MAX783CSLASHD datasheet(15/28 Pages) MAXIM

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+5V Inductor (L1)

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, 5V

VIN(MAX) = maximum input voltage (V)

f = switching frequency, normally 300kHz

IOUT = maximum +5V 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 current, 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 3 and 4). 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.

+3.3V Transformer (L2)

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 +5V

inductor, using VOUT = +3.3V; 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 +3.3V power,

PTOTAL, is the sum of these powers:

PTOTAL = P3 + PDD

where:

P3 = (VOUT) (IOUT);

PDD = (VDD) (IDD);

and:

VOUT = output voltage, 3.3V;

IOUT = maximum +3.3V load current (A);

VDD = VDD output voltage, 15V;

IDD = maximum VDD load current (A);

so:

PTOTAL = (3.3V x IOUT) + (15V x IDD)

and the equivalent +3.3V output current, ITOTAL, is:

ITOTAL = PTOTAL / 3.3V

= [(3.3V x IOUT) + (15V x IDD)] / 3.3V.

The primary inductance, LP, is given by:

(VOUT) (VIN(MAX) - VOUT)

LP = —————————————

(VIN(MAX)) (f) (ITOTAL) (LIR)

where:

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) (VIN(MAX) - VOUT)

ILPEAK = ITOTAL + —————————————.

(2) (f) (LP) (VIN(MAX))

Choose a core with an LI2 parameter greater than

(LP) (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 (RP).

The minimum turns ratio, NMIN, is 3.3V:(15V-3.3V). Use

1:4 to accommodate the tolerance of the +3.3V 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 +3.3V load is heavy, and

there is no load on VDD.

Triple-Output Power-Supply Controller

for Notebook Computers

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