TLF50241EL_15 Datasheet, PDF (14/30 Pages) Marl International Limited – 2.2 MHz Step-Down Regulator 500 mA, 5 V, low quiescent current

TLF50241EL
Buck Regulator
Frequency Variation during PWM/PFM Transition:
Figure 6 is showing the transition from Pulse Frequency Modulation to Pulse Width Modulation (and vice versa)
in relation to output current and switching frequency. The diagram shows the principle, in reality the signals might
be slightly different. The diagram is without scale in respect of frequency and current values.
The transition from PWM to PFM is shown in a grey line. Starting from right the switching frequency f_PWM is
constant as set by the frequency setting resistor R_FREQ. The output current I_CC is decreasing.
As soon as the output current I_CC is below the imaginary current threshold for transition from PWM to PFM
i_PWM/PFM, the regulator will be switched from PWM to PFM mode depending on the output voltage V_CC. With the
output current I_CC decreasing, the switching frequency will also decrease, as the pulses are triggered by
monitoring the output voltage V_CC at capacitor C_OUT.
The transition from PFM to PWM is shown in a black line. Starting from left the switching frequency is increasing
with the increasing output current I_CC.
As soon as the switching frequency is crossing the frequency threshold for transition from PFM to PWM f_PFM/PWM
(which is above the switching frequency f_OSC set by the frequency setting resistor R_FREQ) the regulator will switch
from PFM to PWM.
t
PWM to PFM
PFM to PWM
f_PFM/PWM
f_PWM
i_PWM/PFM
Figure 6 PWM <-> PFM Transitions
i_PFM/PWM
Output Current
(log.scale)
Data Sheet
14
Rev. 1.0, 2013-06-19