LTC2920-1/LTC2920-2
10
292012fa
APPLICATIO S I FOR ATIO
WUU
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For any desired V
MARGIN
:
I
TRIM
= V
MARGIN
/K
TRIM
R
SET
can now be calculated for the LTC2920.
For 5μA ≤ I
TRIM
≤ 167μA:
R
SET
= 1V/I
TRIM
Connect R
SET
between the R
S
pin and the LTC2920 ground
pin.
For 167μA < I
TRIM
≤ 2mA:
R
SET
= 1V/(I
TRIM
/30)
Connect R
SET
between the R
S
pin and the LTC2920 V
CC
pin.
If I
TRIM
falls outside of this range, the LTC2920 cannot be
used for this application.
The LTC2920 can source or sink current only when the
voltage at the I
M
pin is between 0.6 and (V
CC
– 0.6) volts.
In order to be sure that the LTC2920 will operate correctly
in this application, ensure that the V
T
node will stay within
these limits. To do this, calculate the effective output
resistance of the power supply module’s trim output pin,
R
VT
(refer to Figure 10). Using the measurements taken
above, the open circuit voltage is:
V
REF
= V
TNOM
To calculate R
VT
, subtract the untrimmed V
TNOM
and
trimmed V
TTRIM
voltages measured above:
V
TDELTA
= V
TNOM
– V
TTRIM
The effective TRIM pin source resistance can then be
calculated by:
R
VT
= V
TDELTA
/I
TRIM
The voltage at the LTC2920 I
MARGIN
pin for any I
TRIM
can
now calculated for both voltage margin directions. Refering
to Figure 10:
V
TSINK
= V
REF
– (R
VT
• I
TRIM
)
V
TSOURCE
= V
REF
+ (R
VT
• I
TRIM
)
Note: be sure to use these equations to verify that V
TSINK
and V
TSOURCE
are within LTC2920 V
M
voltages specified in
Even though the manufacturer does not directly supply the
equation for the trim current, a simple measurement can
be made to calculate an equation for V
TRIM
as a function
of I
TRIM
.
To do this, select the trim resistor configuration which
places the trim resistor between the trim pin and ground
(see Figure 10).
With the trim resistor connected to ground, note the
direction of the power module output voltage change. This
is the direction that the power module output voltage will
change when the LTC2920 IN control pin is HIGH, above
V
IH
. Remember that the direction of the voltage trim for
this configuration can vary among power modules, even
among power modules from the same manufacturer.
Calculate a resistor value from the manufacturer’s equa-
tion, or select it from a chart (if a chart is supplied by the
manufacturer). Pick a value near the middle of the trim
resistor range. Obtain and measure the selected resistor
with an ohmmeter or use a precision 0.1% resistor.
Knowing the correct value of this resistance is critical to
obtaining good results. Make provisions to connect and
disconnect this test resistor between the trim pin and the
power supply module’s negative output pin. (Figure 10.)
Carefully follow all other manufacturer’s application notes
regarding power supply input voltage, minimum and
maximum output voltages, sense pin connections (if any),
minimum and maximum current loads, etc. Failure to do
so may permanently damage the power supply module!
Apply the specified input voltage to the power supply
module. Measure the power supply output voltage V
PS
and the V
T
voltages before and after connecting the trim
resistor.
Subtract the untrimmed (V
PSNOM
) and trimmed (V
PSTRIM
)
power supply output voltages to obtain the trim voltage
(V
DELTA
):
V
DELTA
= V
PSNOM
– V
PSTRIM
and the trim current:
I
TRIM
= V
TRIM
/R
TRIM
Calculate the linear current trim constant K
TRIM
:
K
TRIM
= V
DELTA
/I
TRIM
