LTC2970-1 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC2970-1' PDF : 36 Pages View PDF
LTC2970/LTC2970-1
APPLICATIO S I FOR ATIO
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 VDC0,NOM, and
solve for R10.
VDC0,NOM is the desired output voltage of the DC/DC con-
verter when the LTC2970’s VOUT0 pin is in a high impedance
state. VFB0 is the voltage at the converter’s feedback node
when the loop is in regulation, and IFB0 is the feedback
node’s input current.
R10 =
R20 • VFB0
(1)
VDC,NOM − IFB0 •R20 − VFB0
2. Solve for the maximum value of R30 that yields the
maximum required DC/DC converter output voltage
VDC0,max.
When VOUT0 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)
VDC,MAX − VDC,NOM
3. Solve for the minimum value of R40 that’s needed
to yield the minimum required DC/DC converter output
voltage VDC0,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.
( ) R40 ≥
VDC,NOM − VDC,MIN
•
R30
R20
+
VFB
+
10mV
(3)
236μA
4. Re-calculate the minimum, nominal, and maximum
DC/DC converter output voltages and the resulting mar-
gining resolution.
28
VDC0,NOM
=
VFB
•
⎛
⎝⎜
1+
R20
R10
⎞
⎠⎟
+ IFB
• R20
(4)
VDC0,MIN
≤
VDC0,NOM
−
R20
R30
•
(5)
(R40 • 236μA − VFB0 − 10mV)
( ) VDC0,MAX
≥
VDC0,NOM
+
R20
R30
•
VFB0 − 10mV
(6)
The margining resolution is bounded by:
R20 •R40 • 276μA
VRES ≤ R30 256
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 VOUT0 pin connects directly to the TRIM
pin through resistor R30 and the IOUT0 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 specified independently.
Following power-up, the LTC2970's VOUT0 pin defaults to a
high-impedance state. If the soft-connect feature is used,
VIN
VO+
TRIM
DC/DC
CONVERTER
VSENSE+
ON/OFF
VSENSE–
VO–
8V TO 15V
R30
+
LOAD VDC0
–
12VIN
VDD
1/2 LTC2970
GPIO_CFG
VOUT0
VIN0_AP
ALERT
SCL
SDA
IOUT0
R40
GPIO_0
VIN0_AM
REF
GND ASEL0 ASEL1
0.1μF
0.1μF
I2C BUS
0.1μF
Figure 2. LTC2970 Application Circuit for
DC/DC Converters with a TRIM Pin
29701 F02
29701fc
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