LTC4080X datasheet(13/20 Pages) LINER

13 / 20 page

LTC4080X

4080Xf

Timer and Recharge

The LTC4080X’s battery charger has an internal charge

termination timer that starts when the input voltage is

greater than the undervoltage lockout threshold and at

least 82mV above BAT, and the battery charger is leaving

shutdown.

At power-up or when exiting shutdown, the charge time

is set to 4.5 hours. Once the charge cycle terminates, the

battery charger continuously monitors the BAT pin voltage

using a comparator with a 2ms ﬁlter time. When the aver-

age battery voltage falls below 4.1V (which corresponds

to 80%-90% battery capacity), a new charge cycle is initi-

ated and a 2.25 hour timer begins. This ensures that the

battery is kept at, or near, a fully charged condition and

eliminates the need for periodic charge cycle initiations.

The

⎯C⎯H⎯R⎯G output assumes a strong pulldown state dur-

ing recharge cycles until C/10 is reached or the recharge

cycle terminates.

SWITCHING REGULATOR OPERATION:

The switching buck regulator in the LTC4080X can be turned

on by pulling the EN_BUCK pin above VIH. It has two user-

selectable modes of operation: constant-frequency (PWM)

mode and Burst Mode Operation. The constant-frequency

mode operation offers low noise at the expense of efﬁciency

whereas the Burst Mode operation offers higher efﬁciency

at light loads at the cost of increased noise, higher output

voltage ripple, and less output current. A detailed descrip-

tion of different operating modes and different aspects of

operation follow. Operations can best be understood by

referring to the Block Diagram.

Constant-Frequency (PWM) Mode Operation

The switching regulator operates in constant-frequency

(PWM) mode when the MODE pin is pulled below VIL. In

this mode, it uses a current mode architecture including

an oscillator, an error ampliﬁer, and a PWM comparator

for excellent line and load regulation. The main switch

MP2 (P-channel MOSFET) turns on to charge the inductor

at the beginning of each clock cycle if the FB pin voltage

is less than the 0.8V reference voltage. The current into

the inductor (and the load) increases until it reaches the

peak current demanded by the error amp. At this point,

the main switch turns off and the synchronous switch

MN1 (N-channel MOSFET) turns on allowing the inductor

current to ﬂow from ground to the load until either the

next clock cycle begins or the current reduces to the zero

current (IZERO) level.

Oscillator: In constant-frequency mode, the switching

regulator uses a dedicated oscillator which runs at a

ﬁxed frequency of 2.25MHz. This frequency is chosen to

minimize possible interference with the AM radio band.

Error Ampliﬁer: The error ampliﬁer is an internally com-

pensated transconductance (gm) ampliﬁer with a gm

of 65 µmhos. The internal 0.8V reference voltage is

compared to the voltage at the FB pin to generate a

current signal at the output of the error ampliﬁer. This cur-

rent signal represents the peak inductor current required

to achieve regulation.

PWM Comparator: Lossless current sensing converts

the PMOS switch current signal to a voltage which is

summed with the internal slope compensation signal.

The PWM comparator compares this summed signal to

determine when to turn off the main switch. The switch

current sensing is blanked for ~12ns at the beginning of

each clock cycle to prevent false switch turn-off.

Burst Mode Operation

Burst Mode operation can be selected by pulling the

MODE pin above VIH. In this mode, the internal oscil-

lator is disabled, the error ampliﬁer is converted into a

comparator monitoring the FB voltage, and the inductor

current swings between a ﬁxed IPEAK (~100mA) and IZERO

(35mA) irrespective of the load current as long as the FB

pin voltage is less than or equal to the reference voltage

of 0.8V. Once VFB is greater than 0.8V, the control logic

shuts off both switches along with most of the circuitry

and the regulator is said to enter into SLEEP mode. In

SLEEP mode, the regulator only draws about 20µA from

the BAT pin provided that the battery charger is turned

off. When the output voltage droops about 1% from its

nominal value, the regulator wakes up and the inductor

current resumes swinging between IPEAK and IZERO. The

output capacitor recharges and causes the regulator to

re-enter the SLEEP state if the output load remains light

OPERATIO

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