Datasheet

LTC2974
86
2974fc
For more information www.linear.com/LTC2974
applicaTions inForMaTion
Figure 31. Application Circuit for DC/DC Converters with External Feedback Resistors
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
OUT
R20
R30
R10
2974 F31
RUN/SS
SGND
V
FB
GND
DC/DC
CONVERTER
LOAD
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
DAC0
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
DAC0
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
DAC0(F/S)
output voltage:
V
DAC0(F/S)
> V
DC(NOM)
– V
DC(MIN)
( )
•
R30
R20
+ V
FB
(3)
4. Re-calculate the minimum, nominal, and maximum
DC/DC converter output voltages and the resulting
margining resolution.
V
DC(NOM)
= V
FB
• 1+
R20
R10






+I
FB
• R20 (4)
V
DC(MIN)
= V
DC(NOM)
–
R20
R30
• V
DAC0(F /S)
– V
FB
( )
(5)
V
DC(MAX)
= V
DC(NOM)
+
R20
R30
• V
FB
(6)
V
RES
=
R20
R30
• V
DAC0(F /S)
1024
V/DAC LSB (7)
Trimming and Margining DC/DC Converters with a
TRIM Pin
Figure 32 illustrates a typical application circuit for trim
-
ming/margining the output voltage of a DC/DC converter
with a TRIM Pin. The LTC2974’s V
DAC0
pin connects to
the TRIM pin through resistor R30. For this configuration,
set the DAC polarity bit Mfr_config_dac_pol in MFR_CON
-
FIG_LTC2974 to 1.