MAX786CSLASHD datasheet(14/20 Pages) MAXIM

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Dual-Output Power-Supply

Controller for Notebook Computers

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VIN(MAX), the maximum input (battery) voltage. This

value should include the worst-case conditions under

which the power supply is expected to function, such

as no-load (standby) operation when a battery charger

is connected but no battery is installed. VIN(MAX) cannot

exceed 30V.

VIN(MIN), the minimum input (battery) voltage. This

value should be taken at the full-load operating cur-

rent under the lowest battery conditions. If VIN(MIN) is

below about 6.5V, the filter capacitance required to

maintain good AC load regulation increases, and the

current limit for the +5V supply has to be increased

for the same load level.

Inductor (L1, L2)

Three inductor parameters are required: the inductance

value (L), the peak inductor current (ILPEAK), and the

coil resistance (RL). The inductance is:

(VOUT) (VIN(MAX) - VOUT)

L = ————————————

(VIN(MAX)) (f) (IOUT) (LIR)

where:

VOUT = output voltage (3.3V or 5V);

VIN(MAX) = maximum input voltage (V);

f = switching frequency, normally 300kHz;

IOUT = maximum DC load current (A);

LIR = ratio of inductor peak-to-peak AC

current to average DC load current, typically 0.3.

A higher value of LIR allows smaller inductance, but

results in higher losses and higher ripple.

The highest peak inductor current (ILPEAK) equals the DC

load current (IOUT) plus half the peak-to-peak AC inductor

current (ILPP). The peak-to-peak AC inductor current is

typically chosen as 30% of the maximum DC load cur-

rent, so the peak inductor current is 1.15 times IOUT.

The peak inductor current at full load is given by:

(VOUT) (VIN(MAX) - VOUT)

ILPEAK = IOUT + —————————————.

(2) (f) (L) (VIN(MAX))

The coil resistance should be as low as possible,

preferably in the low milliohms. The coil is effectively in

series with the load at all times, so the wire losses alone

are approximately:

Power loss = (IOUT2) (RL).

In general, select a standard inductor that meets the L,

ILPEAK, and RL requirements (see Tables 1 and 2). If a

standard inductor is unavailable, choose a core with an

LI2 parameter greater than (L) (ILPEAK2), and use the

largest wire that will fit the core.

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 +3.3V supply, R1 =

80mV / (1.15 x IOUT); for the +5V supply, R2 =

80mV/(1.15 x IOUT), 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 con-

nect the CS_ and FB_ pins to the sense resistors; i.e.,

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 iden-

tical 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 (C6, C7, C12)

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 (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).

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