PM6680 Datasheet, PDF (22/49 Pages) STMicroelectronics – No Rsense dual step-down controller with adjustable voltages for notebook system power

Device description
Figure 28. Constant ON time PWM control
PM6680
22/49
The duty cycle of the buck converter in steady state is:
Equation 2
D= ^V-----^O----^U---^T--
V_IN
The PWM control works at a nearly fixed frequency f_SW:
Equation 3
V_{-----O----U---T--}
f_SW=
-----------^V-----^I-^N-------------
K_on
×
V_{-----O----U---T--}
V_IN
=
1 ⁄ K_on
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