Datasheet

LTM4628
11
4628fe
For more information www.linear.com/LTM4628
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.
V
IN
to V
OUT
Step-Down Ratios
There are restrictions in the maximum V
IN
and V
OUT
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 V
IN
to V
OUT
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 t
ON(MIN)
is another consideration in
operating at a specified duty cycle while operating at a
certain frequency due to the fact that t
ON(MIN)
< D/f
SW
,
where D is duty cycle and f
SW
is the switching frequency.
t
ON(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 V
OUTS1
to V
FB1
and V
OUTS2
to V
FB2
. It is very important that these pins be connected
to their respective outputs for proper feedback regulation.
Overvoltage can occur if these V
OUTS1
and V
OUTS2
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 V
FB1
or V
FB2
. Adding a resistor R
FB
from V
FB
pin to
GND programs the output voltage:
V
OUT
= 0.6V •
60.4k + R
FB
R
FB
Table 1. V
FB
Resistor Table vs Various Output Voltages
V
OUT
0.6V 1.0V 1.2V 1.5V 1.8V 2.5V 3.3V 5.0V
R
FB
Open 90.9k 60.4k 40.2k 30.2k 19.1k 13.3k 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 V
OUTS1
to the output as
shown in Figure 2, thus tying one of the internal 60.4k
resistors to the output. All of the V
FB
pins tie together with
one programming resistor as shown in Figure 2.
APPLICATIONS INFORMATION
In parallel operation the V
FB
pins have an I
FB
current of
20nA maximum each channel. To reduce output voltage
error due to this current, an additional V
OUTS
pin can be
tied to V
OUT
, and an additional R
FB
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 V
FB
pin is (60.4k // R
FB
), which
is 30.2k where R
FB
is equal to 60.4k for a 1.2V output.
Four phases connected in parallel equates to a worse case
feedback current of 4•I
FB
equals 80nA maximum. The volt-
age error is 80nA • 30.2k = 2.4mV. If V
OUTS2
is connected
as shown in Figure 2 to V
OUT
, and another 60.4k resistor is
connected from V
FB2
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 V
FB
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.
Figure 2. 4-Phase Parallel Configurations
4628 F02
60.4k
TRACK1
TRACK2
V
OUT1
V
OUTS1
V
FB1
V
FB2
COMP1
4 PARALLELED OUTPUTS
FOR 1.2V AT 32A
OPTIONAL CONNECTION
COMP2
V
OUTS2
V
OUT2
60.4k
60.4k
TRACK1
TRACK2
0.1µF
V
OUT1
V
OUTS1
V
FB1
V
FB2
COMP1
COMP2
V
OUTS2
V
OUT2
60.4k
LTM4628
LTM4628
R
FB
60.4k
OPTIONAL
R
FB
60.4k
USED TO LOWER TOTAL
THEVENIN EQUIVALENT TO
LOWER I
FB
VOLTAGE ERROR
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