TLF50211EL_15 Datasheet, PDF(13/27 Page) Marl International Limited – 2.2 MHz Step-Down Regulator 500 mA, 5 V, low quiescent current

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TLF50211EL_15 Datasheet, PDF (13/27 Pages) Marl International Limited – 2.2 MHz Step-Down Regulator 500 mA, 5 V, low quiescent current

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TLF50211EL

Buck Regulator

Transition from PFM to PWM:

Figure 5 is showing the transition from Pulse Frequency Modulation to Pulse Width Modulation under the

assumption, that the input voltage VS will be constant, and only the output current ICC will vary. The diagram shows

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 i3, is applied to the regulator output. i3 shall be

below the imaginary current threshold for transition from PFM to PWM iPFM/PWM. The regulator is in PFM mode,

the output voltage is VREF_PFM, which is equal to VFB in PFM mode (or slightly higher).

Pulses of the duration TON,min are triggered whenever the output voltage VCC touches the PFM reference voltage

VREF_PFM.

At point t5 the output current increases from i3 to a higher i2, that shall be above the imaginary current threshold

for transition from PFM to PWM iPFM/PWM. Due to the higher output current more pulses of the duration TON,min have

to be triggered, the frequency of these pulses is monitored. The frequency of these pulses increases until it is

higher than the switching frequency fOSC set by the frequency setting resistor RFREQ. The regulator is still in PFM

mode

At point t6 the frequency monitoring detects that the frequency of the PFM pulses is being higher than the

frequency threshold for transition from PFM to PWM fPFM/PWM. Therefore the regulator switches back to PWM

mode. This results in a certain duty cycle D2 with the on-time TON2 of the internal power stage. The time period

TPWM is as adjusted by the frequency setting resistor RFREQ.

TON,min

iPFM/PWM

time

TON,min

TPWM

TON2

VPWM/PFM

Figure 5

PFM to PWM Transition (Timing Diagram)

time

Switch to PWM mode

VREF_PFM

VREF_PWM

time

Data Sheet

Rev. 1.0, 2013-06-19

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