SOT-89 Step-up Dual-Mode Switcher with Shutdown
Functions and Operation
Dual Mode Operation
The ILC6380 performs boost DC-DC conversion by controlling the
switch element shown in the circuit below
But there are downsides of PWM approaches, especially at very
low currents. Because the PWM technique relies on constant
switching and varying duty cycle to match the load conditions,
there is some point where the load current gets too small to be
handled efficiently. An actual switch consumes some finite amount
of current to switch on and off; at very low currents this can be of
the same magnitude as the load current itself, driving switching
efficiencies down to 50% and below. The ILC6380 and ILC6381
overcome this limitation by automatically switching over to a PFM,
or Pulse Frequency Modulation, technique at low currents. This
technique conserves power loss by only switching the output if the
current drain requires it. As shown in the diagram below, the wave-
form actually skips pulses depending on the power needed by the
output. [This technique is also called “pulse skipping” because of
this characteristic.]
When the switch is closed, current is built up through the inductor.
When the switch opens, this current has to go somewhere and is
forced through the diode to the output. As this on and off switch-
ing continues, the output capacitor voltage builds up due to the
charge it is storing from the inductor current. In this way, the out-
put voltage gets boosted relative to the input. The ILC6380 moni-
tors the voltage on the output capacitor to determine how much
and how often to drive the switch.
Switch Waveform
In general, the switching characteristic is determined by the output
voltage desired and the current required by the load. Specifically
the energy transfer is determined by the power stored in the coil
during each switching cycle.
VSET
PL = ƒ(tON, VIN)
VOUT
The ILC6380 and ILC6381 use a PWM or Pulse Width Modulation
technique. The parts come in one of three fixed internal frequen-
cies: 50, 100, or 180kHz. The switches are constantly driven at
these frequencies. The control circuitry varies the power being
delivered to the load by varying the on-time, or duty cycle, of the
switch. Since more on-time translates to higher current build-up in
the inductor, the maximum duty cycle of the switch determines the
maximum load current that the device can support. The ILC6380
and ILC6381 both support up to 87% duty cycles, for maximum
usable range of load currents.
In the ILC6380 and ILC6381, this switchover is internally set to be
at the point where the PWM waveform hits approximately 10%
duty cycle. So the PFM mode is running at 10% duty cycle at the
rated frequency; for 100kHz part this means a constant on-time of
1msec. This not only is ideal for efficiency at these low currents,
but a 10% duty cycle will have much better output ripple charac-
teristics than a similarly configured PFM part, such as the ILC6390
and ILC6391.
The Dual-Mode architecture was designed specifically for those
applications, like communications, which need the spectral pre-
dictability of a PWM-type DC-DC converter, yet which also needs
the highest efficiencies possible, especially in Shutdown or
Standby mode. [For other conversion techniques, please see the
ILC6370/71 and ILC6390/91 datasheets.]
There are two key advantages of the PWM type controllers. First,
because the controller automatically varies the duty cycle of the
switch’s on-time in response to changing load conditions, the
PWM controller will always have an optimized waveform for a
steady-state load. This translates to very good efficiency at high
currents and minimal ripple on the output. [Ripple is due to the out-
put cap constantly accepting and storing the charge received from
the inductor, and delivering charge as required by the load. The
“pumping” action of the switch produces a sawtooth-shaped volt-
age as seen by the output.]
Other Considerations
The other limitation of PWM techniques is that, while the funda-
mental switching frequency is easier to filter out since it’s constant,
the higher order harmonics of PWM will be present and may have
to be filtered out, as well. Any filtering requirements, though, will
vary by application and by actual system design and layout, so
generalizations in this area are difficult, at best.
The other key advantage of the PWM type controllers is that the
radiated noise due to the switching transients will always occur at
the (fixed) switching frequency. Many applications do not care
much about switching noise, but certain types of applications,
especially communication equipment, need to minimize the high
frequency interference within their system as much as is possible.
Using a boost converter requires a certain amount of higher fre-
quency noise to be generated; using a PWM converter makes that
noise highly predictable; thus easier to filter out.
However, PWM control for boost DC-DC conversion is widely
used, especially in audio-noise sensitive applications or applica-
tions requiring strict filtering of the high frequency components.
Impala’s products give very good efficiencies of 85% at 50mA out-
put (5V product), 87% maximum duty cycles for high load condi-
tions, while maintaining very low shutdown current levels of
Impala Linear Corporation
(408) 574-3939
ILC6380/1 1.4
www.impalalinear.com
Sept 1999
4
IMPALA [ Impala Linear Corporation ]
