SOT-89 Step up Switching Regulator with Shutdown
Functions and Operation
some point where the load current gets to small to be handled effi-
The ILC6370 performs boost DC-DC conversion by controlling the
switch element shown in the circuit below.
ciently. If the ILC6370 had an ideal switch, this would not be such
a problem. But an actual switch consumes some finite amount of
current to switch on and off; at very low current this can be of the
same magnitude as the load current itself, driving switching effi-
ciencies down to 50% and below.
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 rquirements will vary by appli-
cation and by actual system design and layout, so generalizations
in this area are difficult, at best. [For other boost converter tech-
niques, please see the ILC6380/81 and ILC6390/91 data sheets.]
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 ILC6370 mon-
itors the voltage on the output capacitor to determine how much
and how often to drive the switch.
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 operation), 87% maximum duty cycles for high load con-
ditions, while maintaining very low shutdown current levels of
0.5µA. The only difference between the ILC6370 and ILC6371
parts is that the 6371 is configured to drive an external transistor
as the switch element. Since larger transistors can be selected for
this element, higher effective loads can be regulated.
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.
PL = ƒ(tON, VIN)
The ILC6370 and ILC6371 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 maxmim duty cycle of the switch determines the
maximum load current that the device can support. The ILC6370
and ILC6371 both support up to 87% duty cycles, for maximum
usable range of load currents.
Start-up Mode
The ILC6370 has an internal soft-start mode which suppresses
ringing or overshoot on the output during start-up. The following
diagram illustrates this start-up condition’s typical performance
VOUT MIN
VIN - Vf
There are two key advantages of PWM type controllers. First,
because the controller automatically varies the duty cycle of the
switche’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
output cap constanty accepting and storing the charge recieved
from the inductor, and delivering charge as required by the load.
The “pumping” action of the switch produces a sawtooth-shaped
voltage as seen by the output.]
T SOFT-START
(~10msec)
t = 0
External Components and Layout Consideration
The ILC6370 is designed to provide a complete DC-DC convertor
solution with a minmum of external components. Ideally, only
three externals are required: the inductor, a pass diode, and an
output capacitor.
The other key advatage of the PWM type controllers is that the
radiated noise due to the swtiching 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 minimze 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.
The inductor needs to be of low DC Resistance type, typically 1Ω
value. Toroidal wound inductors have better field containment (less
high frequency noise radiated out) but tend to be more expensive.
Some manufacturers like Coilcraft have new bobbin-wound induc-
tors with shielding included, which may be an ideal fit for these
applications. Contact the manufacturer for more information.
The inductor size needs to be in the range of 47µH to 1mH. In
general, larger inductor sizes deliver less current, so the load cur-
rent wil determine the inductor size used.
There are downsides of PWM approaches, especially at very low
currents. Because the PWM technique relies on constant switch-
ing and varying duty cycle to match the load conditions, there is
Impala Linear Corporation
(408) 574-3939
ILC6370/1 1.3
www.impalalinear.com
July 1999
5
IMPALA [ Impala Linear Corporation ]
