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MAX782

Triple-Output Power-Supply Controller for Notebook Computers

Maxim Integrated 

Triple-Output Power-Supply

Controller for Notebook Computers

+5V Transformer (T1)

Table 3 lists two commercially available transformers

and parts for a custom transformer. The following

instructions show how to determine the transformer

parameters required for a custom design:

LP, the primary inductance value

ILPEAK, the peak primary current

LI2, the core’s energy rating

RP and RS, the primary and secondary resistances

N, the primary-to-secondary turns ratio.

The transformer primary is specified just as the +3.3V

inductor, using VOUT = +5.0V; but the secondary output

(VDD) power must be added in as if it were part of the

primary. VDD current (IDD) usually includes the VPPA

and VPPB output currents. The total +5V power, PTOTAL,

is the sum of these powers:

where:

and:

PTOTAL = P5 + PDD

P5 = VOUT x IOUT;

PDD = VDD x IDD;

VOUT = output voltage, 5V;

IOUT = maximum +5V load current (A);

VDD = VDD output voltage, 15V;

IDD = maximum VDD load current (A);

so:

PTOTAL = (5V x IOUT) + (15V x IDD)

and the equivalent +5V output current, ITOTAL, is:

ITOTAL = PTOTAL / 5V

= [(5V x IOUT) + (15V x IDD)] / 5V.

The primary inductance, LP, is given by:

VOUT x (VIN(MAX) - VOUT)

LP = ———————————————

VIN(MAX) x f x ITOTAL x LIR

where:

VOUT = output voltage, 5V;

VIN(MAX) = maximum input voltage;

f = switching frequency, normally 300kHz;

ITOTAL = maximum equivalent load current (A);

LIR = ratio of primary peak-to-peak AC

current to average DC load current, typically 0.3.

The highest peak primary current (ILPEAK) equals the

total DC load current (ITOTAL) plus half the peak-to-peak

AC primary current (ILPP). The peak-to-peak AC primary

current is typically chosen as 30% of the maximum DC

load current, so the peak primary current is 1.15 times

ITOTAL. A higher value of LIR allows smaller inductance,

but results in higher losses and higher ripple.

The peak current in the primary at full load is given by:

VOUT x (VIN(MAX) - VOUT)

ILPEAK = ITOTAL + —————————————.

2 x f x LP x VIN(MAX)

Choose a core with an LI2 parameter greater than LP x

ILPEAK2.

The winding resistances, RP and RS, should be as low

as possible, preferably in the low milliohms. Use the

largest gauge wire that will fit on the core. The coil is

effectively in series with the load at all times, so the

resistive losses in the primary winding alone are

approximately (ITOTAL)2 x RP.

The minimum turns ratio, NMIN, is 5V:(15V-5V). Use 1:2.2

to accommodate the tolerance of the +5V supply. A

greater ratio will reduce efficiency of the VPP regulators.

Minimize the diode capacitance and the interwinding

capacitance, since they create losses through the

VDD shunt regulator. These are most significant when

the input voltage is high, the +5V load is heavy, and

there is no load on VDD.

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 measuring the

VDD voltage when VDD is unloaded and the input voltage

(VIN) is varied over its full range. Correct connection is

indicated if VDD is maintained between 13V and 20V.

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 ITOTAL), assuming that LIR = 0.3.

Since the sense resistance values (e.g. R1 = 25mΩ 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;

i.e., use separate traces not carrying any of the induc-

tor 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.

16 ______________________________________________________________________________________

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