PM6680ATR STMicroelectronics, PM6680ATR Datasheet - Page 19

• Manufacturer Part Number: PM6680ATR
• Description: IC CTRLR DUAL STEP DOWN 32VFQFPN
• Manufacturer: STMicroelectronics

Specifications of PM6680ATR

• Applications: Controller, Embedded Computer System
• Voltage - Input: 6 ~ 36 V
• Number Of Outputs: 2
• Voltage - Output: 0.9 ~ 5 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 32-VFQFN, 32-VFQFPN
• 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
• Other names: 497-6896-2; PM6680ATR

PM6680A
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 input
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 = 3.3 V (kHz)
f
SW
195
295
390
=
------------------------------ -
K
on
D
V
--------------
=
V
×
OUT
IN
V
--------------
V
--------------
V
V
OUT
OUT
IN
IN
=
1 K
SW
⁄
:
on
Fsw@OUT2 = 1.8 V (kHz)
Device description
335
440
600
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