PM6685 STMicroelectronics, PM6685 Datasheet - Page 20

• 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

Device description
20/48
Figure 28. Constant on time PWM control
The duty cycle D of the buck converter in steady state is:
Equation 2
The PWM control works at a nearly fixed frequency f
Equation 3
As mentioned the steady state switching frequency is theoretically independent from battery
voltage and from output voltage. Actually the frequency depends on parasitic voltage drops
that are present during the charging path (high side switch resistance, inductor resistance
(DCR)) and discharging path (low side switch resistance, DCR). As a result the switching
frequency increases as a function of the load current. Standard switching frequency values
can be selected for both sections by pin FSEL as shown in the following table:
Table 6.
The values in the table are measured with V
The other output are unloaded.
SGND
VREF
LDO5
FSEL
FSEL pin selection
Frequency
323kHz
432kHz
212kHz
SMPS 5V
f
sw
Doc ID 11674 Rev 8
=
T
D
ON
=
D =
K
2.31µs
4,7µs
K
ON
in
3µs
ON
V
V
=12 V, operation mode = PWM and I
V
×
V
OUT
OUT
IN
IN
V
V
OUT
IN
SW
=
:
1
K
ON
Frequency
297,6kHz
400kHz
500kHz
SMPS 3.3V
3.36µs
load
2.5µs
2.0µs
K
PM6685
ON
= 2 A.