Texas Instruments LM3671 Evaluation Board LM3671LC-1.8EV/NOPB LM3671LC-1.8EV/NOPB Datenbogen
Produktcode
LM3671LC-1.8EV/NOPB
High PFM Threshold
~1.017*Vout
Low1 PFM Threshold
~1.006*Vout
PFM Mode at Light Load
PWM Mode at
Moderate to Heavy
Loads
Pfet on
until
Ipfm limit
reached
Nfet on
drains
inductor
current
until
I inductor = 0
High PFM
Voltage
Threshold
reached,
go into
sleep mode
Low PFM
Threshold,
turn on
PFET
Current load
increases,
draws Vout
towards
Low2 PFM
Threshold
Low2 PFM Threshold,
switch back to PWMmode
Load current
increases
increases
Low2 PFM Threshold
Vout
Z
-
A
xi
s
Z-
Axi
s
SNVS294Q – NOVEMBER 2004 – REVISED NOVEMBER 2013
During PFM operation, the converter positions the output voltage slightly higher than the nominal output voltage
during PWM operation, allowing additional headroom for voltage drop during a load transient from light to heavy
load. The PFM comparators sense the output voltage via the feedback pin and control the switching of the output
FETs such that the output voltage ramps between ~0.6% and ~1.7% above the nominal PWM output voltage. If
the output voltage is below the ‘high’ PFM comparator threshold, the PMOS power switch is turned on. It remains
on until the output voltage reaches the ‘high’ PFM threshold or the peak current exceeds the I
during PWM operation, allowing additional headroom for voltage drop during a load transient from light to heavy
load. The PFM comparators sense the output voltage via the feedback pin and control the switching of the output
FETs such that the output voltage ramps between ~0.6% and ~1.7% above the nominal PWM output voltage. If
the output voltage is below the ‘high’ PFM comparator threshold, the PMOS power switch is turned on. It remains
on until the output voltage reaches the ‘high’ PFM threshold or the peak current exceeds the I
PFM
level set for
PFM mode. The typical peak current in PFM mode is: I
PFM
= 112 mA + V
IN
/27
Ω .
Once the PMOS power switch is turned off, the NMOS power switch is turned on until the inductor current ramps
to zero. When the NMOS zero-current condition is detected, the NMOS power switch is turned off. If the output
voltage is below the ‘high’ PFM comparator threshold (see
to zero. When the NMOS zero-current condition is detected, the NMOS power switch is turned off. If the output
voltage is below the ‘high’ PFM comparator threshold (see
), the PMOS switch is again turned on and
the cycle is repeated until the output reaches the desired level. Once the output reaches the ‘high’ PFM
threshold, the NMOS switch is turned on briefly to ramp the inductor current to zero and then both output
switches are turned off and the part enters an extremely low power mode. Quiescent supply current during this
‘sleep’ mode is 16 µA (typ.), which allows the part to achieve high efficiency under extremely light load
conditions.
threshold, the NMOS switch is turned on briefly to ramp the inductor current to zero and then both output
switches are turned off and the part enters an extremely low power mode. Quiescent supply current during this
‘sleep’ mode is 16 µA (typ.), which allows the part to achieve high efficiency under extremely light load
conditions.
If the load current should increase during PFM mode (see
) causing the output voltage to fall below the
‘low2’ PFM threshold, the part will automatically transition into fixed-frequency PWM mode. When V
IN
=2.7V the
part transitions from PWM to PFM mode at ~35 mA output current and from PFM to PWM mode at ~85 mA ,
when V
when V
IN
=3.6V, PWM to PFM transition happens at ~50 mA and PFM to PWM transition happens at ~100 mA,
when V
IN
=4.5V, PWM to PFM transition happens at ~65 mA and PFM to PWM transition happens at ~115 mA.
Figure 41. Operation in PFM Mode and Transfer to PWM Mode
Shutdown Mode
Setting the EN input pin low (<0.4V) places the LM3671 in shutdown mode. During shutdown the PFET switch,
NFET switch, reference, control and bias circuitry of the LM3671 are turned off. Setting EN high (>1.0V) enables
normal operation. It is recommended to set EN pin low to turn off the LM3671 during system power up and
undervoltage conditions when the supply is less than 2.7V. Do not leave the EN pin floating.
NFET switch, reference, control and bias circuitry of the LM3671 are turned off. Setting EN high (>1.0V) enables
normal operation. It is recommended to set EN pin low to turn off the LM3671 during system power up and
undervoltage conditions when the supply is less than 2.7V. Do not leave the EN pin floating.
Soft Start
The LM3671 has a soft-start circuit that limits in-rush current during startup. During startup the switch current limit
is increased in steps. Soft start is activated only if EN goes from logic low to logic high after Vin reaches 2.7V.
Soft start is implemented by increasing switch current limit in steps of 70 mA, 140 mA, 280 mA and 1020 mA
(typical switch current limit). The startup time thereby depends on the output capacitor and load current
demanded at startup. Typical startup times with a 10 µF output capacitor and 300 mA load is 400 µs and with
1mA load is 275 µs.
is increased in steps. Soft start is activated only if EN goes from logic low to logic high after Vin reaches 2.7V.
Soft start is implemented by increasing switch current limit in steps of 70 mA, 140 mA, 280 mA and 1020 mA
(typical switch current limit). The startup time thereby depends on the output capacitor and load current
demanded at startup. Typical startup times with a 10 µF output capacitor and 300 mA load is 400 µs and with
1mA load is 275 µs.
Copyright © 2004–2013, Texas Instruments Incorporated
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