LT1511CSW datasheet(9/16 Pages) LINER

9 / 16 page

LT1511

APPLICATIONS INFORMATION

Input and Output Capacitors

In the 3A Lithium Battery Charger (Figure 1), the input

capacitor (CIN) 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 capacitors

such as the AVX TPS and Sprague 593D series have high

ripple current rating in a relatively small surface mount

package, but

caution must be used when tantalum capaci-

tors are used for input bypass. High input surge currents

can be created when the adapter is hot-plugged to the

charger and solid tantalum capacitors have a known

failure mechanism when subjected to very high turn-on

surge currents. Highest possible voltage rating on the

capacitor will minimize problems. Consult with the manu-

facturer before use. Alternatives include new high capacity

ceramic (5

µF to 20µF) from Tokin or United Chemi-Con/

Marcon, et al., and the old standby, aluminum electrolytic,

which will require more microfarads to achieve adequate

ripple rating. Sanyo OS-CON can also be used.

The output capacitor (COUT) is also assumed to absorb

output switching current ripple. The general formula for

capacitor current is:

IRMS =

(L1)(f)

VBAT

VCC

()

0.29 (VBAT) 1 –

For example, VCC = 16V, VBAT = 8.4V, L1 = 20µH,

and f = 200kHz, IRMS = 0.3A.

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 200kHz

switching frequency. Switching ripple current splits be-

tween the battery and the output capacitor depending on

the ESR of the output capacitor and the battery imped-

ance. If the ESR of COUT is 0.2Ω and the battery impedance

is rased to 4

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

current ripple will flow in the battery.

Soft-Start

The LT1511 is soft started by the 0.33

µF capacitor on the

VC pin. On start-up, VC pin voltage will rise quickly to 0.5V,

then ramp at a rate set by the internal 45

µA pull-up current

and the external capacitor. Battery charging current starts

ramping up when VC voltage reaches 0.7V and full current

is achieved with VC at 1.1V. With a 0.33µF capacitor, time

to reach full charge current is about 10ms and it is

assumed that input voltage to the charger will reach full

value in less than 10ms. The capacitor can be increased up

to 1

µF if longer input start-up times are needed.

In any switching regulator, conventional timer-based soft

starting can be defeated if the input voltage rises much

slower than the time out period. This happens because the

switching regulators in the battery charger and the com-

puter power supply are typically supplying a fixed amount

of power to the load. If input voltage comes up slowly

compared to the soft start time, the regulators will try to

deliver full power to the load when the input voltage is still

well below its final value. If the adapter is current limited,

it cannot deliver full power at reduced output voltages and

the possibility exists for a quasi “latch” state where the

adapter output stays in a current limited state at reduced

output voltage. For instance, if maximum charger plus

computer load power is 30W, a 15V adapter might be

current limited at 2.5A. If adapter voltage is less than

(30W/2.5A = 12V) when full power is drawn, the adapter

voltage will be sucked down by the constant 30W load until

it reaches a lower stable state where the switching regu-

lators can no longer supply full load. This situation can be

prevented by utilizing

undervoltage lockout, set higher

than the minimum adapter voltage where full power can be

achieved.

A fixed undervoltage lockout of 7V is built into the VCC pin,

but an additional adjustable lockout is also available on the

UV pin. Internal lockout is performed by clamping the VC

pin low. The VC pin is released from its clamped state when

the UV pin rises above 6.7V and is pulled low when the UV

pin drops below 6.2V (0.5V hysteresis). At the same time

UVOUT goes high with an external pull-up resistor. This

signal can be used to alert the system that charging is

about to start. The charger will start delivering current

about 4ms after VC is released, as set by the 0.33µF

LT1511CSW Datasheet(PDF) 9 Page - Linear Technology

LT1511CSW Datasheet(HTML) 9 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Datasheet Download

Link URL