in the inductor discharge path, including synchronous

is the on-time calculated by the MAX1844.

Automatic Pulse-Skipping Switchover

In skip mode (SKIP low), an inherent automatic

switchover to PFM takes place at light loads (Table 3).

This switchover is effected by a comparator that trun-

cates the low-side switch on-time at the inductor current’s

zero crossing. This mechanism causes the threshold

between pulse-skipping PFM and nonskipping PWM

operation to coincide with the boundary between continu-

ous and discontinuous inductor-current operation (also

known as the “critical conduction” point; see the

Continuous to Discontinuous Inductor Current vs. Input

Voltage graph in the Typical Operating Characteristics).

In low-duty-cycle applications, this threshold is relatively

constant, with only a minor dependence on battery

voltage.

where K is the on-time scale factor (Table 5). The load-

current level at which PFM/PWM crossover occurs,

rent, which is a function of the inductor value (Figure 3).

For example, in the standard application circuit with

6.8µH, switchover to pulse-skipping operation occurs at

occurs at an even lower value if a swinging (soft-satura-

tion) 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-transient response

(especially at low input voltage levels).

DC output accuracy specifications refer to the threshold

of the error comparator. When the inductor is in continu-

ous conduction, the output voltage will have a DC regu-

lation level higher than the trip level by 50% of the ripple.

In discontinuous conduction (SKIP = GND, light load),

the output voltage will have a DC regulation level higher

than the error-comparator threshold by approximately

1.5% due to slope compensation.

Forced-PWM Mode (SKIP = High)

The low-noise forced-PWM mode (SKIP = high) disables

the zero-crossing comparator, which controls the low-

side switch on-time. This causes the low-side gate-drive

waveform to become the complement of the high-side

gate-drive waveform. This in turn causes the inductor

current to reverse at light loads while DH maintains a

mode is to keep the switching frequency fairly constant,

but it comes at a cost: the no-load battery current can be

10mA to 40mA, depending on the external MOSFETs.

Forced-PWM mode is most useful for reducing audio-

frequency noise, improving load-transient response, pro-

viding 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.

I

KV

2L

V- V

V

LOAD(SKIP)

OUT

IN

OUT

IN

≈×

High-Speed Step-Down Controller with

Accurate Current Limit for Notebook Computers

______________________________________________________________________________________

13

Figure 3. Pulse-Skipping/Discontinuous Crossover Point

ON-TIME

0

TIME

L

∆i

∆t

=

Figure 4. ‘‘Valley’’ Current-Limit Threshold Point

0

TIME