ltc2978 Linear Technology Corporation, ltc2978 Datasheet - Page 57

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

Trimming and Margining DC/DC Converters
with a TRIM Pin
Figure 17 illustrates a typical application circuit for trim-
ming/margining the output voltage of a DC/DC converter
with a TRIM Pin. The LTC2978’s V
the TRIM pin through resistor R30, and the V
connected to the converter’s point-of-load ground.
DC/DC converters with a TRIM pin are typically margined
high or low by connecting an external resistor between
the TRIM pin and either the V
relationships between these resistors and the ∆% change
in the output voltage of the DC/DC converter are typically
expressed as:
where R
V
V
applicaTions inForMaTion
REF
DC
R
R
R R
TRIM DOWN
TRIM UP
TRIM
is the DC/DC converter’s nominal output voltage.
is the TRIM pin’s open-circuit output voltage and
TRIM
_
_
•




V
=
DC
is the resistance looking into the TRIM pin,
2
•
=
•
V
(
R
REF
100
∆
TRIM
DOWN
•
+
∆
∆
•
UP
4.5V < V
%
50
UP
%
SENSEP
%
–
Figure 17. Application Circuit for DC/DC Converters with Trim Pin
IBUS
R
)
–
TRIM
DACPn
< 15V



V
or V
0.1µF
0.1µF
∆
IN
UP
50
SENSEM
pin connects to
%


 
V
V
V
V
DACM0
PWR
DD33
DD33
DD25
–
1
pin. The




LTC2978*
pin is
GND
( )
( )
8
9
V
V
V
SENSEM0
OUT_EN0
*SOME DETAILS OMITTED FOR CLARITY
ONLY ONE OF EIGHT CHANNELS SHOWN
V
SENSEP0
V
V
IN_SNS
DACM0
DACP0
∆
converter’s output voltage when margining up or down,
respectively.
2-Step Resistor and DAC Full-Scale Voltage Selection
Procedure for DC/DC Converters with a TRIM Pin
The following 2-step procedure should be used to calculate
the resistor value for R30 and the required full-scale DAC
voltage (refer to Figure 17).
1. Solve for R30:
2. Calculate the maximum required output voltage for
UP
V
R
V
% and ∆
DACP0
30
DACP
≤
LOAD
0
R
:
≥
TRIM
DOWN



R30
1
+
•
% denote the percentage change in the



∆
V
V
TRIM
V
V
ON/OFFB
50
∆
DOWN
IN
OUT
SENSE
CONVERTER
SENSE
UP
DC/DC
∆
+
–
GND
DOWN
%
+
–
∆
%
2978 F17
DOWN



%
•
V
%
REF



LTC2978

( )
2978fa
( )
10
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