MAX783CSLASHD datasheet(16/28 Pages) MAXIM

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Ensure the transformer secondary is connected with the

right polarity: A VDD supply will be generated with either

polarity, but proper operation is possible only with the cor-

rect polarity. Test for correct connection by observing the

phase relationship between the LX3 switching node and

the transformer secondary under load. The two wave-

forms must be 180° out of phase.

Current-Sense Resistors (R1, R2)

The sense resistors must carry the peak current in the

inductor, which exceeds the full DC load current.

The internal current limiting starts when the voltage

across the sense resistors exceeds 100mV nominally,

80mV minimum. Use the minimum value to ensure

adequate output current capability: For the +5V sup-

ply, R1 = 80mV / (1.15 x IOUT); for the +3.3V supply,

R2 = 80mV/(1.15 x ITOTAL), assuming that LIR = 0.3.

Since the sense resistance values (e.g., R1 = 25m

Ω for

IOUT = 3A) are similar to a few centimeters of narrow

traces on a printed circuit board, trace resistance can

contribute significant errors. To prevent this, Kelvin

connect the CS_ and FB_ pins to the sense resistors;

use separate traces not carrying any of the inductor or

load current, as shown in Figure 5. Run these traces

parallel at minimum spacing from one another. The

wiring layout for these traces is critical for stable, low-

ripple outputs (see the

Layout and Grounding section).

MOSFET Switches (N1-N4)

The four N-channel power MOSFETs are usually identi-

cal and must be “logic-level” FETs; that is, they must be

fully on (have low rDS(ON)) with only 4V gate-source

drive voltage. The MOSFET rDS(ON) should ideally be

about twice the value of the sense resistor. MOSFETs

with even lower rDS(ON) have higher gate capacitance,

which increases switching time and transition losses.

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 (C3–C6)

The output filter capacitors determine the loop stability

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

mum capacitance and maximum ESR values are:

VREF

CF > —————————————

(VOUT) (RCS) (2) (π) (GBWP)

and,

(VOUT) (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)) [(ESRCF+1/(2 x π x f x CF)].

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

component:

(4 x 10-4) (L)

VOUT(RPL)(C) = ——————— x

(RCS2) (CF)

(———+—————)Volts

VOUT

VIN - VOUT

(0.02) (ESRCF)

VOUT(RPL)(R) = ——————- 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).

Triple-Output Power-Supply Controller

for Notebook Computers

16 _______________________________________________________________________________________

MAX783

KELVIN SENSE TRACES

SENSE RESISTOR

MAIN CURRENT PATH

FAT, HIGH-CURRENT TRACES

Figure 5. Kelvin Connections for the Current-Sense Resistors

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