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MAX782

Triple-Output Power-Supply Controller for Notebook Computers

Maxim Integrated 

Triple-Output Power-Supply

Controller for Notebook Computers

The comparators are always active when V+ is above

+4V, even when VH is 0V. Thus, Q1-Q3 will sink current

to GND even when VH is 0V, but they will only source

current from VH when VH is above approximately 1.5V.

If Q1, Q2, or Q3 is externally pulled above VH, an inter-

nal diode conducts, pulling VH a diode drop below the

output and powering anything connected to VH. This

voltage will also power the other comparator outputs.

Internal VL and REF Supplies

An internal linear regulator produces the 5V used by the

internal control circuits. This regulator’s output is avail-

able on pin VL and can source 5mA for external loads.

Bypass VL to GND with 4.7µF. To save power, when

the +5V switch-mode supply is above 4.5V, the internal

linear regulator is turned off and the high-efficiency +5V

switch-mode supply output is connected to VL.

The internal 3.3V bandgap reference (REF) is powered

by the internal 5V VL supply, and is always on. It can

furnish up to 5mA. Bypass REF to GND with 0.22µF,

plus 1µF/mA of load current.

Both the VL and REF outputs remain active, even when

the switching regulators are turned off, to supply mem-

ory keep-alive power.

These linear-regulator ouputs can be directly connected

to the corresponding step-down regulator outputs (i.e.,

REF to +3.3V, VL to +5V) to keep the main supplies alive

in standby mode. However, to ensure start-up, standby

load currents must not exceed 5mA on each supply.

Fault Protection

The +3.3V and +5V PWM supplies, the high-side sup-

ply, and the comparators are disabled when either of

two faults is present: VL < +4.0V or REF < +2.8V (85%

of its nominal value).

__________________Design Procedure

Figure 1’s schematic and Table 2’s component list

show values suitable for a 3A, +5V supply and a 3A,

+3.3V supply. This circuit operates with input voltages

from 6.5V to 30V, and maintains high efficiency with

output currents between 5mA and 3A (see the Typical

Operating Characteristics). This circuit’s components

may be changed if the design guidelines described in

this section are used – but before beginning the design,

the following information should be firmly established:

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) can-

not 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 power available from the

VDD supply will be reduced. In addition, the filter

capacitance required to maintain good AC load reg-

ulation increases, and the current limit for the +5V

supply has to be increased for the same load level.

+3.3V 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 x (VIN(MAX) - VOUT)

L = ————————————-

VIN(MAX) x f x IOUT x LIR

where:

VOUT = output voltage, 3.3V;

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

f = switching frequency, normally 300kHz;

IOUT = maximum +3.3V 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 x (VIN(MAX) - VOUT)

ILPEAK = IOUT + —————————————.

2 x f x L x 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 x 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 x ILPEAK2, and use the

largest wire that will fit the core.

______________________________________________________________________________________ 15

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