LTC4008EGN#TRPBF Datasheet LTC4008EGN#PBF, LTC4008EGN-1#PBF, LTC4008EGN#TRPBF | P13-P15

LTC4008

4008fb

APPLICATIONS INFORMATION

Charger Current Programming

The basic formula for charging current is:

VkR V

CHARGE MAX

REF PROG

SENSE

()

•. / –.

Ω3 01 0 035

REF

= 1.19V. This leaves two degrees of freedom: R

SENSE

and R

PROG

. The 3.01k input resistors must not be altered

since internal currents and voltages are trimmed for this

value. Pick R

SENSE

by setting the average voltage between

CSP and BAT to be close to 100mV during maximum

charger current. Then R

PROG

can be determined by solving

the above equation for R

PROG

RI V

PROG

REF

SENSE CHARGE MAX

•.

()

301

0 035

Table 2. Recommended R

SNS

and R

PROG

Resistor Values

MAX

(A) R

SENSE

(Ω) 1% R

SENSE

(W) R

PROG

(kΩ) 1%

1.0 0.100 0.25 26.7

2.0 0.050 0.25 26.7

3.0 0.033 0.5 26.7

4.0 0.025 0.5 26.7

Charging current can be programmed by pulse width

modulating R

PROG

with a switch Q1 to R

PROG

at a frequency

higher than a few kHz (Figure 5). C

PROG

must be increased

to reduce the ripple caused by the R

PROG

switching. The

compensation capacitor at I

will probably need to be

increased also to improve stability and prevent large

overshoot currents during start-up conditions. Charging

current will be proportional to the duty cycle of the switch

with full current at 100% duty cycle and zero current when

Q1 is off.

Maintaining C/10 Accuracy

The C/10 comparator threshold that drives the FLAG pin

has a ﬁ xed threshold of approximately V

PROG

= 400mV.

This threshold works well when R

PROG

is 26.7k, but will

not yield a 10% charging current indication if R

PROG

a different value. There are situations where a standard

value of R

SENSE

will not allow the desired value of charging

current when using the preferred R

PROG

value. In these

cases, where the full-scale voltage across R

SENSE

is within

±20mV of the 100mV full-scale target, the input resistors

connected to CSP and BAT can be adjusted to provide the

desired maximum programming current as well as the

correct FLAG trip point.

For example, the desired max charging current is 2.5A but

the best R

SENSE

value is 0.033Ω. In this case, the volt-

age across R

SENSE

at maximum charging current is only

82.5mV, normally R

PROG

would be 30.1k but the nominal

FLAG trip point is only 5% of maximum charging current.

If the input resistors are reduced by the same amount as

the full-scale voltage is reduced then, R4 = R5 = 2.49k

and R

PROG

= 26.7k, the maximum charging current is

still 2.5A but the FLAG trip point is maintained at 10%

of full scale.

There are other effects to consider. The voltage across the

current comparator is scaled to obtain the same values as

the 100mV sense voltage target, but the input referred sense

voltage is reduced, causing some careful consideration of

the ripple current. Input referred maximum comparator

threshold is 117mV, which is the same ratio of 1.4x the

DC target. Input referred I

REV

threshold is scaled back to

–24mV. The current at which the switcher starts will be

reduced as well so there is some risk of boost activity.

These concerns can be addressed by using a slightly larger

inductor to compensate for the reduction of tolerance to

ripple current.

102k

PROG

4008 F05

LTC4008

PROG

2N7002

PROG

Figure 5. PWM Current Programming

LTC4008

4008fb

APPLICATIONS INFORMATION

Battery Conditioning

Some batteries require a small charging current to condi-

tion them when they are severely depleted. The charging

current is switched to a high rate after the battery voltage

has reached a “safe” voltage to do so. Figure 6 illustrates

how to do this 2-level charging. When Q1 is on, the charger

current is set to maximum. When Q1 is off, the charging

current is set to 10% of the maximum.

53.6k

PROG

0.0047μF

4008 F06

LTC4008

2N7002

26.7k

PROG

Figure 6. 2-Level Current Programming

Charger Voltage Programming

A resistor divider, R8 and R9 (see Figure 10), programs

the ﬁ nal ﬂ oat voltage of the charger. The equation for ﬂ oat

voltage is (the input bias current of EA is typically –4nA

and can be ignored):

FLOAT

= V

REF

(1 + R8/R9)

It is recommended that the sum of R8 and R9 not be less

than 100k. Accuracy of the LTC4008 voltage reference is

±0.8% at 25°C, and ±1% over the full temperature range.

This leads to the possibility that very accurate (0.1%)

resistors might be needed for R8 and R9. Actually, the

temperature of the LTC4008 will rarely exceed 50°C near the

ﬂ oat voltage because charging currents have tapered to a low

level, so 0.25% resistors will normally provide the required

level of overall accuracy. Table 3 contains recommended

values for R8 and R9 for popular ﬂ oat voltages.

Table 3

FLOAT VOLTAGE (V) R9 (kΩ) 0.25% R8 (kΩ) 0.25%

8.2 24.9 147

8.4 26.1 158

12.3 15 140

12.6 16.9 162

16.4 11.5 147

16.8 13.3 174

Soft-Start

The LTC4008 is soft started by the 0.12μF capacitor on

the I

pin. On start-up, I

pin voltage will rise quickly to

0.5V, then ramp up at a rate set by the internal 40μA pull-up

current and the external capacitor. Battery charging current

starts ramping up when I

voltage reaches 0.8V and full

current is achieved with I

at 2V. With a 0.12μF capacitor,

time to reach full charge current is about 2ms and it is

assumed that input voltage to the charger will reach full

value in less than 2ms. The capacitor can be increased up

to 1μF if longer input start-up times are needed.

Input and Output Capacitors

The input capacitor (C2) is assumed to absorb all input

switching ripple current in the converter, so it must have

adequate ripple current rating. Worst-case RMS ripple

current will be equal to one-half of output charging current.

Actual capacitance value is not critical. Solid tantalum

low ESR capacitors have high ripple current rating in a

relatively small surface mount package, but caution must

be used when tantalum capacitors are used for input or

output bypass. High input surge currents can be created

when the adapter is hot-plugged to the charger or when

a battery is connected to the charger. Solid tantalum

capacitors have a known failure mechanism when subjected

to very high turn-on surge currents. Kemet T495 series

of “Surge Robust” low ESR tantalums are rated for high

surge conditions such as battery to ground.

LTC4008

4008fb

APPLICATIONS INFORMATION

The relatively high ESR of an aluminum electrolytic for

C1, located at the AC adapter input terminal, is helpful

in reducing ringing during the hot-plug event. Refer to

Application Note 88 for more information.

Highest possible voltage rating on the capacitor will

minimize problems. Consult with the manufacturer before

use. Alternatives include high capacity ceramic (at least

20μF) from Tokin, United Chemi-Con/Marcon, et al. Other

alternative capacitors include OS-CON capacitors from

Sanyo.

The output capacitor (C3) is also assumed to absorb

output switching current ripple. The general formula for

capacitor current is:

RMS

BAT

DCIN

()

⎛

⎝

⎜

⎞

⎠

⎟

()()

029 1

.–

For example:

DCIN

= 19V, V

BAT

= 12.6V, L1 = 10μH, and

f = 300kHz, I

RMS

= 0.41A.

EMI considerations usually make it desirable to minimize

ripple current in the battery leads, and beads or inductors

may be added to increase battery impedance at the 300kHz

switching frequency. Switching ripple current splits between

the battery and the output capacitor depending on the ESR

of the output capacitor and the battery impedance. If the

ESR of C3 is 0.2Ω and the battery impedance is raised to

4Ω with a bead or inductor, only 5% of the current ripple

will ﬂ ow in the battery.

Inductor Selection

Higher operating frequencies allow the use of smaller

inductor and capacitor values. A higher frequency gener-

ally results in lower efﬁ ciency because of MOSFET gate

charge losses. In addition, the effect of inductor value

on ripple current and low current operation must also be

considered. The inductor ripple current ΔI

decreases with

higher frequency and increases with higher V

Δ=

()()

⎛

⎝

⎜

⎞

⎠

⎟

L OUT

OUT

1–

Accepting larger values of ΔI

allows the use of low

inductances, but results in higher output voltage ripple

and greater core losses. A reasonable starting point for

setting ripple current is ΔI

= 0.4(I

MAX

). In no case should

ΔI

exceed 0.6(I

MAX

) due to limits imposed by I

REV

and

CA1. Remember the maximum ΔI

occurs at the maxi-

mum input voltage. In practice 10μH is the lowest value

recommended for use.

Lower charger currents generally call for larger inductor

values. Use Table 4 as a guide for selecting the correct

inductor value for your application.

Table 4

MAXIMUM AVERAGE

CURRENT (A)

INPUT

VOLTAGE (V)

MINIMUM INDUCTOR

VALUE (μH)

1 ≤20 40 ±20%

1 >20 56 ±20%

2 ≤20 20 ±20%

2 >20 30 ±20%

3 ≤20 15 ±20%

3 >20 20 ±20%

4 ≤20 10 ±20%

4 >20 15 ±20%

Charger Switching Power MOSFET

and Diode Selection

Two external power MOSFETs must be selected for use

with the charger: a P-channel MOSFET for the top (main)

switch and an N-channel MOSFET for the bottom (syn-

chronous) switch.

The peak-to-peak gate drive levels are set internally. This

voltage is typically 6V. Consequently, logic-level threshold

MOSFETs must be used. Pay close attention to the BV

DSS

speciﬁ cation for the MOSFETs as well; many of the logic

level MOSFETs are limited to 30V or less.

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P21 P22-P24

LTC4008EGN#TRPBF

Mfr. #:

Buy LTC4008EGN#TRPBF

Manufacturer:

Analog Devices / Linear Technology

Description:

Battery Management 4A, High Efficiency Li-Ion Charger

Lifecycle:

New from this manufacturer.

Delivery:

DHL FedEx Ups TNT EMS

Payment:

T/T Paypal Visa MoneyGram Western Union

LTC4008EGN#TRPBF

LTC4008EGN#TRPBF

Products related to this Datasheet