LM3677_0810 Datasheet, PDF (15/24 Pages) National Semiconductor (TI) – 3MHz, 600mA Miniature Step-Down DC-DC Converter for Ultra Low Voltage Circuits

FIGURE 7. Typical PFM Operation
30008479
During PFM operation, the converter positions the output volt-
age 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 ~0.2% and ~1.8% 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/20Ω .
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 be-
low the ‘high’ PFM comparator threshold (see Figure 8), 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 ex-
tremely low-power mode. Quiescent supply current during
this ‘sleep’ mode is 16 µA (typ.), which allows the part to
achieve high efficiencies under extremely light load condi-
tions.
If the load current should increase during PFM mode (Figure
8) causing the output voltage to fall below the ‘low2’ PFM
threshold, the part will automatically transition into fixed-fre-
quency 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 ~ 95 mA , when V_IN=3.6V, PWM to PFM
transition occurs at ~ 42 mA and PFM to PWM transition oc-
curs at ~ 115 mA, when V_IN =4.5V, PWM to PFM transition
occurs at ~ 60 mA and PFM to PWM transition occurs at ~
135 mA.
FIGURE 8. Operation in PFM Mode and Transfer to PWM Mode
30008403
SHUTDOWN MODE
Setting the EN input pin low (<0.4V) places the LM3677 in
shutdown mode. During shutdown the PFET switch, NFET
switch, reference, control and bias circuitry of the LM3677 are
turned off. Setting EN high (>1.0V) enables normal operation.
It is recommended to set EN pin low to turn off the LM3677
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 LM3677 has a soft-start circuit that limits in-rush current
during start-up. During start-up the switch current limit is in-
creased in steps. Soft start is activated only if EN goes from
logic low to logic high after V_IN reaches 2.7V. Soft start is im-
plemented by increasing switch current limit in steps of 200
mA, 400 mA, 600 mA and 1220 mA (typical switch current
limit). The start-up time thereby depends on the output ca-
pacitor and load current demanded at start-up. Typical start-
15
www.national.com