LTC3553EUD-2#PBF Datasheet LTC3553EUD-2#PBF, LTC3553EUD-2#TRPBF | P16-P18

LTC3553-2

35532f

P-channel MOSFET whenever the voltage at V

OUT

is ap-

proximately 15mV (V

FWD

) below the voltage at BAT. The

resistance of the internal ideal diode is approximately

240m.

Suspend Mode

When the SUSP pin is pulled high the LTC3553-2 enters

suspend mode to comply with the USB speciﬁ cation. In

this mode, the power path between V

BUS

and V

OUT

is put

in a high impedance state to reduce the V

BUS

input current

to 15A. The system load connected to V

OUT

is supplied

through the ideal diode connected to BAT.

BUS

Undervoltage Lockout (UVLO) and Undervoltage

Current Limit (UVCL)

An internal undervoltage lockout circuit monitors V

BUS

and keeps the input current limit circuitry off until V

BUS

rises above the rising UVLO threshold (3.8V) and at least

200mV above V

BAT

. Hysteresis on the UVLO turns off the

input current limit circuitry if V

BUS

drops below 3.6V or

wi thin 50 mV of V

BAT

. When this happens, system power at

OUT

will be drawn from the battery via the ideal diode. To

minimize the possibility of oscillation in and out of UVLO

when using resistive input supplies, the input current limit

is reduced as V

BUS

falls below 4.45V typical.

Battery Charger

The LTC3553-2 includes a constant-current/constant-volt-

age battery charger with automatic recharge, automatic

termination by safety timer, low voltage trickle charging,

bad cell detection and thermistor sensor input for out of

temperature charge pausing. When a battery charge cycle

begins, the battery charger ﬁ rst determines if the battery

is deeply discharged. If the battery voltage is below V

TRKL

typically 2.9V, an automatic trickle charge feature sets the

battery charge current to 10% of the programmed value. If

the low voltage persists for more than 1/2 hour, the battery

charger automatically terminates. Once the battery voltage

is above 2.9V, the battery charger begins charging in full

power constant current mode. The current delivered to

the battery will try to reach 750V/R

PROG

. Depending on

available input power and external load conditions, the

battery charger may or may not be able to charge at the

full programmed current. The external load will always be

prioritized over the battery charge current. The USB cur-

rent limit programming will always be observed and only

additional current will be available to charge the battery.

When system loads are light, battery charge current will

be maximized.

Charge Termination

The battery charger has a built-in safety timer. When the

battery voltage approaches the ﬂ oat voltage, the charge

current begins to decrease as the LTC3553-2 enters

constant-voltage mode. Once the battery charger detects

that it has entered constant-voltage mode, the four hour

safety timer is started. After the safety timer expires,

charging of the battery will terminate and no more current

will be delivered to the battery.

Automatic Recharge

After the battery charger terminates, it will remain off

drawing only microamperes of current from the battery.

If the portable product remains in this state long enough,

the battery will eventually self discharge. To ensure that

the battery is always topped off, a charge cycle will au-

tomatically begin when the battery voltage falls below

RECHRG

(typically 4.1V). In the event that the safety timer

is running when the battery voltage falls below V

RECHRG

the timer will reset back to zero. To prevent brief excur-

sions below V

RECHRG

from resetting the safety timer, the

battery voltage must be below V

RECHRG

for approximately

2ms. The charge cycle and safety timer will also restart

if the V

BUS

UVLO cycles low and then high (e.g., V

BUS

, is

removed and then replaced).

Charge Current

The charge current is programmed using a single resis-

tor from PROG to ground. 1/750th of the battery charge

current is delivered to PROG which will attempt to servo

to 1.000V. Thus, the battery charge current will try to

reach 750 times the current in the PROG pin. The program

resistor and the charge current are calculated using the

following equations:

PROG

750V

CHG

750V

PROG

OPERATION

LTC3553-2

35532f

In either the constant-current or constant-voltage charg-

ing modes, the PROG pin voltage will be proportional to

the actual charge current delivered to the battery. There-

fore, the actual charge current can be determined at any

time by monitoring the PROG pin voltage and using the

following equation:

BAT

PROG

• 750

In many cases, the actual bat ter y charge current, I

BAT

, will

be lower than I

CHG

due to limited input current available

and prioritization with the system load drawn from V

OUT

Thermal Regulation

To prevent thermal damage to the IC or surrounding compo-

nen t s, an in te rn al t her ma l f ee dback lo op will au tom at ic all y

decrease the programmed charge current if the die tem-

perature rises to approximately 110°C. Thermal regulation

protects the LTC3553-2 from excessive temperature due to

high power operation or high ambient thermal conditions

and allows the user to push the limits of the power handling

capability with a given circuit board design without risk

of damaging the LTC3553-2 or external components. The

beneﬁ t of the LTC3553-2 thermal regulation loop is that

charge current can be set according to the desired charge

rate rather than worst-case conditions with the assurance

that the battery charger will automatically reduce the cur-

rent in worst-case conditions.

Charge Status Indication

The CHRG pin indicates the status of the battery charger. An

open-drain output, the CHRG pin can drive an indicator LED

through a current limiting resistor for human interfacing or

simply a pull-up resistor for microprocessor interfacing.

When charging begins, CHRG is pulled low and remains

low for the duration of a normal charge cycle. When charg-

ing is complete, i.e., the charger enters constant-voltage

mode and the charge current has dropped to one-tenth

of the programmed value, the CHRG pin is released (high

impedance). The CHRG pin does not respond to the C/10

threshold if the LTC3553-2 reduces the charge current due

to excess load on the V

OUT

pin. This prevents false end

of charge indications due to insufﬁ cient power available

to the battery charger. Even though charging is stopped

during an NTC fault the CHRG pin will stay low indicating

that charging is not complete.

Battery Charger Stability Considerations

The LTC3553-2’s battery charger contains both a constant-

voltage and a constant-current control loop. The constant-

voltage loop is stable without any compensation when a

battery is connected with low impedance leads. Excessive

lead length, however, may add enough series inductance

to require a bypass capacitor of at least 1F from BAT to

GND. Furthermore, a 100F 1210 ceramic capacitor in

series with a 0.3 resistor from BAT to GND is required

to keep ripple voltage low if operation with the battery

disconnected is allowed.

High value, low ESR multilayer ceramic chip capacitors

reduce the constant-voltage loop phase margin, possibly

resulting in instability. Ceramic capacitors up to 22F may

be used in parallel with a battery, but larger ceramics

should be decoupled with 0.2 to 1 of series resist ance.

In constant-current mode, the PROG pin is in the feed-

back loop rather than the battery voltage. Because of the

additional pole created by any PROG pin capacitance,

capacitance on this pin must be kept to a minimum. With

no additional capacitance on the PROG pin, the battery

charger is stable with program resistor values as high

as 25k. However, additional capacitance on this node

reduces the maximum allowed program resistor. The pole

frequency at the PROG pin should be kept above 100kHz.

Therefore, if the PROG pin has a parasitic capacitance,

PROG

, the following equation should be used to calculate

the maximum resistance value for R

PROG

≤

2π • 100kHz •C

PROG

OPERATION

LTC3553-2

35532f

NTC Thermistor

The battery temperature is measured by placing a nega-

tive temperature coefﬁ cient (NTC) thermistor close to the

battery pack. To use this feature connect the NTC therm-

istor, R

NTC

, between the NTC pin and ground and a bias

resistor, R

NOM

, from V

BUS

to NTC, as shown in Figure 1.

NOM

should be a 1% resistor with a value equal to the

value of the chosen NTC thermistor at 25°C (R25). The

LTC3553-2 will pause charging when the resistance of

the NTC thermistor drops to 0.54 times the value of R25

or approximately 54k (for a Vishay curve 1 thermistor,

this corresponds to approximately 40°C). If the battery

charger is in constant-voltage mode, the safety timer also

pauses until the thermistor indicates a return to a valid

temperature. As the temperature drops, the resistance of

the NTC thermistor rises. The LTC3553-2 is also designed

to pause charging when the value of the NTC thermistor

increases to 3.17 times the value of R25. For a Vishay

curve 1 thermistor this resistance, 317k, corresponds to

approximately 0°C. The hot and cold comparators each

have approximately 3°C of hysteresis to prevent oscillation

about the trip point.

Alternate NTC Thermistors and Biasing

The LTC3553-2 provides temperature qualiﬁ ed charging if

a grounded thermistor and a bias resistor are connected

to NTC. By using a bias resistor whose value is equal to

the room temperature resistance of the thermistor (R25)

the upper and lower temperatures are preprogrammed

to approximately 40°C and 0°C, respectively (assuming

a Vishay curve 1 thermistor).

The upper and lower temperature thresholds can be

adjusted by either a modiﬁ cation of the bias resistor value

or by adding a second adjustment resistor to the circuit.

If only the bias resistor is adjusted, then either the upper

or the lower threshold can be modiﬁ ed but not both. The

other trip point will be determined by the characteristics

of the thermistor. Using the bias resistor in addition to an

adjustment resistor, both the upper and the lower tem-

perature trip points can be independently programmed

with the constraint that the difference between the upper

and lower temperature thresholds cannot decrease.

Examples of each technique are given below.

NTC thermistors have temperature characteristics which

are indicated on resistance-temperature conversion tables.

The Vishay-Dale thermistor NTHS0603N011-N1003F, used

in the following examples, has a nominal value of 100k

and follows the Vishay curve 1 resistance-temperature

characteristic.

In the explanation below, the following notation is used.

R25 = Value of the thermistor at 25°C

NTC|COLD

= Value of thermistor at the cold trip point

NTC|HOT

= Value of the thermistor at the hot trip point

COLD

= Ratio of R

NTC|COLD

to R25

HOT

= Ratio of R

NTC|HOT

to R25

NOM

= Primary thermistor bias resistor (see Figure 2)

R1 = Optional temperature range adjustment resistor (see

Figure 2)

OPERATION

Figure 1. Typical NTC Thermistor Circuit

–

NOM

100k

NTC

100k

NTC

BUS

NTC_ENABLE

35532 F01

NTC BLOCK

TOO_COLD

TOO_HOT

t7

BUS

(NTC RISING)

t7

BUS

(NTC FALLING)

t7

BUS

(NTC FALLING)

–

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LTC3553EUD-2#PBF

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Manufacturer:

Analog Devices / Linear Technology

Description:

Battery Management Micropower USB Power Manager with Li-Ion Charger, LDO, and Buck Regulator

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LTC3553EUD-2#PBF

LTC3553EUD-2#PBF

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