PM6681A STMicroelectronics, PM6681A Datasheet - Page 19

• Manufacturer Part Number: PM6681A
• Description: Dual synchronous step down controller with adjustable LDO
• Manufacturer: STMicroelectronics

PM6681A
Figure 30. 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 connecting pin
FSEL to SGND, VREF or LDO5 pin. The following table shows the typical switching
frequencies that can be obtained as a function of the programmed output voltage. The
measures are referred to switching sections with 2 A load, 12 V input voltage and working in
continuous conduction mode.
Table 7.
FSEL = VREF
FSEL = LDO5
FSEL = GND
FSEL pin selection: typical switching frequency
Fsw @ OUT1 = 1.5 V (kHz)
f
sw
=
T
200
290
390
D
on
=
D =
K
on
Vout
Vout
Vin
×
Vin
Vout
Vin
SW
=
:
1
K
on
Fsw @ OUT2 = 1.05 V (kHz)
Device description
325
425
590
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