5

LT1620/LT1621

used as integration nodes to facilitate averaging of the

current sense amplifier signal. (Note: filter capacitors on

of these signals enables direct sensing and control of DC

load current, eliminating the inclusion of ripple current in

load determination.

Transconductance Amplifier

The transconductance amplifier converts the difference

unidirectional and only sinks current. The amplifier is

designed to operate at a typical output current of 130

µA

(Refer to the Functional Block Diagram)

loop on the mated PWM controller IC to maintain a

programmed load current.

Comparator

The comparator circuit (available only in the LT1620GN)

may be used as an end-of-cycle sensor in a Li-Ion battery

charging system. The comparator detects when the charg-

ing current has fallen to a small value (typically 20% of the

maximum charging current). The comparator drives an open

the programmed threshold).

In Figure 2, an LT1620MS8 is coupled with an LTC1435

switching regulator in a high performance lithium-ion

battery charger application. The LTC1435 switching regu-

lator delivers extremely low dropout as it is capable of

approximately 99% duty cycle operation. No additional

power supply voltage is required for the LT1620 in this

application; it is powered directly from a 5V local supply

generated by the LTC1435. The DC charge current control

and high common mode current sense range of the

LT1620 combine with the low dropout capabilities of the

LTC1435 to make a 4-cell Li-Ion battery charger with over

96% efficiency, and only 0.5V input-to-output drop at 3A

charging current. Refer to the LTC1435 data sheet (available

from the LTC factory) for additional information on IC func-

tionality, performance and associated component selection.

This LT1620/LTC1435 battery charger is designed to yield

a 16.8V float voltage with a battery charge current of 3.2A.

switch MOSFETs). The charger provides a constant 3.2A

charge current until the battery voltage reaches the pro-

grammed float voltage. Once the float voltage is achieved,

a precision voltage regulation loop takes control, allowing

the charge current to fall as required to complete the

battery charge cycle.

The LT1620 will operate throughout a current program-

current setting program voltage of – 0.8V, corresponding

to 80mV differential voltage across the current sense

amplifier inputs. Given the desired current requirement,

possible. For the desired 3.2A charge current;

At the programmed 3.2A charge current, the sense resis-

tor will dissipate (0.08V)(3.20A) = 0.256W, and must be

rated accordingly.

Current Sense

The current sense inputs are connected on either side of

given average charging current flow. The sense resistor to

pair or minimum PC trace spacing for noise immunity.

Keep lead lengths short and away from noise sources for

best performance.