Datasheet

LTM4620A
12
4620afb
For more information www.linear.com/LTM4620A
The typical LTM4620A application circuit is shown in
Figure 26. External component selection
is primarily
determined by the maximum load current and output
voltage. Refer to Table 5 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 LTM4620A is capable of 95% duty cycle
at 500kHz, 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.
See Note 8 in the Electrical Characteristics section for
output current guideline.
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 5V
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
APPLICATIONS INFORMATION
resistors to the output. All of the V
FB
pins tie together with
one programming resistor as shown in Figure 2.
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
= 80nA maximum. The voltage 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 connec-
tions 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 regulator. The soft-start equation
will need to have the soft-start current parameter increased
by the number of paralleled channels. See TRACK/Soft-
Start Pin section.
Figure 2. 4-Phase Parallel Configurations
4620A F02
60.4k
TRACK1
TRACK2
V
OUT1
V
OUTS1
V
FB1
V
FB2
COMP1
4 PARALLELED OUTPUTS
FOR 1.2V AT 50A
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
LTM4620A
LTM4620A
R
FB
60.4k
OPTIONAL
R
FB
60.4k
USE TO LOWER
TOTAL EQUIVALENT
RESISTANCE TO LOWER
I
FB
VOLTAGE ERROR