Datasheet
LTC2977
83
2977fa
For more information www.linear.com/LTC2977
APPLICATIONS INFORMATION
Four-Step Resistor Selection Procedure for DC/DC
Converters with External Feedback Resistors
The following four-step procedure should be used to
calculate the resistor values required for the application
circuit shown in Figure 23.
1. Assume values for feedback resistor R20 and the nominal
DC/DC converter output voltage V
DC(NOM)
, and solve
for R10.
V
DC(NOM)
is the output voltage of the DC/DC converter
when the LTC2977’s V
DACP0
pin is in a high impedance
state. R10 is a function of R20, V
DC(NOM)
, the voltage at
the feedback node (V
FB
) when the loop is in regulation,
and the feedback node’s input current (I
FB
).
R10 =
R20• V
FB
V
DC(NOM)
–I
FB
• R20 – V
FB
(1)
2. Solve for the value of R30 that yields the maximum
required DC/DC converter output voltage V
DC(MAX)
.
When V
DACP0
is at 0V, the output of the DC/DC converter
is at its maximum voltage.
R30 ≤
R20 • V
FB
V
DC(MAX)
– V
DC(NOM)
(2)
3. Solve for the minimum value of V
DACP0
that is needed
to yield the minimum required DC/DC converter output
voltage V
DC(MIN)
.
The DAC has two full-scale settings, 1.38V and 2.65V.
In order to select the appropriate full-scale setting,
calculate the minimum required V
DACP0(F/S)
output
voltage:
V
DACP0(F/S)
> V
DC(NOM)
– V
DC(MIN)
( )
•
R30
R20
+ V
FB
(3)
4. Recalculate the minimum, nominal, and maximum DC/
DC converter output voltages and the resulting margin
-
ing resolution.
V
DC(NOM)
= V
FB
• 1+
R20
R10
⎛
⎝
⎜
⎞
⎠
⎟
+ I
FB
• R20 (4)
V
DC(MIN)
= V
DC(NOM)
–
R20
R30
• V
DACP0(F/S)
– V
FB
( )
(5)
V
DC(MAX)
= V
DC(NOM)
+
R20
R30
• V
FB
(6)
V
RES
=
R20
R30
• V
DACP0(F/S)
1024
V/DACLSB (7)