a
function of its load current and will limit output current
capability related to high duty cycle on the top side switch.
Minimum on-time tON(MIN) is another consideration in
operating at a specified duty cycle while operating at a
certain frequency due to the fact that tON(MIN) < D/fSW,
where D is duty cycle and fSW is the switching frequency.
tON(MIN) is specified in the electrical parameters as 90ns.
Output Voltage Programming
The PWM controller has an internal 0.6V reference voltage.
As shown in the Block Diagram, a 60.4kΩ internal feedback
resistor connects between the VOUTS1 to VFB1 and VOUTS2
to VFB2. It is very important that these pins be connected
to their respective outputs for proper feedback regulation.
Overvoltage can occur if these VOUTS1 and VOUTS2 pins are
left floating when used as individual regulators, or at least
one of them is used in paralleled regulators. The output
voltage will default to 0.6V with no feedback resistor on
either VFB1 or VFB2. Adding a resistor RFB from VFB pin to
GND programs the output voltage:
VOUT
=
0.6V
•
60.4k + RFB
RFB
Table 1. VFB Resistor Table vs Various Output Voltages
VOUT 0.6V 1.0V 1.2V 1.5V 1.8V 2.5V 3.3V
RFB Open 90.9k 60.4k 40.2k 30.2k 19.1k 13.3k
5.0V
8.25k
For parallel operation of multiple channels the same feed-
back setting resistor can be used for the parallel design.
This is done by connecting the VOUTS1 to the output as
shown in Figure 2, thus tying one of the internal 60.4k
resistors to the output. All of the VFB pins tie together with
one programming resistor as shown in Figure 2.
In parallel operation the VFB pins have an IFB current of
20nA maximum each channel. To reduce output voltage
error due to this current, an additional VOUTS pin can be
tied to VOUT, and an additional RFB resistor can be used
to lower the total Thevenin equivalent resistance seen by
this current. For example in Figure 2, the total Thevenin
equivalent resistance of the VFB pin is (60.4k // RFB), which
is 30.2k where RFB is equal to 60.4k for a 1.2V output.
Four phases connected in parallel equates to a worse case
feedback current of 4 • IFB equals 80nA maximum. The volt-
age error is 80nA • 30.2k = 2.4mV. If VOUTS2 is connected
as shown in Figure 2 to VOUT, and another 60.4k resistor is
connected from VFB2 to ground, then the voltage error is
reduced to 1.2mV. If the voltage error is acceptable then
no additional connections are necessary. The onboard
60.4k resistor is 0.5% accurate and the VFB resistor can
be chosen by the user to be as accurate as needed.
All COMP pins are tied together for current sharing between
the phases. The TRACK pins can be tied together and a single
soft-start capacitor can be used to soft-start the regula-
tor. The soft-start equation will need to have the soft-start
current parameter increased by the number of paralleled
channels. See the Output Voltage Tracking section.
0.1µF
COMP1 LTM4628
COMP2
VOUT1
VOUT2
TRACK1
TRACK2
60.4k
60.4k
VOUTS1
VOUTS2
VFB1
VFB2
COMP1 LTM4628
COMP2
VOUT1
VOUT2
TRACK1
TRACK2
60.4k
60.4k
VOUTS1
VOUTS2
VFB1
VFB2
4628 F02
4 PARALLELED OUTPUTS
FOR 1.2V AT 32A
OPTIONAL CONNECTION
OPTIONAL
RFB
60.4k
USED TO LOWER TOTAL
THEVENIN EQUIVALENT TO
LOWER IFB VOLTAGE ERROR
RFB
60.4k
Figure 2. 4-Phase Parallel Configurations
4628fc
11
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