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LTC1759

voltage will be pulled 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 the DCDIV resistor divider, set

higher than the minimum adapter voltage where full power

can be achieved.

Input and Output Capacitors

In the 4A Lithium Battery Charger (Figure 10), the input

capacitor (C14) 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. Only Kemet T495 series of “Surge

Robust” low ESR tantalums are rated for high surge

conditions such as battery to ground.

The relatively high ESR of an aluminum electrolytic for

C15, located at the AC adapter input terminal, is helpful in

reducing ringing during the hot-plug event.

Highest possible voltage rating on the capacitor will mini-

mize problems. Consult with the manufacturer before use.

Alternatives include new high capacity ceramic (at least

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

ever, using ceramic capacitors in the output filter of the

charger can lead to acoustic noise radiation that can be

confused with instability. At low charge currents, the

charger operates in discontinuous current mode at an

audible frequency. Other alternative capacitors include

OSCON capacitors from Sanyo.

The output capacitor (C3) is also assumed to absorb

output switching current ripple. The general formula for

capacitor current is:

APPLICATIONS INFORMATION

IRMS =

(L1)(f)

VBAT

VCC

()

0.29 (VBAT) 1 –

For example, VCC = 19V, VBAT = 12.6V, L1 = 10µH, and

f = 200kHz, IRMS = 0.6A.

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 C3 is 0.2

Ω and the battery impedance

is raised to 4

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

current ripple will flow in the battery.

Charger Crowbar Protection

If VIN connector of Figure 1 charger can be instantaneously

shorted (crowbarred) to ground, then a small

P-channel FET M4 should be used to fast turn off the input

P-channel FET M3 (see Figure 11), otherwise, high surge

current might damage M3. M3 can also be replaced by a

diode if dropout voltage and heat dissipation are not

problems.

Note that LT1759 will operate even when VBAT is grounded.

If VBAT of Figure 1 charger gets shorted to ground very

quickly (crowbarred) from a high battery voltage, slow

loop response may allow charge current to build up and

damage the topside N-channel FET M1. A small diode D5

(see Figure 12) from SDB pin to VBAT will shut down

switching and protect the charger.

Note that M4 and/or D5 are needed only if the charger

system can be potentially crowbarred.

Protecting SMBus Inputs

The SMBus inputs, SCL and SDA, are exposed to uncon-

trolled transient signals whenever a battery is connected

to the system. If the battery contains a static charge, the

SMBus inputs are subjected to ESD which can cause

damage after repeated exposure. Also, if the battery’s

positive terminal makes contact to the connector before

the negative terminal, the SMBus inputs can be forced

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