ADP3198 datasheet(28/32 Pages) AD

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ADP3198

Rev. A | Page 28 of 32

The compensation values can then be solved using

524

μs

(40)

524

μs

(41)

560

(42)

(43)

These are the starting values prior to tuning the design that

account for layout and other parasitic effects (see the Tuning the

ADP3198 section). The final values selected after tuning are

CA = 560 pF

RA = 10.0 kΩ

CB = 560 pF

CFB = 27 pF

Figure 13 and Figure 14 show the typical transient response

using these compensation values.

CH1 50mV

M 10µs

A CH1

–36mV

Figure 13. Typical Transient Response for Design Example Load Step

CH1 50mV

M 10µs

A CH1

–36mV

Figure 14. Typical Transient Response for Design Example Load Release

CIN SELECTION AND INPUT CURRENT

di/dt REDUCTION

In continuous inductor current mode, the source current of the

high-side MOSFET is approximately a square wave with a duty

ratio equal to n × VOUT/VIN and an amplitude of one-nth the

maximum output current. To prevent large voltage transients,

a low ESR input capacitor, sized for the maximum rms current,

must be used. The maximum rms capacitor current is given by

14.7

0.108

108

−

CRMS

(44)

The capacitor manufacturer’s ripple-current ratings are often

based on only 2000 hours of life. As a result, it advisable to

further derate the capacitor or to choose a capacitor rated at a

higher temperature than required. Several capacitors can be

placed in parallel to meet size or height requirements in the

design. In this example, the input capacitor bank is formed by

two 2700 μF, 16 V aluminum electrolytic capacitors and eight

4.7 μF ceramic capacitors.

To reduce the input current di/dt to a level below the recom-

mended maximum of 0.1 A/μs, an additional small inductor

(L > 370 nH at 18 A) should be inserted between the converter

and the supply bus. This inductor also acts as a filter between

the converter and the primary power source.

THERMAL MONITOR DESIGN

A thermistor is used on the TTSENSE input of the ADP3198

for monitoring the temperature of the VR. A constant current

of 123 μA is sourced out of this pin and runs through a

thermistor network such as the one shown in Figure 15.

VRHOT

VRFAN

TTSENSE

ADP3198

0.1µF

RTTSENSE

OPTIONAL

TEMPERATURE

ADJUST RESISTOR

PLACE

THERMISTOR

NEAR CLOSEST

PHASE

Figure 15. VR Thermal Monitor Circuit

A voltage is generated from this current through the thermistor

and sensed inside the IC. When the voltage reaches 1.105 V,

the VRFAN output gets set. When the voltage reaches 0.81 V,

the VRHOT gets set. This corresponds to RTTSENSE values of

8.98 kΩ for VRFAN and 6.58 kΩ for VRHOT.

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