Datasheet

LTC2977
84
2977fa
For more information www.linear.com/LTC2977
APPLICATIONS INFORMATION
Trimming and Margining DC/DC Converters with a
TRIM Pin
Figure 24 illustrates a typical application circuit for
trimming/margining the output voltage of a DC/DC
converter with a TRIM Pin. The LTC2977’s V
DACP0
pin
connects to the TRIM pin through resistor R30, and the
V
DACM0
pin is connected to the converter’s point-of-load
ground. For this configuration, set the DAC polarity bit
Mfr_config_ dac_pol in MFR_CONFIG_LT C 2977 to 1.
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
Δ
DOWN
%
– R
TRIM
(8)
R
TRIM_UP
=
R
TRIM
•
V
DC
• 100+ Δ
UP
%
( )
2 • V
REF
• Δ
UP
%
–
50
Δ
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.
Tw o -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 24).
1. Solve for R30:
R30 ≤ R
TRIM
•
50 – Δ
DOWN
%
Δ
DOWN
%






(10)
2. Calculate the maximum required output voltage for
V
DACP0
:
V
DAC
P0
≥ 1+
Δ
UP
%
Δ
DOWN
%






• V
REF
(11)
Note: Not all DC/DC’s converters follow these trim equa-
tions especially newer
bricks. Consult LT C Field Application
Engineering.
Measuring Current
Odd numbered ADC channels may be used to measure
supply current. Set the ADC to high resolution mode to
configure for current measuring and improve sensitivity.
Note that no OV or UV faults or warnings are reported in
this mode, but telemetry is available from the READ_VOUT
command using the 11-bit signed mantissa plus 5-bit
signed exponent L11 data format. Set the MFR_CONFIG_
Figure 24. Application Circuit for DC/DC Converters with Trim Pin
V
PWR
V
DD33
V
DD33
V
DD25
V
IN_SNS
V
DACP0
V
SENSEP0
V
DACM0
V
SENSEM0
V
OUT_EN0
LTC2977*
0.1µF
0.1µF
V
IN
4.5V < V
IBUS
< 15V
GND
*SOME DETAILS OMITTED FOR CLARITY
ONLY ONE OF EIGHT CHANNELS SHOWN
V
IN
V
OUT
+
TRIM
2977 F24
ON/OFFB
V
SENSE
–
V
SENSE
+
GND
DC/DC
CONVERTER
LOAD
R30
0.1µF