PM6685TR View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
PM6685TR
STMicroelectronics
'PM6685TR' PDF : 48 Pages View PDF
Device description
PM6685
Table 6. FSEL pin selection
FSEL
SMPS 5V
Frequency
KON
SGND
212kHz
4,7us
VREF
323kHz
3us
LDO5
432kHz
2.31us
SMPS 3.3V
Frequency
297,6kHz
400kHz
500kHz
KON
3.36us
2.5us
2.0us
The values in the table are measured with Vin =12V, operation mode = PWM and Iload = 2A.
The other output are unloaded.
7.2
Constant on time architecture
Figure 29 on page 23 shows the simplified block diagram of a Constant On Time controller.
A minimum off-time constrain (380ns typ) is introduced to allow inductor valley current
sensing on the synchronous switch. A minimum on-time(150ns typ) is also introduced to
assure the start-up switching sequence.
PM6685 has a one-shot generator for each power section that turns on the high side
MOSFET when the following conditions are satisfied simultaneously: the PWM comparator
is high, the synchronous rectifier current is below the current limit threshold, and the
minimum off-time has timed out. Once the on-time has timed out, the high side switch is
turned off, while the synchronous switch is turned on according to the anti-cross conduction
circuitry management. When the negative input voltage at the PWM comparator, which is a
scaled-down replica of the output voltage ripple (see the Rfb1/Rfb2 divider in Figure 29),
reaches the valley limit (determined by internal reference Vr=0.9V), the low-side MOSFET is
turned off according to the anti-cross conduction logic once again, and a new cycle begins.
22/52
Share Link: 