LTC2920-1CS5 datasheet(10/16 Pages) LINER

Part #	LTC2920-1CS5

Description	Single/Dual Power Supply Margining Controller

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Manufacturer	LINER [Linear Technology]

Direct Link	http://www.linear.com

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LTC2920-1/LTC2920-2

10

292012fa

APPLICATIO S I FOR ATIO

For any desired VMARGIN:

ITRIM = VMARGIN/KTRIM

RSET can now be calculated for the LTC2920.

For 5

μA ≤ ITRIM ≤ 167μA:

RSET = 1V/ITRIM

Connect RSETbetween the RS pin and the LTC2920 ground

pin.

For 167

μA < ITRIM ≤ 2mA:

RSET = 1V/(ITRIM/30)

Connect RSET between the RS pin and the LTC2920 VCC

pin.

If ITRIM 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 IM pin is between 0.6 and (VCC – 0.6) volts.

In order to be sure that the LTC2920 will operate correctly

in this application, ensure that the VT node will stay within

these limits. To do this, calculate the effective output

resistance of the power supply module’s trim output pin,

RVT (refer to Figure 10). Using the measurements taken

above, the open circuit voltage is:

VREF = VTNOM

To calculate RVT, subtract the untrimmed VTNOM and

trimmed VTTRIM voltages measured above:

VTDELTA = VTNOM – VTTRIM

The effective TRIM pin source resistance can then be

calculated by:

RVT = VTDELTA/ITRIM

The voltage at the LTC2920 IMARGIN pin for any ITRIM can

now calculated for both voltage margin directions. Refering

to Figure 10:

VTSINK = VREF – (RVT • ITRIM)

VTSOURCE = VREF + (RVT • ITRIM)

Note: be sure to use these equations to verify that VTSINK

and VTSOURCE are within LTC2920 VM 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 VTRIM as a function

of ITRIM.

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

VIH. 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 VPS

and the VT voltages before and after connecting the trim

resistor.

Subtract the untrimmed (VPSNOM) and trimmed (VPSTRIM)

power supply output voltages to obtain the trim voltage

(VDELTA):

VDELTA = VPSNOM – VPSTRIM

and the trim current:

ITRIM = VTRIM/RTRIM

Calculate the linear current trim constant KTRIM:

KTRIM = VDELTA/ITRIM

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LTC2920-1CS5 Datasheet(PDF) 10 Page - Linear Technology

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