8

LT1620/LT1621

reduces battery charging current until the external load

subsides.

In Figure 4 the LT1620 is coupled with an LT1513 SEPIC

battery charger IC to create an input overcurrent protected

charger circuit.

supply to ground. In this configuration, if the input current

drawn by the battery charger

combined with the system

load requirements exceeds a current limit threshold of 3A,

the battery charger current will be reduced by the LT1620

such that the total

input supply current is limited to 3A.

Refer to the LT1513 data sheet for additional information.

PROGRAMMING ACCURACY CONSIDERATIONS

PWM Controller Error Amp Maximum Source Current

In a typical battery charger application, the LT1620 con-

trols charge current by servoing the error amplifier output

pin of the associated PWM controller IC. Current mode

control is achieved when the LT1620 sinks all of the

current available from the error amplifier. Since the LT1620

has finite transconductance, the voltage required to gen-

erate its necessary output current translates to input

current of 130

µA to help reduce this term. Knowing the

current source capability of the associated PWM control-

ler in a given application will enable adjustment of the

required programming voltage to accommodate the de-

vs PWM source capability is shown in Figure 5a. For

example, the LTC1435 has a current source capability of

about 75

µA. This translates to about –15mV of induced

PROG pin must be 15mV lower).

The LT1620 sense amplifier circuit has an inherent input

loop operation during light load conditions. This offset vs

input voltage has a linear characteristic, crossing 0V as

output (times a factor of 10), and thus to the programming

desired load current corresponds to 100mV across the

absolute voltage at the PROG pin must be 7.5mV higher).

This error term should be taken into consideration when

(LT1620GN Only)

well below the zero offset point of 1.6V, so adjustment for this

typically requires an additional –50mV offset (the absolute

voltage at the PROG2 pin must be 50mV lower).

The LT1620 sense amplifier has additional input offset

tolerance when the inputs are pulled significantly below

referred offset of up to 11mV when the inputs are at 0V

common-mode. This additional offset term reduces roughly

applications, this offset increases the charge current tol-

from ground. The resulting output current shift is generally

negative; however, this offset is not precisely controlled.

Precision operation should not be attempted with sense

The LT1620 sense amplifier induces a small additional

a linear characteristic and amounts to about

±0.33mV

(input-referred) over the recommended operating range

and AVG2 outputs (times factors of 10 and 20), and thus

to the programming voltages. A plot of programming