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MAX782

Triple-Output Power-Supply Controller for Notebook Computers

Maxim Integrated 

Triple-Output Power-Supply

Controller for Notebook Computers

FAT, HIGH-CURRENT TRACES

MAIN CURRENT PATH

KELVIN SENSE TRACES

SENSE RESISTOR

MAX782

Figure 5. Kelvin Connections for the Current-Sense Resistors

MOSFETs with low gate-threshold voltage specifica-

tions (i.e., maximum VGS(TH) = 2V rather than 3V) are

preferred, especially for high-current (5A) applications.

Output Filter Capacitors (C6, C7, C14)

The output filter capacitors determine the loop stability

and output ripple voltage. To ensure stability, the mini-

mum capacitance and maximum ESR values are:

and,

VREF

CF > —————————————

VOUT x RCS x 2 x π x GBWP

VOUT x RCS

ESRCF < ——————

VREF

where:

CF = output filter capacitance, C6 or C7 (F);

VREF = reference voltage, 3.3V;

VOUT = output voltage, 3.3V or 5V;

RCS = sense resistor (Ω);

GBWP = gain-bandwidth product, 60kHz;

ESRCF = output filter capacitor ESR (Ω).

Be sure to select output capacitors that satisfy both the

minimum capacitance and maximum ESR require-

ments. To achieve the low ESR required, it may be

appropriate to use a capacitance value 2 or 3 times

larger than the calculated minimum.

The output ripple in continuous-current mode is:

VOUT(RPL) = ILPP(MAX) x (ESRCF+1/(2 x π x f x CF)).

In idle-mode, the ripple has a capacitive and resistive

component:

4 x 10-4 x L

VOUT(RPL)(C) = ——————— x

RCS2 x CF

( ) 1

1

——— + ————— Volts

VOUT VIN - VOUT

VOUT(RPL)(R) = —0.—02—x—E—SR—CF- Volts

RCS

The total ripple, VOUT(RPL), can be approximated as fol-

lows:

if VOUT(RPL)(R) < 0.5 VOUT(RPL)(C),

then VOUT(RPL) = VOUT(RPL)(C),

otherwise, VOUT(RPL) = 0.5 VOUT(RPL)(C) +

VOUT(RPL)(R).

Diode D2

The voltage rating of D2 should be at least 2 x VIN +

5V plus a safety margin. A rating of at least 75V is

necessary for the maximum 30V supply. A Schottky

diode is preferable for lower input voltages, and is

required for input voltages under 7V. Use a high-

speed silicon diode (with a higher breakdown voltage

and lower capacitance) for high input voltages. D2’s

current rating should exceed twice the maximum cur-

rent load on VDD.

Diodes D3 and D4

Use 1N5819s or similar Schottky diodes. D3 and D4

conduct only about 3% of the time, so the 1N5819’s

1A current rating is conservative. The voltage rating

of D3 and D4 must exceed the maximum input supply

voltage from the battery. These diodes must be

Schottky diodes to prevent the lossy MOSFET body

diodes from turning on, and they must be placed

physically close to their associated synchronous recti-

fier MOSFETs.

Soft-Start Capacitors (C8, C9)

A capacitor connected from GND to either SS pin caus-

es that supply to ramp up slowly. The ramp time to full

current limit, tSS, is approximately 1ms for every nF of

capacitance on SS_, with a minimum value of 10µs.

Typical capacitor values are in the 10nF to 100nF

range; a 5V rating is sufficient.

______________________________________________________________________________________ 17

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