TLF50281EL Infineon Technologies, TLF50281EL Datasheet - Page 12

• Manufacturer Part Number: TLF50281EL
• Manufacturer: Infineon Technologies

Specifications of TLF50281EL

• Packages: PG-SSOP-14
• Comment: Asyncronous low-Iq-Buck; 5V/500mA; Iq
• Vq (max): 5V
• Iq (max): 500.0 mA
• Iq (typ): 45.0 µA
• Output: Buck Converter

5.1.2
Under normal conditions the TLF50281EL will operate with a constant switching frequency
The ratio between switch-on-time
voltage
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
the ratio between input voltage
to the minimum value switch-on-time
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
To optimize the efficiency and to reduce the current consumption, the TLF50281EL 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
on the output current and/or the ratio between input voltage
Transition from PWM to PFM:
Figure 4
assumption, that the input voltage
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
in a duty cycle
set by the frequency setting resistor
is equal to
At point
of the internal power stage. Due to the reduced output load the on-time
Discontinuous Conduction Mode DCM) than
frequency setting resistor
to
to smaller conduction losses.
At point
reduced also. The output current
does not allow a on-time smaller than
imaginary current threshold for transition from PWM to PFM
output voltage
Data Sheet
V
FB
in PWM mode. In Continuous Conduction Mode CCM the variation from
V
t
t
1
2
S
is showing the transition from Pulse Width Modulation to Pulse Frequency Modulation under the
the output current decreases from
V
the output current decreases again from
and the output voltage
FB
PWM (Pulse Width Modulation) Mode
PFM (Pulse Frequency Modulation) Mode
in PWM mode.
V
D
CC
1
with the on-time
will rise. The regulator is still in PWM mode, but the output voltage rises.
T
ON
should theoretically be below the minimum threshold
R
FREQ
V
. The regulator is still in PWM mode, the output voltage is
S
i
3
T
and output voltage
V
V
ON
is so low, that the on-time
T
S
CC
R
ON1
will be constant and only the output current
and switch-off-time
FREQ
and is influenced by the output current
T
of the internal power stage. The switching frequency
T
ON,min
ON,min
. The regulator is in PWM mode, the output voltage is
i
1
and stay there. As a consequence the output voltage
to a lower
T
. Therefore we can say that the output current
ON1
C
. The switching frequency
OUT
V
i
2
CC
12
to a lower
. Thus in PFM mode the repetition time of pulses depend
depending on the switching frequency), it will be reduced
T
i
OFF
2
. This results in a duty cycle
i
V
T
PWM/PFM
S
ON3
is mainly determined by the ratio between the input
and output voltage
i
would be smaller than the
1
i
3
, is applied to the regulator output. This results
. As a consequence the on-time
. With the pulse staying at on-time
T
I
ON2
CC
T
T
ON1
I
f
.
ON,min
CC
OSC
is shorter (the regulator is in
V
to
will vary. The diagram shows
CC
is constant as set by the
T
.
(due to low load or due to
ON2
D
V
2
REF_PWM
will be very small due
Rev. 1.0, 2011-10-24
with the on-time
T
f
ON,min
OSC
f
OSC
Buck Regulator
TLF50281EL
V
i
in PWM mode.
. The regulator
3
REF_PWM
which is equal
is constant as
is under the
T
T
ON
ON,min
V
will be
CC
which
T
will
ON2
the