MAX783CSLASHD datasheet(14/28 Pages) MAXIM

14 / 28 page

These codes are compatible with many popular PCMCIA

digital controllers such as the Intel 82365SL. For other

interfaces, one of the inputs can be permanently wired

high or low and the other toggled to turn the supply on

and off. The truth table shows that either a “0” or “1” can

be used to turn each supply on. The two VPP outputs can

be safely connected in parallel for increased load capabil-

ity if the control inputs are also tied together (i.e., DA0 to

DB0, DA1 to DB1). If VPAA and VPPB are connected in

parallel, some devices may exhibit several milliamps of

increased quiescent supply current when enabled, due to

slightly mismatched output voltage set points.

Comparators

Two noninverting comparators can be used as preci-

sion voltage comparators or high-side drivers. The

supply for these comparators (VH) is brought out and

may be connected to any voltage between +3V and

+19V. The noninverting inputs (D1-D2) are high imped-

ance, and the inverting input is internally connected to

a 1.650V reference. Each output (Q1-Q2) sources

20µA from VH when its input is above 1.650V, and

sinks 500µA to GND when its input is below 1.650V.

The Q1-Q2 outputs can be fixed together in wired-OR

configuration since the pull-up current is only 20µA.

Connecting VH to a logic supply (5V or 3V) allows the

comparators to be used as low-battery detectors. For dri-

ving N-channel power MOSFETs to turn external loads on

and off, VH should be 6V to 12V higher than the load volt-

age. This enables the MOSFETs to be fully turned on and

results in low rDS(ON). VDD is a convenient source for VH.

Internal VREF and VL 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 VL linear

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

switch-mode supply output is internally connected to VL.

The 3.3V precision reference (REF) is powered from the

internal 5V VL supply. It can furnish up to 5mA for exter-

nal loads. Bypass REF to GND with 0.22µF, plus 1µF/mA

of load current. The main switch-mode outputs track the

reference voltage. Loading the reference reduces the

main output voltages slightly, according to the reference

voltage load regulation error.

Both the VL and REF supplies can remain active—even

when the switch-mode regulators are turned off—to supply

memory keep-alive power (see

Shutdown Mode section).

These linear regulator outputs can be directly connected

to the corresponding switch-mode regulator outputs (i.e.,

REF to +3.3V, VL to +5V) to hold up the main supplies in

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

load currents must not exceed 5mA on each supply.

Shutdown Mode

Shutdown (S

—

–

= low) forces both PWMs off and dis-

ables the REF output and the auxiliary comparators

including R

—

–

. Supply current in shutdown mode is

typically 25µA. The VL supply remains active and can

source 25mA for external loads. VL load capability is

higher in shutdown and standby modes than when the

PWMs are operating (25mA vs. 5mA).

Standby mode is achieved by holding ON3 and ON5

low while S

—

–

is high. This disables both PWMs, but

keeps VL, REF, and the precision comparators alive.

Supply current in standby mode is typically 70µA.

Other ways to shut down the MAX783 are suggested in

the applications section of the MAX782 data sheet.

__________________Design Procedure

Figure 1’s predesigned application circuit contains the

correct component values for 3A output currents and a

6V to 20V input range. Use the design procedure that

follows to optimize this basic schematic for different

voltage or current requirements.

Before beginning a design, firmly establish the following:

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 6V, 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.

Triple-Output Power-Supply Controller

for Notebook Computers

______________________________________________________________________________________

Table 2. VPP Program Codes

DA0

DA1

VPPA

12V

3.3V

DB0

DB1

VPPB

12V

3.3V

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