Qdatasheet_Logo
Integrated circuits, Transistor, Semiconductors Search and Datasheet PDF Download Site

PM6670 View Datasheet(PDF) - STMicroelectronics

Part Name
Description
MFG CO.
'PM6670' PDF : 53 Pages View PDF
Device description
PM6670
7.1
VDDQ section - constant On-Time PWM controller
The PM6670 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
where KOSC is a constant value (130ns 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 = 0.9V), 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
20/53
Share Link: GO URL

All Rights Reserved © qdatasheet.com  [ Privacy Policy ] [ Contact Us ]