MAX782CSLASHD datasheet(23/32 Pages) MAXIM

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Triple-Output Power-Supply

Controller for Notebook Computers

______________________________________________________________________________________

The MAX782 has three comparators/level-shifters that

can be used for this purpose, and two that are needed for

each PCMCIA port. Two transistors can be used as

shown in Figure 11 to provide two additional TTL-input

MOSFET gate drivers for a second PCMCIA slot. The

component values have been carefully chosen to provide

smooth switching from 5V to 3.3V without make-before-

break glitches, and without a break in the VCC supply.

Low-Voltage (6-Cell) Operation

Low input voltages, such as the 6V end-of-life voltage of

a 6-cell NiCd battery, place extra demands on the +5V

buck regulator because of the very low input-output dif-

ferential voltage. The standard application circuit works

well with supply voltages down to 6.5V; at input voltages

less than 6.5V, some component changes are needed

(see Table 5), and the operating frequency must be set

to 200kHz. The two main issues are load-transient

response and load capability of the +15V VDD supply.

The +5V supply’s load-transient response is impaired due

to reduced inductor-current slew rate, which is in turn

caused by reduced voltage applied across the buck induc-

tor during the high-side switch-on time. So, the +5V output

sags when hit with an abrupt load current change, unless

the +5V filter capacitor value is increased. Note that only the

capacitance is affected and ESR requirements don’t

change. Therefore, the added capacitance can be sup-

plied by an additional low-cost bulk capacitor in parallel with

the normal low-ESR switching-regulator capacitor. The

equation for voltage sag under a step-load change follows:

ISTEP2 x L

VSAG = —————————————————

2 x CF x (VIN(MIN) x DMAX - VOUT)

where DMAX is the maximum duty cycle. Higher duty

cycles are possible when the oscillator frequency is

reduced to 200kHz, due to fixed propagation delays

through the PWM comparator becoming a lesser part of

the whole period. The tested worst-case limit for DMAX

is 92% at 200kHz. Lower inductance values can reduce

the filter capacitance requirement, but only at the

expense of increased noise at high input voltages (due

to higher peak currents).

The components shown in Table 5 allow the main +5V

supply to deliver 2A from VIN = 5.5V, or alternatively

allow the +15V supply to deliver 70mA while simultane-

ously providing +5V at 2A from VIN = 5.7V. Note:

Components for +3.3V don’t need to be changed.

The +15V supply’s load capability is also affected by

low input voltages, especially under heavy loads. When

the +5V supply is heavily loaded, there simply isn’t

enough extra duty cycle left for the flyback winding

controller to deliver energy to the secondary. VDD load-

current limitations are thus determined by the worst-

case duty-cycle limits, and also by any parasitic resis-

tance or inductance on the transformer secondary.

These parasitics, most notably the transformer leakage

inductance and the forward impedance of the +15V

rectifier diode, limit the rate-of-rise of current in the sec-

ondary during the brief interval when primary current

reverses and the transformer conducts in the forward

mode. See the

Typical Operating Characteristics. For

low-voltage applications that require heavy +15V load

currents (for example, 6-cell circuits where +12V VPP

must deliver 120mA or more), see the MAX783 data

sheet. This device is similar to the MAX782 except the

+15V flyback winding controller has been shifted from

the +5V side to the +3.3V side.

Table 5. Components for Low-Voltage

Operation

(Circuit of Figure 1, f = 200kHz, VIN Range = 5.5V to 12V)

Transformer L2:

Filter Capacitor C6:

660µF

Flyback Rectifier D2:

1N5819 or equivalent

Schottky diode

Coiltronics CTX03-12062

(low-leakage inductance,

10µH primary)

Sense Resistor:

25m

Ω

PCMCIA 2.0

DIGITAL

CONTROLLER

VCC_EN0

VCC_EN1

MAX782

100

µF

SLOT

VCC

+5V

VDD

FB5

FB3

+3.3V

2N3904

510k

NOTE: MOSFET BODY DIODES SHOWN FOR CLARITY.

Figure 11. Using Discrete Circuitry to Switch PCMCIA 2.0 Slot VCC

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