9

LT1500/LT1501

value, the peak-to-peak inductor current (set internally)

output voltage in continuous mode by adjusting the aver-

age value of inductor current while maintaining the peak-

to-peak value of the current relatively constant, hence, the

name “current mode architecture.”

The LT1500 sets the peak-to-peak value of switch current

internally to establish operating frequency. This peak-to-

peak value is scaled down somewhat at light load currents

to avoid as long as possible the characteristic of other

micropower converters wherein their switching frequency

drops very low (into the audio range) at less than full load

currents. At extremely light loads, even the LT1500 can no

longer maintain higher frequency operation, and utilizes a

Burst Mode operation to control output voltage.

Details of Continuous Mode Operation

At the start of a switch cycle, inductor current has de-

less than the internally generated voltage across Rh. This

causes the current comparator output to go high and turn

on the switch. At the same time, extra current is added to

Rh via S1 to create hysteresis in the trip point of the

comparator. This extra current is composed of a fixed

amount (I1), and an amount proportional to average

inductor current (I2). The presence of a variable I2 in-

creases switching frequency at lighter loads to extend the

load current range where high frequency operation is

maintained and no Burst Mode operation exists.

With the switch turned on, inductor current will increase

voltage across Rh. Then the comparator output will go

low, the switch will turn off and the current through Rh will

be switched back to its lower value. Inductor current will

decrease until the original condition is reached, complet-

ing one switch cycle.

Control of output voltage is maintained by adjusting the

continuous current flowing through Rh. This affects both

upper and lower inductor current trip levels at the same

time. Continuous Rh current is controlled by the error

amplifier which is comparing the voltage on the Feedback

pin to the internal 1.265V reference. An internal frequency

compensation capacitor filters out most the ripple voltage

at the amplifier output.

Operation at Light Loads

At light load currents the lower trip level (switch turn-on)

for inductor current drops below zero. At first glance, this

would seem to initiate a permanent switch off-state be-

cause the inductor current cannot reverse in a boost

topology. In fact, what happens is that output voltage

drops slightly between switch cycles, causing the error

amplifier output to increase and bring the current trip level

back up to zero. The switch then turns back on and

inductor current increases to a value set by I1 (I2 is near

zero at this point). The switch then turns off, and the

inductor energy is delivered to the output, causing it to rise

back up slightly. One or more switch cycles may be needed

to raise the output voltage high enough that the amplifier

output drops enough to force a sustained switch off

period. The output voltage then slowly drops back low

enough to cause the amplifier output to rise high enough

to initiate a switch turn-on. Switching operation now

consists of a series of bursts where the switch runs at

normal frequency for one or more cycles, then turns off for

a number of cycles. This Burst Mode operation is what

allows the LT1500 to have micropower operation and high

efficiency at very light loads.

Saving Current in Burst Mode Operation

Internal current drain for the LT1500 control circuitry is

about 400

µA when everything is operating. To achieve

higher efficiency at extremely light loads, a special oper-

ating mode is initiated when the error amplifier output is

toward the low end of its range. The adaptive bias circuit

comparator detects that the error amplifier output is below

a predetermined level and turns off the current comparator

and switch driver biasing. This reduces current drain to

about 200

µA, and forces a switch off state. Hysteresis in

the comparator forces the device to remain in this

micropower mode until the error amplifier output rises up

beyond the original trip point. The regulated output volt-

age will fall slightly over a relatively long period of time

(remember that load current is very low) until the error

amplifier output rises enough to turn off the adaptive bias