High-Efficiency, Quad-Output, Main Power-

Supply Controllers for Notebook Computers

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19

tinuous and discontinuous inductor-current operation

(also known as the critical conduction point):

where K is the on-time scale factor (see the On-Time

the peak-to-peak ripple current, which is a function of the

inductor value (Figure 5). For example, in the MAX8732A

L = 7.6µH, and K = 5µs, switchover to pulse-skipping

The crossover point occurs at an even lower value if a

swinging (soft-saturation) inductor is used.

The switching waveforms may appear noisy and asyn-

chronous when light loading causes pulse-skipping

operation, but this is a normal operating condition that

results in high light-load efficiency. Trade-offs in PFM

noise vs. light-load efficiency are made by varying the

inductor value. Generally, low inductor values produce

a broader efficiency vs. load curve, while higher values

result in higher full-load efficiency (assuming that the

coil resistance remains fixed) and less output voltage

ripple. Penalties for using higher inductor values

include larger physical size and degraded load-tran-

sient response (especially at low input-voltage levels).

DC output accuracy specifications refer to the trip level of

the error comparator. When the inductor is in continuous

conduction, the output voltage has a DC regulation higher

than the trip level by 50% of the ripple. In discontinuous

conduction (SKIP = GND, light load), the output voltage

has a DC regulation higher than the trip level by approxi-

mately 1.5% due to slope compensation.

Forced-PWM Mode

ables the zero-crossing comparator, which controls the

low-side switch on-time. Disabling the zero-crossing

detector causes the low-side, gate-drive waveform to

become the complement of the high-side, gate-drive

waveform. The inductor current reverses at light loads

as the PWM loop strives to maintain a duty ratio of

the switching frequency fairly constant, but it comes at

a cost: the no-load battery current can be 10mA to

50mA, depending on switching frequency and the

external MOSFETs.

Forced-PWM mode is most useful for reducing audio-

frequency noise, improving load-transient response,

providing sink-current capability for dynamic output

voltage adjustment, and improving the cross-regulation

of multiple-output applications that use a flyback trans-

former or coupled inductor.

Enhanced Ultrasonic Mode

(25kHz (min) Pulse Skipping)

Leaving SKIP unconnected or connecting SKIP to REF

activates a unique pulse-skipping mode with a mini-

mum switching frequency of 25kHz. This ultrasonic

pulse-skipping mode eliminates audio-frequency mod-

ulation that would otherwise be present when a lightly

loaded controller automatically skips pulses. In ultra-

sonic mode, the controller automatically transitions to

fixed-frequency PWM operation when the load reaches

occurs when normally pulse skipping.

An ultrasonic pulse occurs when the controller detects

that no switching has occurred within the last 28µs.

Once triggered, the ultrasonic controller pulls DL high,

turning on the low-side MOSFET to induce a negative

inductor current. After the inductor current reaches the

negative ultrasonic current threshold, the controller

turns off the low-side MOSFET (DL pulled low) and trig-

gers a constant on-time (DH driven high). When the on-

time has expired, the controller reenables the low-side

MOSFET until the controller detects that the inductor

current dropped below the zero-crossing threshold.

Starting with a DL pulse greatly reduces the peak out-

put voltage when compared to starting with a DH pulse.

The output voltage at the beginning of the ultrasonic

pulse determines the negative ultrasonic current thresh-

old, resulting in the following equation:

synchronous rectifier (MAX8734A) or the current-sense

resistor value (MAX8732A/MAX8733A).

VI R

V

V

ISONIC

L

ON

REF

FB

==

−

.

I

KV

L

VV

V

LOAD SKIP

OUT

OUT

()

__

=

×

×

+−

+









2

ON-TIME

0

TIME

L

∆i

∆t

=

Figure 5. Pulse-Skipping/Discontinuous Crossover Point