LTC2978
APPLICATIONS INFORMATION
APPLICATION CIRCUITS
V
is the output voltage of the DC/DC converter
DC(NOM)
when the LTC2978’s V
pin is in a high impedance
DACP±
Trimming and Margining DC/DC Converters with
External Feedback Resistors
state. R1± is a function of R2±, V
, the voltage at
DC(NOM)
the feedback node (V ) when the loop is in regulation,
FB
and the feedback node’s input current (I ).
FB
Figure 23 shows a typical application circuit for trimming/
margining a power supply with an external feedback
R2± • VFB
R1± =
(1)
network. The V
and V
differential inputs
SENSEP±
SENSEM±
VDC(NOM) –IFB • R2± – V
FB
sense the load voltage directly, and a correction voltage
is developed between the V and V pins by the
DACP±
DACM±
is Kelvin connected
2. Solve for the value of R3± that yields the maximum
required DC/DC converter output voltage V
closed-loop servo algorithm. V
DACM±
.
DC(MAX)
to the point-of-load GND in order to minimize the effects
of load induced grounding errors. The V output
WhenV
is at its maximum voltage.
isat±V, theoutputoftheDC/DCconverter
DACP±
DACP±
is connected to the DC/DC converter’s feedback node
through resistor R3±. For this configuration, set b[±] in
MFR_CONFIG_LTC2978 = ±.
R2± • VFB
R3± ≤
(2)
VDC(MAX) – VDC(NOM)
Four-Step Resistor Selection Procedure for DC/DC
Converters with External Feedback Resistors
3. Solve for the minimum value of V
that’s needed
DACP±
to yield the minimum required DC/DC converter output
The following four-step procedure should be used to
calculate the resistor values required for the application
circuit shown in Figure 23.
voltage V
.
DC(MIN)
The DAC has two full-scale settings, 1.38V and 2.6ꢀV.
In order to select the appropriate full-scale setting,
calculate the minimum required V
voltage:
1. AssumevaluesforfeedbackresistorR2±andthenominal
output
DACP±(F/S)
DC/DC converter output voltage V
for R1±.
, and solve
DC(NOM)
R3±
R2±
VDACP±(F /S) > VDC(NOM) – VDC(MIN)
•
+ VFB
(
)
(3)
V
IN
V
V
IN
4.5V < V
< 15V
IBUS
V
V
PWR
IN_SNS
OUT
0.1µF
V
DACP0
0.1µF
R30
DC/DC
CONVERTER
V
V
V
V
SENSEP0
DD33
DD33
DD25
R20
R10
LTC2978*
V
LOAD
FB
V
DACM0
0.1µF
V
SGND
SENSEM0
V
RUN/SS
OUT_EN0
GND
2978 F23
GND
*SOME DETAILS OMITTED FOR CLARITY
ONLY ONE OF EIGHT CHANNELS SHOWN
Figure 23. Application Circuit for DC/DC Converters with External Feedback Resistors
2978fc
69
Linear Systems [ Linear Systems ]
