ISL6237 Datasheet, PDF (26/35 Pages) Intersil Corporation – High-Efficiency, Quad-Output, Main Power Supply Controllers for Notebook Computers

ISL6237
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 non skipping 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 19). 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 67). For example, in the ISL6237
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.
DI
=
t
VIN -VOUT
L
I_PEAK
I_LOAD = I_PEAK/2
0 ON-TIME
TIME
FIGURE 67. ULTRASONIC CURRENT WAVEFORMS
40µs (MAX)
INDUCTOR
CURRENT
0A
FB < REG.POINT
ZZeErRo-OC-rCoRsOsiSnSgING
DETECTION
ON-TIME (t _ON)
FIGURE 68. 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
26
FN6418.4
March 18, 2008