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PM6680 データシート(PDF) 22 Page - STMicroelectronics

部品番号 PM6680
部品情報  No Rsense dual step-down controller with adjustable voltages for notebook system power
PDF  49 Pages
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メーカー  STMICROELECTRONICS [STMicroelectronics]
ホームページ  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

PM6680 データシート(HTML) 22 Page - STMicroelectronics

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Device description
PM6680
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 fSW:
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 pin selection: typical switching frequency
Fsw@OUT1=1.5V(kHz)
Fsw@OUT2=1.05V(kHz)
FSEL = GND
200
325
FSEL = VREF
290
425
FSEL = LDO5
390
590
D
V
OUT
V
IN
--------------
=
f
SW
V
OUT
V
IN
--------------
K
on
V
OUT
V
IN
--------------
×
-------------------------------
1K
⁄
on
=
=



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