9
LT1500/LT1501
APPLICATIONS INFORMATION
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value, the peak-to-peak inductor current (set internally)
and the values for V
IN
and V
OUT
. The LT1500 controls
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-
creased to the point where the voltage across R
SENSE
is
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
until the voltage drop across R
SENSE
is equal to the higher
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 400µA 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 200µA, 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
