ISL6236A Datasheet, PDF (27/37 Pages) Intersil Corporation – High-Efficiency, Quad-Output, Main Power Supply Controllers for Notebook Computers

ISL6236A
Automatic Pulse-Skipping Switchover
(Idle Mode)
In Idle Mode (SKIP = GND), an inherent automatic
switchover to PFM takes place at light loads. This switchover
is affected by a comparator that truncates 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 continuous and discontinuous
inductor-current operation (also known as the critical
conduction point):
I_LOAD(SKIP)
=
-^K-----^â‹---^V----^O-----^U----^T-- -^V----^I--^N-----–----^V-----^O----^U----^T--
2⋅L
V_IN
(EQ. 3)
where K is the on-time scale factor (see “ON-TIME ONE-
SHOT (t_ON)” on page 20). The load-current level at which
PFM/PWM crossover occurs, I_LOAD(SKIP), is equal to half
the peak-to-peak ripple current, which is a function of the
inductor value (Figure 72). For example, in the ISL6236A
typical application circuit with V_OUT1 = 5V, V_IN = 12V,
L = 7.6µH, and K = 5µs, switchover to pulse-skipping
operation occurs at I_LOAD = 0.96A or about on-fifth full load.
The crossover point occurs at an even lower value if a
swinging (soft-saturation) inductor is used.
ΔI
=
t
VIN -VOUT
L
IPEAK
I_LOAD = I_PEAK/2
0 ON-TIME
TIME
FIGURE 72. ULTRASONIC CURRENT WAVEFORMS
The switching waveforms may appear noisy and
asynchronous 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 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 approximately
1.0% due to slope compensation.
Forced-PWM Mode
The low-noise, forced-PWM (SKIP = VCC) mode disables
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 V_OUT/V_IN. The benefit of
forced-PWM mode is to keep 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 transformer 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 minimum
switching frequency of 25kHz. This ultrasonic pulse-skipping
mode eliminates audio-frequency modulation that would
otherwise be present when a lightly loaded controller
automatically skips pulses. In ultrasonic mode, the controller
automatically transitions to fixed-frequency PWM operation
when the load reaches the same critical conduction point
(ILOAD(SKIP)).
An ultrasonic pulse occurs when the controller detects that
no switching has occurred within the last 20µs. Once
triggered, the ultrasonic controller pulls LGATE high, turning
on the low-side MOSFET to induce a negative inductor
current. After FB drops below the regulation point, the
controller turns off the low-side MOSFET (LGATE pulled low)
and triggers a constant on-time (UGATE driven high). When
the on-time has expired, the controller re-enables the
low-side MOSFET until the controller detects that the
inductor current dropped below the zero-crossing threshold.
Starting with a LGATE pulse greatly reduces the peak output
voltage when compared to starting with a UGATE pulse, as
long as VFB < VREF, LGATE is off and UGATE is on, similar
to pure SKIP mode.
27
FN6453.3
March 18, 2008