Datasheet

LTC2970/LTC2970-1
28
29701fc
APPLICATIO S I FOR ATIO
WUU
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4-Step Resistor Selection Procedure for DC/DC
Converters with External Feedback Resistors
The following 4-step procedure should be used to quickly
calculate the resistor values shown for the Typical Applica-
tion Circuit shown in Figure 1.
1. Assume values for feedback resistor R20 and the
nominal DC/DC converter output voltage V
DC0,NOM
, and
solve for R10.
V
DC0,NOM
is the desired output voltage of the DC/DC con-
verter when the LTC2970’s V
OUT0
pin is in a high impedance
state. V
FB0
is the voltage at the converter’s feedback node
when the loop is in regulation, and I
FB0
is the feedback
node’s input current.
R
RV
VIRV
FB
DC NOM FB FB
10
20
20
0
00
=
•
−•−
,
(1)
2. Solve for the maximum value of R30 that yields the
maximum required DC/DC converter output voltage
V
DC0,max
.
When V
OUT0
is at 0V, the output of the DC/DC converter
is at its maximum voltage. Note that the 10mV term cor-
responds to the maximum offset voltage of the IDAC 1X
voltage buffer.
R
RV mV
VV
FB
DC MAX DC NOM
30
20 10
≤
•−
()
−
,,
(2)
3. Solve for the minimum value of R40 that’s needed
to yield the minimum required DC/DC converter output
voltage V
DC0,MIN
.
The DC/DC converter output voltage will be a minimum
when IDAC0 is at its full-scale current. In order to guarantee
that R40 is large enough, assume that IDAC0’s full-scale
current is at the datasheet minimum of 236μA.
R
VV
R
R
VmV
A
DC NOM DC MIN FB
40
30
20
10
236
≥
−
()
•++
,,
μ
(3)
4. Re-calculate the minimum, nominal, and maximum
DC/DC converter output voltages and the resulting mar-
gining resolution.
VV
R
R
IR
DC NOM FB FB0
1
20
10
20
,
=•+
⎛
⎝
⎜
⎞
⎠
⎟
+•
(4)
VV
R
R
RAVm
DC MIN DC NOM
FB
00
0
20
30
40 236 10
,,
≤−•
•−−μ VV
()
(5)
VV
R
R
VmV
DC MAX DC NOM FB00 0
20
30
10
,,
≥+•−
()
(6)
The margining resolution is bounded by:
V
R
R
RA
RES
≤
••
20
30
40 276
256
μ
volts/DAC LSB (7)
Margining DC/DC Converters with a TRIM Pin
Figure 2 illustrates a typical application circuit for margining
the output voltage of a DC/DC converter with a TRIM Pin.
The LTC2970’s V
OUT0
pin connects directly to the TRIM
pin through resistor R30 and the I
OUT0
pin is terminated
at the converter's point-of-load ground throught R40.
Resistors R30 and R40 give this application circuit two
degrees of freedom so that the margin-up and margin-
down percentages can be specifi ed independently.
Following power-up, the LTC2970's V
OUT0
pin defaults to a
high-impedance state. If the soft-connect feature is used,
Figure 2. LTC2970 Application Circuit for
DC/DC Converters with a TRIM Pin
VO+
V
SENSE–
V
SENSE+
R40
0.1μF
0.1μF
TRIM
1/2 LTC2970
V
OUT0
V
IN0_AP
V
DC0
+
–
I
OUT0
V
IN0_AM
ALERT
SCL I
2
C BUS
SDA
GPIO_0
REF
12V
IN
GND ASEL0 ASEL1
LOAD
V
DD
GPIO_CFG
DC/DC
CONVERTER
VO–
R30
29701 F02
ON/OFF
V
IN
8V TO 15V
0.1μF