PM6685 STMicroelectronics, PM6685 Datasheet - Page 21

• Manufacturer Part Number: PM6685
• Description: IC CTLR DUAL SYNC STDN 5X5VFQFPN
• Manufacturer: STMicroelectronics

Specifications of PM6685

• Applications: Controller, Notebook Power System
• Voltage - Input: 6 ~ 28 V
• Number Of Outputs: 2
• Voltage - Output: -3.3V, 5V
• Operating Temperature: 0°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 32-VFQFN, 32-VFQFPN
• Output Voltage: 5 V, 3.3 V
• Output Current: 0.1 A
• Input Voltage: 5.5 V to 28 V
• Mounting Style: SMD/SMT
• Maximum Operating Temperature: + 125 C
• Minimum Operating Temperature: - 10 C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

PM6685
7.2
7.3
Constant on time architecture
Figure 29 on page 21
minimum off-time constrain (380 ns typ) is introduced to allow inductor valley current
sensing on the synchronous switch. A minimum on-time(150 ns 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 R
reaches the valley limit (determined by internal reference V
is turned off according to the anti-cross conduction logic once again, and a new cycle
begins.
Figure 29. Constant ON-time block diagram
Output ripple compensation
In a classic constant on time control, the system regulates the valley value of the output
voltage and not the average value, as shown in
output voltage ripple is source of a DC static error.
To compensate this error, an integrator network can be introduced in the control loop, by
connecting the output voltage to the COMP5/COMP3(for the 5 V and 3.3 V sections
respectively) pin through a capacitor C
shows the simplified block diagram of a constant on time controller. A
Doc ID 11674 Rev 8
INT
as in
Figure 30 on page
Figure 28 on page
fb1
r
=0.9 V), the low-side MOSFET
/R
fb2
divider in
22.
20. In this condition, the
Device description
Figure
29),
21/48