ltc2978 Linear Technology Corporation, ltc2978 Datasheet - Page 56

• Manufacturer Part Number: ltc2978
• Description: Octal Pmbus Power Supply Monitor And Controller With Eeprom
• Manufacturer: Linear Technology Corporation

4-Step Resistor Selection Procedure for DC/DC
Converters with External Feedback Resistors
The following 4-step procedure should be used to calcu-
late the resistor values required for the application circuit
shown in Figure 16.
1. Assume values for feedback resistor R20 and the nominal
2. Solve for the value of R30 that yields the maximum
LTC2978
applicaTions inForMaTion

DC/DC converter output voltage V
for R10.
V
when the LTC2978’s V
state. R10 is a function of R20, V
the feedback node (V
and the feedback node’s input current (I
required DC/DC converter output voltage V
When V
is at its maximum voltage.
DC0(NOM)
R
R
10
30
DACP0
=
≤
V
V
is the output voltage of the DC/DC converter
DC NOM
DC MAX
is at 0V, the output of the DC/DC converter
(
(
R
20
R
)
)
–
20
–
•
FB
I
DACP0
V
V
FB
) when the loop is in regulation,
•
DC NOM
FB
0
V
(
FB
•
R
pin is in a high impedance
0
20
)
DC0(NOM)
–
DC0(NOM)
V
FB
, the voltage at
FB
).
, and solve
DC0(MAX)
(1)
(2)
.
3. Solve for the minimum value of V
4. Recalculate the minimum, nominal, and maximum
V
V
V V
V
DC
DC MAX
RES
DC NOM
to yield the minimum required DC/DC converter output
voltage V
The voltage-buffered current DAC has two full-scale
settings, 1.33V ±3.7% and 2.7V ±3.7%. In order to
select the appropriate full-scale setting, calculate the
maximum required V
DC/DC converter output voltages and the resulting
margining resolution.
0 M M IN
0
(
(
(
V
=
DACP
R
R
)
)
)
20
3
=
=
=
0 0
DC0(MIN)
0
V
V
>
V
•
FB
DC NOM
DC NOM
1024
V
(
DACP F S
V
•
0
(
(
DC NOM
1
.
(
+
0
)
( / )
)
R
R
+
DAC0P
–
20
10
)
R
R
R
R
–
20
30
V/DAC LSB
20
30
V
+
DC MIN
output voltage:
•
•
I
FB
(
V
(
FB
V
•
DACP F S
R
)
DACP0
)
20
•
0
R
R
( / )
30
20
that’s needed
+
–
V
V
FB
FB
)
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( )
( )
( )
( )
( )
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