LTC2970-1_15 Datasheet, PDF (28/38 Pages) Linear Technology – Dual I2C Power Supply Monitor and Margining Controller

LTC2970/LTC2970-1
Applications Information
4-Step Resistor Selection Procedure for DC/DC
Converters with External Feedback Resistors
V_DC0,NOM
=
V_FB
•


1+
R20
R10


+I_FB
•R20
(4)
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.
V_DC0,MIN
≤
V_DC0,NOM
−
R20
R30
•
(5)
1. Assume values for feedback resistor R20 and the
(R40•236µA − V_FB0 −10mV)
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
( ) V_DC0,MAX
≥
V_DC0,NOM
+
R20
R30
•
V_FB0 − 10mV
(6)
converter when the LTC2970’s V_OUT0 pin is in a high The margining resolution is bounded by:
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.
V_RES
≤
R20
R30
•R40•
256
276µA
volts/DAC LSB
(7)
R10
=
R20• V_FB0
V_DC,NOM −I_FB0 •R20 −
V_FB0
(1)
Margining DC/DC Converters with a TRIM Pin
2. Solve for the maximum value of R30 that yields the maxi-
mum 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.
R30 ≤ ^{R20•(VFB −10mV)}
(2)
V_DC,MAX − V_DC,NOM
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 data sheet minimum of 236µA.
( ) _{R40 ≥}
V_DC,NOM − V_DC,MIN
•
R30
R20
+
V_FB
+
10mV
(3)
236µA
4. Re-calculate the minimum, nominal, and maximum DC/
DC converter output voltages and the resulting margining
resolution.
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 through R40. Resistors
R30 and R40 give this application circuit two degrees of
freedom so that the margin-up and margin-down percent-
ages can be specified independently.
Following power-up, the LTC2970's V_OUT0 pin defaults
to a high-impedance state. If the soft-connect feature
V_IN
VO+
TRIM
DC/DC
CONVERTER
V_SENSE+
ON/OFF
V_SENSE–
VO–
8V TO 15V
R30
+
LOAD V_DC0
–
12V_IN
V_DD
1/2 LTC2970
GPIO_CFG
V_OUT0
V_{IN0_AP}
ALERT
SCL
SDA
I_OUT0
R40
GPIO_0
V_{IN0_AM}
REF
GND ASEL0 ASEL1
0.1µF
0.1µF
I²C BUS
0.1µF
29701 F02
Figure 2. LTC2970 Application Circuit for
DC/DC Converters with a TRIM Pin
29701fd
28
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