PM6670S View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'PM6670S' PDF : 54 Pages View PDF
PM6670S
Device description
7.1
VDDQ section - constant on-time PWM controller
The PM6670S uses a pseudo-fixed frequency, constant on-time (COT) controller as the core
of the switching section. It is well known that the COT controller uses a relatively simple
algorithm and uses the ripple voltage derived across the output capacitor’s ESR to trigger
the on-time one-shot generator. In this way, the output capacitor’s ESR acts as a current
sense resistor providing the appropriate ramp signal to the PWM comparator. Nearly
constant switching frequency is achieved by the system’s loop in steady-state operating
conditions by varying the on-time duration, avoiding thus the need for a clock generator. The
on-time one shot duration is directly proportional to the output voltage, sensed at VSNS pin,
and inversely proportional to the input voltage, sensed at the VOSC pin, as follows:
Equation 1
TON
= K OSC
VSNS
VOSC
+τ
where KOSC is a constant value (130 ns typ.) and τ is the internal propagation delay (40ns
typ.). The one-shot generator directly drives the high-side MOSFET at the beginning of each
switching cycle allowing the inductor current to increase; after the on-time has expired, an
Off-Time phase, in which the low-side MOSFET is turned on, follows. The off-time duration
is solely determined by the output voltage: when lower than the set value (i.e. the voltage at
VSNS pin is lower than the internal reference VR = 0.9 V), the synchronous rectifier is turned
off and a new cycle begins (Figure 28).
Figure 28. Inductor current and output voltage in steady state conditions
Inductor
current
Output
voltage
Vreg
Ton
Toff
t
Doc ID 14432 Rev 4
21/54
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