ADP3430

http://onsemi.com

21

DC gain = 120 dB, Equation 30 to Equation 33 are intended

to yield an optimal starting point for the design; some

adjustments may be necessary to account for PCB and

component parasitic effects (see the Tuning the ADP3430

section).

First, compute the time constants for all the poles and zeros in the system using Equation 30 to Equation 33.

1

1

(eq. 30)

1

1.24 kW

10

120

20

+ 806 pF

(eq. 31)

1

0.5

+

1

0.5

1

6

2 p

300 kHz

806 pF

+ 7.9 kW

(eq. 32)

1

1.5

+

1

1.5

2 p

300 kHz

7.9 kW

+ 44 pF

(eq. 33)

1

2

+

1

2

1

6

2 p

300 kHz

1.24 kW

+ 1.28 nF

Figure 9 and Figure 10 show the typical transient response

using these compensation values.

Figure 9. Typical Transient Response for Design

Example Load Step

Figure 10. Typical Transient Response for Design

Example Load Release

1−Vo, 3−COMP, 4−TRDET, D0~D2−PWM1~3

In continuous inductor current mode, the source current of

the high­side MOSFET is approximately a square wave with

a duty ratio equal to n

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:

1

N

D *

1

(eq. 34)

1

3

0.1266 *

1 + 9.1 A

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 three 680

mF, 16 V aluminum

electrolytic capacitors and twelve 4.7

mF ceramic capacitors.

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

recommended maximum of 0.1 A/

ms, 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

The thermistor is used on the TTSENSE input of the

ADP3430 for monitoring the temperature of the VR. A

constant current of 123

mA is sourced out of this pin and runs

through a thermistor network such as the one shown in

Figure 11.