PM6670AS View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'PM6670AS' PDF : 54 Pages View PDF
Device description
PM6670AS
7.1.3
Pulse-skip and no-audible pulse-skip modes
High efficiency at light load conditions is achieved by PM6670AS entering the pulse-skip
mode (if enabled). When one of the two fixed output voltages is set, pulse-skip power saving
is a default feature. At light load conditions the zero-crossing comparator truncates the low-
side switch on-time as soon as the inductor current becomes negative; in this way the
comparator determines the on-time duration instead of the output ripple (see Figure 31).
Figure 31. Inductor current and output voltage at light load with pulse-skip
Inductor
current
VDDQ
Output
Vreg
28/53
TON TOFF
t
TIDLE
As a consequence, the output capacitor is left floating and its discharge depends solely on
the current drained from the load. When the output ripple on the pin COMP falls under the
reference, a new shot is triggered and the next cycle begins. The pulse-skip mode is
naturally obtained enabling the zero-crossing comparator and automatically takes part in the
COT algorithm when the inductor current is about half the ripple current amount, i.e.
migrating from continuous conduction mode (C.C.M.) to discontinuous conduction mode
(D.C.M.).
The output current threshold related to the transition between PWM mode and pulse-skip
mode can be approximately calculated as:
Equation 20
ILOAD (PWM2Skip)
=
VIN − VOUT
2⋅L
⋅ TON
At higher loads, the inductor current never crosses zero and the device works in pure PWM
mode with a switching frequency around the nominal value.
A physiological consequence of pulse-skip mode is a more noisy and asynchronous (than
normal conditions) output, mainly due to very low load. If the pulse-skip is not compatible
with the application, the PM6670AS, when set in adjustable mode-of-operation, allows the
user to choose between forced-PWM and no-audible pulse-skip alternative modes (see
4.1.4 for details).
No-audible pulse-skip mode
Some audio-noise sensitive applications cannot accept the switching frequency to enter the
audible range as is possible in pulse-skip mode with very light loads. For this reason, the
PM6670AS implements an additional feature to maintain a minimum switching frequency of
33 kHz despite a slight efficiency loss. At very light load conditions, if any switching cycle
Doc ID 14436 Rev 2
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