Datasheet

LTC2974
87
2974fc
For more information www.linear.com/LTC2974
applicaTions inForMaTion
DC/DC converters with a TRIM pin may be margined
high or low by connecting an external resistor between
the TRIM pin and either the V
SENSEP
or V
SENSEM
pin. The
relationships between these resistors and the ∆% change
in the output voltage of the DC/DC converter are typically
expressed as:
R
TRIM_DOWN
=
R
TRIM
• 50
∆
DOWN
%
– R
TRIM
(8)
R
TRIM_UP
=
R
TRIM
•
V
DC
• 100 + ∆
UP
%
( )
2 • V
REF
• ∆
UP
%
–
50
∆
UP
%






– 1






(9)
where R
TRIM
is the resistance looking into the TRIM pin,
V
REF
is the TRIM pin’s open-circuit output voltage and
V
DC
is the DC/DC converter’s nominal output voltage.
∆
UP
% and ∆
DOWN
% denote the percentage change in the
converter’s output voltage when margining up or down,
respectively.
Two-Step Resistor and DAC Full-Scale Voltage Selection
Procedure for DC/DC Converters with a TRIM Pin
The following two-step procedure should be used to cal
-
culate the resistor value for R30 and the required full-scale
DAC voltage (refer to Figure 32).
1.
Solve for R30:
R30 ≤ R
TRIM
•
50 – ∆
DOWN
%
∆
DOWN
%






(10)
2. Calculate the maximum required output voltage for
V
DAC0
:
V
DAC
0
≥ 1+
∆
UP
%
∆
DOWN
%






• V
REF
(11)
Note: Not all DC/DC converters follow these trim equations,
especially newer bricks. Consult LTC Field Application
Engineering.
Figure 32. Application Circuit for DC/DC Converters with Trim Pin
V
PWR
V
DD33
V
DD33
V
DD25
V
DD25
V
IN_SNS
V
DAC0
V
SENSEP0
V
SENSEM0
V
OUT_EN0
LTC2974*
0.1µF
0.1µF
4.5V < V
IBUS
< 15V
GND
*SOME DETAILS OMITTED FOR CLARITY
ONLY ONE OF FOUR CHANNELS SHOWN
V
IN
V
O
+
R30
2974 F32
ON/OFFB
V
SENSE
–
V
SENSE
+
TRIM
V
O
–
DC/DC
CONVERTER
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