LM3880 datasheet(14/20 Pages) NSC

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Current Limiting

A current limit feature allows the LM3686 to protect itself and

external components during overload conditions. PWM mode

implements current limiting using an internal comparator that

trips at 1220mA (typ). If the output is shorted to ground the

device enters a timed current limit mode where the NFET is

turned on for a longer duration until the inductor current falls

below a low threshold. This allows the inductor current more

time to decay, thereby preventing runaway.

PFM Operation

At very light load, the DC-DC converter enters PFM mode and

operates with reduced switching frequency and supply cur-

rent to maintain high efficiency. The part automatically tran-

sitions into PFM mode when either of two conditions occurs

for a duration of 32 or more clock cycles:

A. The NFET current reaches zero.

B. The peak PMOS switch current drops below the I

MODE lev-

el, (typically I

MODE < 75 mA + VBATT / 55Ω ).

30025510

FIGURE 5. Typical PFM Operation

During PFM operation, the DC-DC converter positions the

output voltage slightly higher than the nominal output voltage

during PWM operation, allowing additional headroom for volt-

age 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 + VBATT / 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 6), 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 28µA (typ), which allows the part to achieve

high efficiency under extremely light load conditions.

If the load current should increase during PFM mode (see

Figure 6) causing the output voltage to fall below the ‘low2’

PFM threshold, the part will automatically transition into fixed-

frequency PWM mode.

When V

BATT = 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

BATT= 3.6V, PWM to PFM transition hap-

pens at ~42 mA and PFM to PWM transition happens at ~115

mA, when V

BATT = 4.5V, PWM to PFM transition happens at

~60 mA and PFM to PWM transition happens at ~135 mA.

30025526

FIGURE 6. Operation in PFM Mode and Transfer to PWM Mode

LM3880 Datasheet(PDF) 14 Page - National Semiconductor (TI)

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