Datasheet

LTM4644/LTM4644-1
10
Rev. F
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APPLICATIONS INFORMATION
The typical LTM4644 application circuit is shown in
Figure 33. External component
selection is primarily
determined by the input voltage, the output voltage and
the maximum load current. Refer to Table 7 for specific
external capacitor requirements for a particular application.
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
due to the minimum off-time and minimum on-time limits
of each regulator. The minimum off-time limit imposes a
maximum duty cycle which can be calculated as:
D
MAX
= 1 – t
OFF(MIN)
• f
SW
where t
OFF(MIN)
is the minimum off-time, 70ns typical for
LTM4644, and f
SW
is the switching frequency. Conversely
the minimum on-time limit imposes a minimum duty cycle
of the converter which can be calculated as:
D
MIN
= t
ON(MIN)
• f
SW
where t
ON(MIN)
is the minimum on-time, 40ns typical for
LTM4644. In the rare cases where the minimum duty
cycle is surpassed, the output voltage will still remain
in regulation, but the switching frequency will decrease
from its programmed value. Note that additional thermal
derating may be
applied
. See the Thermal Considerations
and Output Current Derating section in this data sheet.
Output Voltage Programming (LTM4644)
The PWM controller has an internal 0.6V reference voltage.
As shown in the Block Diagram, a 60.4k internal feedback
resistor connects each regulator channel from V
OUT
pin
to FB pin. Adding a resistor R
FB(BOT)
from FB pin to GND
programs the output voltage:
R
FB(BOT)
=
60.4k
V
OUT
0.6
−1
Table 1. V
FB
Resistor Table vs Various Output Voltages
V
OUT
(V) 0.6 1.0 1.2 1.5 1.8 2.5 3.3 5.0
R
FB(BOT)
(k) Open 90.9 60.4 40.2 30.1 19.1 13.3 8.25
For parallel operation of N channels, use the following
equation can be used to solve for R
FB(BOT)
. Tie the V
OUT
and the FB and COMP pins together for each paralleled
output with a single resistor to GND as determined by:
R
FB(BOT)
=
60.4k
N
⎛
⎝
⎜
⎞
⎠
⎟
V
OUT
0.6
−1
⎛
⎝
⎜
⎞
⎠
⎟
OUTPUT VOLTAGE PROGRAMMING (LTM4644-1)
The PWM controller has an internal 0.6V reference voltage.
Adding two resistors R
FB(TOP)
from V
OUT
to FB pin and
R
FB(BOT)
from FB pin to GND programs the output voltage:
R
FB(BOT)
=
R
FB(TOP)
V
OUT
0.6
–1
For parallel operation of N Channels, only one set of
R
FB(TOP)
and R
FB(BOT)
is needed while tying the V
OUT
, FB
and COMP pins from different channels together. See
Figure 1 for example.
Figure 1. LTM4644-1 Feedback Resistor
for Paralleling Application
4644 F01
LTM4644-1
COMP3
R
FB(TOP)
R
FB(BOT)
V
OUT1
FB1
COMP1
V
OUT2
FB2
COMP2
V
OUT3
FB3
LTM4644 LTM4644-1
Top Feedback Resistor
from V
OUT
-to-V
FB
(one resistor per channel)
Integrated
60.4k 0.5%
Resistor
External (to be added on
PCB)
Application General
Applications
To Interface with
PMBus power system
management supervisory
ICs such as the LTC2975
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