LTC4080EMSE View Datasheet(PDF) - Linear Technology

Part Name

Description

MFG CO.

LTC4080EMSE

500mA Standalone Li-Ion Charger with Integrated 300mA Synchronous Buck

Linear Technology 

U

OPERATIO

from zero to full-scale current over a period of approximately

180μs. This has the effect of minimizing the transient cur-

rent load on the power supply during start-up.

Timer and Recharge

The LTC4080’s battery charger has an internal termina-

tion timer that starts when the input voltage is greater

than the undervoltage lockout threshold and at least

80mV 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 regulator in the LTC4080 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 effi-

ciency whereas the Burst Mode operation offers increased

efficiency at light loads at the cost of increased noise and

output voltage ripple. A detailed description of different

operating modes and different aspects of operation fol-

low. 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

LTC4080

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 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

current signal is then converted into a voltage signal

(ITH), and 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 ITH and

determines 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 (~80mA) 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

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