LTC1980 Просмотр технического описания (PDF) - Linear Technology

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LTC1980

Combination Battery Charger and DC/DC Converter

Linear Technology 

LTC1980

U

OPERATIO

Lithium-Ion Battery Charger Operation

With the wall adapter power applied, the LTC1980 oper-

ates as a constant-current/constant-voltage PWM battery

charger, with a portion of the adapter current used for

charging and the rest flowing to the system load through

an optional low dropout regulator.

A charge cycle begins when the voltage at VREG exceeds

the undervoltage lockout threshold level and the IC is

enabled via the MODE pin. If the battery has been deeply

discharged and the battery voltage is less than 2.7V, the

charger will begin with the programmed trickle charge

current.

When the battery exceeds 2.7V, the charger begins the

constant-current portion of the charge cycle with the

charge current equal to the programmed level. As the

battery accepts charge, the voltage increases. When the

battery voltage reaches the recharge threshold, the pro-

grammable timer begins. Constant-current charging con-

tinues until the battery approaches the programmed charge

voltage of 4.1V or 4.2V/cell at which time the charge

current will begin to drop, signaling the beginning of the

constant-voltage portion of the charge cycle. The charger

will maintain the programmed preset float voltage across

the battery until the timer terminates the charge cycle.

During trickle charging, if the battery voltage remains

below 2.7V for 1/4 of the total programmed charge time,

the battery may be defective and the charge cycle ends.

Also, if a battery open circuit is detected, the charge cycle

ends immediately. The charger can be shut down by

pulling the REG pin low, although the timer will continue

until it times out.

Power Converter Operation from Battery

When the AC adapter is removed, the LTC1980 operates as

a DC/DC PWM converter using the battery for input power

to provide a regulated output voltage for the system load.

The LTC1980 is a current mode switcher. This means that

the switch duty cycle is directly controlled by switch

current rather than by output voltage or current. Battery

charger operation will be described for the simplified

diagram (Figure 3). At the start of the oscillator cycle, latch

U9 is set causing M2 to turn on. When switch current

reaches a predetermined level M2 turns off and M1 turns

on. This level is set by the control voltage at the output of

error amplifier U10.

10

U1 VOLTAGE

SELECTION

VBAT

T1

VREG

B1 C1 SN1 SNUBBER

NETWORK

SN2 SNUBBER

R1

NETWORK

C2

–

U4

BDRIVE

DRIVERS

M1 M2

RDRIVE

SENSE

U2

R2

+

DIRECTION

VREF VREF

TYPICAL

WAVEFORM

U5

ZC

CURRENT

AMPLIFIER

R5

++

R4

U6

––

SW1

R7

U7

OSC

R6

C3

+

U8

–

PWM

R9 R8

SW3

– C4

SW2

U10

+

EA

U11

REFERENCE

U12

C5

Figure 3. Simplified Diagram—Power Converter

WALL

ADAPTER

TO SYSTEM

LOAD

R12

C6

R13

S

R

U9

Q

R10

R11

VREF

1980 F03

1980f

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