MAX8734 Maxim Integrated Products, MAX8734 Datasheet - Page 19

• Manufacturer Part Number: MAX8734
• Description: (MAX8732 - MAX8734) Main Power-Supply Controllers
• Manufacturer: Maxim Integrated Products

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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
One-Shot (t
PFM/PWM crossover occurs, I
the peak-to-peak ripple current, which is a function of the
inductor value (Figure 5). For example, in the MAX8732
typical application circuit with V
L = 7.6µH, and K = 5µs, switchover to pulse-skipping
operation occurs at I
load. 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.
The low-noise, forced-PWM (SKIP = V
ables the zero-crossing comparator, which controls the
Figure 5. Pulse- Skipping/Discontinuous Crossover Point
I
LOAD SKIP
Supply Controllers for Notebook Computers
ON
(
High-Efficiency, Quad-Output, Main Power-
0
) section). The load-current level at which
∆i
∆t
ON-TIME
)
=
______________________________________________________________________________________
=
V+ - V
K V
LOAD
L
×
OUT
2
×
OUT
L
= 0.96A or about 1/5th full
TIME
LOAD(SKIP)
_



Forced-PWM Mode
OUT2
V
+ −
-I
PEAK
V
= 5V, V+ = 12V,
V
+
, is equal to 1/2
CC
I
LOAD
OUT
) mode dis-
= I
PEAK
_
/2



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
V
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.
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
the same critical conduction point (I
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 MOFET (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:
where V
synchronous rectifier (MAX8734) or the current-sense
resistor value (MAX8732/MAX8733).
OUT
/V+. The benefit of forced-PWM mode is to keep
FB
V
ISONIC
> V
REF
=
(25kHz (min) Pulse Skipping)
I R
L ON
and R
Enhanced Ultrasonic Mode
ON
=
(
V
is the on-resistance of the
REF
−
V
FB
LOAD(SKIP)
)
× 0 58
.
) that
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