ADP3198 datasheet(29/32 Pages) AD

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ADP3198

Rev. A | Page 29 of 32

These values correspond to a thermistor temperature of ~100°C

and ~110°C when using the same type of 100 kΩ NTC thermistor

used in the current sense amplifier.

where:

VIN(MAX) is the maximum voltage from the 12 V input supply (if

the 12 V input supply is 12 V ± 5%,

VIN(MAX) = 12.6 V; if the 12 V

input supply is 12 V ± 10%,

VIN(MAX) = 13.2 V).

VCC(MIN) is the minimum VCC voltage of the ADP3198. This is

specified as 4.75 V.

RSHUNT is the shunt resistor value.

An additional fixed resistor in parallel with the thermistor allows

tuning of the trip point temperatures to match the hottest tem-

perature in the VR, when the thermistor itself is directly sensing

a proportionately lower temperature. Setting this resistor value

is best accomplished with a variable resistor during thermal

validation and then fixing this value for the final design.

The CECC standard specification for power rating in surface

mount resistors is: 0603 = 0.1 W, 0805 = 0.125 W, 1206 = 0.25 W.

Additionally, a 0.1 μF capacitor should be used for filtering noise.

TUNING THE ADP3198

SHUNT RESISTOR DESIGN

Build a circuit based on the compensation values

computed from the design spreadsheet.

Hook up the dc load to the circuit, turn it on, and verify its

operation. Also, check for jitter at no load and full load.

The ADP3198 uses a shunt to generate 5 V from the 12 V

supply range. A trade-off can be made between the power

dissipated in the shunt resistor and the UVLO threshold.

Figure 16 shows the typical resistor value needed to realize

certain UVLO voltages. It also gives the maximum power

dissipated in the shunt resistor for these UVLO voltages.

DC Load Line Setting

Measure the output voltage at no load (VNL). Verify that it

is within tolerance.

550

150

7.0

11.0

VIN (UVLO)

500

450

400

350

300

250

200

0.50

0.10

0.45

0.40

0.35

0.30

0.25

0.20

0.15

7.5

8.0

8.5

9.0

9.5

10.0

10.5

RSHUNT

PSHUNT

Measure the output voltage at full load cold (

VFLCOLD). Let

the board sit for ~10 minutes at full load, and then measure

the output (

VFLHOT). If there is a change of more than a few mV,

adjust

RCS1 and RCS2 using Equation 46 and Equation 48.

()

FLHOT

FLCOLD

OLD

CS2

NEW

CS2

−

(46)

Repeat Step 4 until the cold and hot voltage measurements

remain the same.

Measure the output voltage from no load to full load using

5 A steps. Compute the load line slope for each change, and

then average to get the overall load line slope (ROMEAS).

Figure 16. Typical Shunt Resistor Value and Power Dissipation

for Different UVLO Voltage

The maximum power dissipated is calculated using Equation 45.

(

)

SHUNT

MIN

MAX

)

(

)

(

−

(45)

ROMEAS is off from RO by more than 0.05 mΩ, use

Equation 47 to adjust the

RPH values.

()

OMEAS

OLD

NEW

(47)

Repeat Step 6 and Step 7 to check the load line. Repeat

adjustments if necessary.

When the dc load line adjustment is complete, do not

change

RPH, RCS1, RCS2, or RTH for the remainder of the

procedure.

10. Measure the output ripple at no load and full load with

a scope, and make sure it is within specifications.

()

(

)

()

(

)

(

)

()

(

)

()

−

OLD

CS1

NEW

CS2

OLD

CS1

OLD

CS1

OLD

CS1

NEW

CS1

(48)

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