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LT1769

APPLICATIONS INFORMATION

Input and Output Capacitors

In the 2A Lithium-Ion 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 the output charge 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

are possible 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. Selecting a high 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. Sanyo OS-CON can also be used.

The output capacitor (COUT) is also assumed to absorb

output switching ripple current. The general formula for

capacitor ripple 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. Beads or inductors can

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 raised to 4

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

ripple current will flow into the battery.

Soft-Start and Undervoltage Lockout

The LT1769 is soft-started by the 0.33

µF capacitor on the

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

0.5V, then ramp at a rate set by the internal 45

µA pull-up

current and the external capacitor. Charge current starts

ramping up when VC pin voltage reaches 0.7V and full

current is achieved with VC at 1.1V. With a 0.33µF capaci-

tor, the 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 time-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 the 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 25W, a 15V adapter might be

current limited at 2A. If adapter voltage is less than

(25W/2A = 12.5V) when full power is drawn, the adapter

voltage will be pulled down by the constant 25W 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 LT1769.

This 7V threshold can be increased by adding a resistive

divider to the UV pin as shown in Figure 2. 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 7V and is pulled low when the UV pin drops below

6.5V (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 0.33

µF capacitor. A resistor divider

is used to set the desired VCC lockout voltage as shown in

Figure 2. A typical value for R6 is 5k and R5 is found from:

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