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ADP3152AR

5-Bit Programmable Synchronous Switching Regulator Controller for Pentium® II Processor

Analog Devices 

ADP3152

CIN Selection and Input Current di/dt Reduction

In continuous-inductor-current mode, the source current of the

high side MOSFET is a square wave with a duty ratio of VO/

VIN. To keep the input ripple voltage at a low value, one or

more capacitors with low equivalent series resistance (ESR) and

adequate ripple-current rating must be connected across the

input terminals. The maximum rms current of the input bypass

capacitors is:

ICINRMS ≈ [VO(VIN – VO)]0.5 IOMAX /VIN = 7 Arms

Let us select the FA-type capacitor with 2700 µF capacitance

and 10 V voltage rating. The ESR of that capacitor is 34 mΩ

and the allowed ripple current at 100 kHz is 1.94 A. At 105°C

we would need to connect at least four such capacitors in paral-

lel to handle the calculated ripple current. At 50°C ambient,

however, the ripple current can be increased, so three capacitors

in parallel are adequate.

The ripple voltage across the three paralleled capacitors is:

VCINRPL = IOMAX [ESRIN/3 + DMAXHF /(3CIN fMIN )] Ϸ140 mV p-p

To further reduce the effect of the ripple voltage on the system

supply voltage bus and to reduce the input-current di/dt to

below the recommended maximum of 0.1 A/µs, an additional

small inductor (L > 1.7 µH @ 10 A) should be inserted between

the converter and the supply bus (see Figure 2).

Feedback Loop Compensation Design

To keep the peak-to-peak output voltage deviation as small as

possible, the low frequency output impedance (i.e., the output

resistance) of the converter should be made equal to the ESR of

the output capacitor. That can be achieved by having a single-pole

roll-off of the voltage gain of the gm error amplifier, where the pole

frequency coincides with the ESR zero of the output capacitor. A

gain with single-pole roll-off requires that the gm amplifier is termi-

nated by the parallel combination of a resistor and capacitor. The

required resistor value can be calculated from the equation:

( ) gm

36 × RSENSE

145 kΩʈRCOMP

= RE

where gm = 2.2 ms and the quantities 36 and 145 kΩ are charac-

teristic of the ADP3152. The calculated compensating resis-

tance is:

R1ʈR2 = RCOMP = 31 kΩ

The compensating capacitance is determined from the equality

of the pole frequency of the error amplifier gain and the zero

frequency of the impedance of the output capacitor.

CCOMP = RECOUT = 5 m × 16.2 mF = 2.6 nF

RCOMP

31 kΩ

In the application circuit we tested, we found that the compen-

sation scheme shown in Figure 2 gave the optimal response to

meet the Pentium II dc/dc static and transient specifications

with sufficient margins including the ADP3152’s initial error

tolerance, the PCB layout trace resistances, and the external

component parasitics. If we increase the load resistance to the

COMP pin, the static regulation will improve. The load transient

response, however, will get worse. In Figure 2, if we decrease the

R1 = 150 kΩ resistor vs. the R2 = 39 kΩ resistor, the regulation

band will shift positive in relation to the 2.8 V. If we increase

the R1 resistor, the regulation band will shift negative. It may be

necessary to adjust these resistor values to obtain the best static

and dynamic regulation compliance depending on the output

capacitor ESR and the parasitic trace resistances of the PCB

layout.

BOARD LAYOUT

A multilayer PCB is recommended with a minimum of two

copper layers. One layer on top should be used for traces inter-

connecting low power SMT components. The ground terminals

of those components should be connected with vias to the bot-

tom traces connecting directly to the ADP3152 ground pins.

One layer should be a power ground plane. If four layers are

possible, one additional layer should be an internal system

ground plane, and one additional layer can be used for other

system interconnections.

When laying out the printed circuit board, the following check-

list should be used to ensure proper operation of the ADP3152.

It is advisable to follow the evaluation board layout as closely as

possible. If necessary, contact Analog Devices Application Engi-

neering for layout suggestions.

–10–

REV. 0

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