MAX8772 datasheet(21/47 Pages) MAXIM

21 / 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

phase operation by alternately triggering the main and

secondary phases after the error comparator drops

below the output voltage set point.

Dual 180

° Out-of-Phase Operation

The two phases in the MAX8770/MAX8771/MAX8772

operate 180

° out-of-phase to minimize input and output

filtering requirements, reduce electromagnetic interfer-

ence (EMI), and improve efficiency. This effectively low-

ers component count—reducing cost, board space,

and component power requirements— making the

MAX8770/MAX8771/MAX8772 ideal for high-power,

cost-sensitive applications.

Typically, switching regulators provide power using

only 1 phase instead of dividing the power among sev-

eral phases. In these applications, the input capacitors

must support high instantaneous current requirements.

The high RMS ripple current can lower efficiency due to

I2R® power loss associated with the input capacitor’s

effective series resistance (ESR). Therefore, the system

typically requires several low-ESR input capacitors in

parallel to minimize input voltage ripple, to reduce ESR-

related power losses, and to meet the necessary RMS

ripple current rating.

With the MAX8770/MAX8771/MAX8772, the controller

shares the current between two phases that operate

180

° out-of-phase, so the high-side MOSFETs never

turn on simultaneously during normal operation. The

instantaneous input current of either phase is effectively

halved, resulting in reduced input voltage ripple, ESR

power loss, and RMS ripple current (see the Input

Capacitor Selection section). Therefore, the same per-

formance may be achieved with fewer or less-expen-

sive input capacitors.

+5V Bias Supply (VCC and VDD)

The Quick-PWM controller requires an external +5V

bias supply in addition to the battery. Typically, this

+5V bias supply is the notebook’s 95% efficient +5V

system supply. Keeping the bias supply external to the

IC improves efficiency and eliminates the cost associat-

ed with the +5V linear regulator that would otherwise be

needed to supply the PWM circuit and gate drivers. If

stand-alone capability is needed, the +5V bias supply

can be generated with an external linear regulator.

The +5V bias supply must provide VCC (PWM con-

troller) and VDD (gate-drive power), so the maximum

current drawn is:

IBIAS = ICC - fSW (QG(LOW) + QG(HIGH))

where ICC is provided in the Electrical Characteristics

Table, fSW is the switching frequency, and QG(LOW)

and QG(HIGH) are the MOSFET data sheet’s total gate-

charge specification limits at VGS = 5V.

VIN and VDD can be tied together if the input power

source is a fixed +4.5V to +5.5V supply. If the +5V

bias supply is powered up prior to the battery supply,

the enable signal (SHDN going from low to high) must

be delayed until the battery voltage is present to

ensure startup.

Switching Frequency (TON)

Connect a resistor (RTON) between TON and VIN to set

the switching period TSW = 1/fSW, per phase:

TSW = CTON (RTON + 6.5k

Ω)

where CTON = 16.26pF.

A 96.75k

Ω to 303.25kΩ corresponds to switching peri-

ods of 167ns (600kHz) to 500ns (200kHz), respectively.

High-frequency (600kHz) operation optimizes the appli-

cation for the smallest component size, trading off effi-

ciency due to higher switching losses. This may be

acceptable in ultra-portable devices where the load

currents are lower and the controller is powered from a

lower voltage supply. Low-frequency (200kHz) opera-

tion offers the best overall efficiency at the expense of

component size and board space.

On-Time One-Shot

The core of each phase contains a fast, low-jitter,

adjustable one-shot that sets the high-side MOSFETs

on-time. The one-shot for the main phase varies the on-

time in response to the input and feedback voltages.

The main high-side switch on-time is inversely propor-

tional to the input voltage (VIN), and proportional to the

feedback voltage (VFB):

where the switching period (TSW = 1/fSW) is set by the

resistor at the TON pin, and 0.075V is an approximation

to accommodate the expected drop across the low-

side MOSFET switch.

The one-shot for the secondary phase varies the on-

time in response to the input voltage and the difference

between the main and secondary inductor currents.

Two identical transconductance amplifiers integrate the

difference between the master and slave current-sense

signals. The summed output is internally connected to

ON MAIN

()

0 075

I2R is a registered trademark of instruments for Research and

Industry, Inc.

MAX8772 датащи(PDF) 21 Page - Maxim Integrated Products

MAX8772 датащи(HTML) 21 Page - Maxim Integrated Products

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

датащи скачать

ссылки URL