TLF50251EL

Buck Regulator

Data Sheet

11

Rev. 1.0, 2013-06-19

5.1.2

PWM (Pulse Width Modulation) Mode

Under normal conditions the TLF50251EL will operate with a constant switching frequency

f

The ratio between switch-on-time

T

voltage

V

In PWM mode the device may operate with 100% duty cycle, in this case the internal PMOS is constantly

conducting current. The current limitation feature is operating under this condition.

If the switch-on-time

T

the ratio between input voltage

V

to the minimum value switch-on-time

T

increase. The PFM comparator detects the PFM threshold and will then switch the device into PFM mode. There

is no possibility to disable the PFM function.

5.1.3

PFM (Pulse Frequency Modulation) Mode

To optimize the efficiency and to reduce the current consumption, the TLF50251EL automatically switches to PFM

mode under low load conditions. In PFM mode the internal power stage including the driver stage is switched off

and will only be switched on for applying pulses to charge the output capacitor. The pulses will be created by

monitoring the voltage of the output filter capacitor

C

on the output current and/or the ratio between input voltage

V

Transition from PWM to PFM:

Figure 4 is showing the transition from Pulse Width Modulation to Pulse Frequency Modulation under the

assumption, that the input voltage

V

the principle, in reality the signals might look slightly different. The diagram is without scale in respect of time,

voltage and current values.

Starting from left of the figure a certain output current, here named

i

in a duty cycle

D

set by the frequency setting resistor

R

is equal to

V

At point

t

of the internal power stage. Due to the reduced output load the on-time

T

Discontinuous Conduction Mode DCM) than

T

frequency setting resistor

R

to

V

to smaller conduction losses.

At point

t

reduced also. The output current

i

does not allow a on-time smaller than

T

imaginary current threshold for transition from PWM to PFM

i

output voltage

V