PM6680 STMicroelectronics, PM6680 Datasheet - Page 22

• Manufacturer Part Number: PM6680
• Description: IC CTRLR DUAL STEP DOWN 32VFQFPN
• Manufacturer: STMicroelectronics

Specifications of PM6680

• Applications: Controller, Notebook Power System
• Voltage - Input: 6 ~ 28 V
• Number Of Outputs: 2
• Voltage - Output: 0.9 ~ 5.5 V
• Operating Temperature: 0°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 32-VFQFN, 32-VFQFPN
• Output Voltage: 5 V, 0.9 V to 5.5 V, 0.9 V to 3.3 V
• Output Current: 0.33 A
• Input Voltage: 5.5 V to 28 V
• Mounting Style: SMD/SMT
• Maximum Operating Temperature: + 125 C
• Minimum Operating Temperature: - 10 C
• For Use With: 497-6379 - BOARD EVALUATION FOR PM6680A497-6378 - BOARD EVALUATION FOR PM6680497-6425 - BOARD EVAL BASED ON PM6680A
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Device description
22/49
Figure 28. Constant ON time PWM control
The duty cycle 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.
FSEL = VREF
FSEL = LDO5
FSEL = GND
FSEL pin selection: typical switching frequency
Fsw@OUT1 = 1.5 V (kHz)
f
SW
200
290
390
=
------------------------------ -
K
on
D
V
--------------
=
V
×
OUT
IN
V
--------------
V
--------------
V
V
OUT
OUT
IN
IN
=
1 K
SW
⁄
:
on
Fsw@OUT2 = 1.05 V (kHz)
325
425
590
PM6680