LTC4080X datasheet(14/20 Pages) LINER

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LTC4080X

4080Xf

OPERATIO

enough. The frequency of this intermittent burst operation

depends on the load current. That is, as the load current

drops further, the regulator turns on less frequently. Thus

Burst Mode operation increases the efﬁciency at light loads

by minimizing the switching and quiescent losses. However,

the output voltage ripple increases to about 2%.

To minimize ripple in the output voltage, the current limits

for both switches in Burst Mode operation are reduced

to about 20% of their values in the constant-frequency

mode. Also the zero current of the synchronous switch

is changed to about 35mA thereby preventing reverse

conduction through the inductor. Consequently, the regu-

lator can only deliver approximately 67mA of load current

while in Burst Mode operation. Any attempt to draw more

load current will cause the output voltage to drop out of

regulation.

Current Limit

To prevent inductor current runaway, there are absolute

current limits (ILIM) on both the PMOS main switch and

the NMOS synchronous switch. These limits are internally

set at 520mA and 700mA respectively for PWM mode. If

the peak inductor current demanded by the error ampliﬁer

ever exceeds the PMOS ILIM, the error ampliﬁer will be

ignored and the inductor current will be limited to PMOS

ILIM. In Burst Mode operation, the PMOS current limit is

reduced to 100mA to minimize output voltage ripple.

Zero Current Comparator

The zero or reverse current comparator monitors the induc-

tor current to the output and shuts off the synchronous

rectiﬁer when this current reduces to a predetermined

value (IZERO). In ﬁxed frequency mode, this is set to nega-

tive 15mA meaning that the regulator allows the inductor

current to ﬂow in the reverse direction (from the output to

ground through the synchronous rectiﬁer) to a maximum

value of 15mA. This is done to ensure that the regulator

is able to regulate at very light loads without skipping any

cycles thereby keeping output voltage ripple and noise low

at the cost of efﬁciency.

However, in Burst Mode operation, IZERO is set to positive

35mA meaning that the synchronous switch is turned off

as soon as the current through the inductor to the output

decreases to 35mA in the discharge cycle. This preserves

the charge on the output capacitor and increases the overall

efﬁciency at light loads.

Soft-Start

The LTC4080X switching regulator provides soft-start in

both modes of operation by slowly charging an internal

capacitor. The voltage on this capacitor, in turn, slowly

ramps the current limits of both switches from a low value

to their respective maximum values over a period of about

400µs. The soft-start capacitor is discharged completely

whenever the regulator is disabled.

Short-Circuit Protection

In the event of a short circuit at the output or during

start-up, VOUT will be near zero volts. Since the downward

slope of the inductor current is ~VOUT/L, the inductor

current may not get a chance to discharge enough to

avoid a runaway situation. Because the current sensing

is blanked for ~12ns at the beginning of each clock cycle,

inductor current can build up to a dangerously high level

over a number of cycles even if there is a hard current

limit on the main PMOS switch. This is why the switching

regulator in the LTC4080X also monitors current through

the synchronous NMOS switch and imposes a hard limit

on it. If the inductor current through the NMOS switch at

the end of a discharge cycle is not below this limit, the

regulator skips the next charging cycle thereby preventing

inductor current runaway.

Switching Regulator Undervoltage Lockout

Whenever VBAT is less than 2.7V, an undervoltage lock-

out circuit keeps the regulator off, preventing unreliable

operation. However, if the regulator is already running

and the battery voltage is dropping, the undervoltage

comparator does not shut down the regulator until VBAT

drops below 2.5V.

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