MAX8772 datasheet(22/47 Pages) MAXIM

22 / 47 page

MAX8770/MAX8771/MAX8772 Dual-Phase, Quick-

PWM Controller for IMVP-6+ CPU Core Power Supplies

______________________________________________________________________________________

CONFIDENTIAL INFORMATION – RESTRICTED TO INTEL® IMVP-6 LICENSEES

CCI, allowing adjustment of the integration time con-

stant with a compensation network connected between

CCI and FB. The resulting compensation current and

voltage are determined by the following equations:

ICCI = GM (VCMP - VCMN) - GM (VCSP - VCSN)

VCCI = VFB + ICCIZCCI

where ZCCI is the impedance at the CCI output. The

secondary on-time one-shot uses this integrated signal

(VCCI) to set the secondary high-side MOSFETs on-

time. When the main and secondary current-sense sig-

nals (VCM = VCMP - VCMN and VCS = VCSP -VCSM)

become unbalanced, the transconductance amplifiers

adjust the secondary on-time, which increases or

decreases the secondary inductor current until the cur-

rent-sense signals are properly balanced:

= (Main On-Time)

+ (Secondary Current Balance Correction)

This algorithm results in a nearly constant switching fre-

quency and balanced inductor currents, despite the lack

of a fixed-frequency clock generator. The benefits of a

constant switching frequency are twofold: first, the fre-

quency can be selected to avoid noise-sensitive regions

such as the 455kHz IF band; second, the inductor ripple-

current operating point remains relatively constant,

resulting in easy design methodology and predictable

output voltage ripple. The on-time one-shots have good

accuracy at the operating points specified in the

Electrical Characteristics table. On-times at operating

points far removed from the conditions specified in the

Electrical Characteristics table can vary over a wider

range. For example, the 600kHz setting typically runs

about 5% slower, with inputs much greater than +12V

due to the very short on-times required.

On-times translate only roughly to switching frequen-

cies. The on-times guaranteed in the Electrical

Characteristics table are influenced by switching

delays in the external high-side MOSFET. Resistive

losses, including the inductor, both MOSFETs, output

capacitor ESR, and PC board copper losses in the out-

put and ground tend to raise the switching frequency at

higher output currents. Also, the dead-time effect

increases the effective on-time, reducing the switching

frequency. It occurs only during forced-PWM operation

and dynamic output-voltage transitions when the induc-

tor current reverses at light or negative load currents.

With reversed inductor current, the inductor’s EMF

causes LX to go high earlier than normal, extending the

on-time by a period equal to the DH-rising dead time.

For loads above the critical conduction point, where the

dead-time effect is no longer a factor, the actual switch-

ing frequency (per phase) is:

where VDIS is the sum of the parasitic voltage drops in

the inductor discharge path, including synchronous

rectifier, inductor, and PC board resistances; VCHG is

the sum of the parasitic voltage drops in the inductor

charge path, including high-side switch, inductor, and

PC board resistances; and tON is the on-time as deter-

mined above.

Current Sense

The output current of each phase is sensed. Low-offset

amplifiers are used for current balance, voltage-posi-

tioning gain, and current limit. Sensing the current at

the output of each phase offers advantages, including

less noise sensitivity, more accurate current sharing

between phases, and the flexibility of using either a

current-sense resistor or the DC resistance of the out-

put inductor.

Using the DC resistance (RDCR) of the output inductor

allows higher efficiency. In this configuration, the initial

tolerance and temperature coefficient of the inductor’s

DCR must be accounted for in the output-voltage

droop-error budget and power monitor. This current-

sense method uses an RC filtering network to extract

the current information from the output inductor (see

Figure 3). The resistive divider used should provide a

current-sense resistance (RCS) low enough to meet the

current-limit requirements, and the time constant of the

RC network should match the inductor’s time constant

(L/RCS):

where RCS is the required current-sense resistance,

and RDCR is the inductor’s series DC resistance. Use

the worst-case inductance and RDCR values provided

by the inductor manufacturer, adding some margin for

the inductance drop over temperature and load. To

and

DCR

⎛

⎝⎜

⎞

⎠⎟

⎡

⎣⎢

⎤

⎦⎥

OUT

DIS

CHG

()

+−

()

ON SEC

CCI

CCI CCI

()

⎛

⎝⎜

⎞

⎠⎟

⎛

⎝⎜

⎞

⎠⎟

⎛

⎝⎜

⎞

⎠⎟

0 075

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