ILC6370 데이터 시트보기 (PDF) - Fairchild Semiconductor
부품명
상세내역
제조사
ILC6370 Datasheet PDF : 9 Pages
| |||
ILC6370/71
Functions and Operation
The ILC6370 performs boost DC-DC conversion by control-
ling the switch element shown in the circuit below.
minimize the high frequency interference within their system
as much as is possible. Using a boost converter requires a
certain amount of higher frequency noise to be generated;
using a PWM converter makes that noise highly predictable;
thus easier to filter out.
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 switching continues, the output capacitor
voltage builds up due to the charge it is storing from the
inductor current. In this way, the output voltage gets boosted
relative to the input. The ILC6370 monitors the voltage on
the output capacitor to determine how much and how often
to drive the switch.
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 frequencies: 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 max-
imum 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.
There are two key advantages of 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 opti-
mized 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 constantly
accepting and storing the charge received from the inductor,
and delivering charge as required by the load. The “pump-
ing” action of the switch produces a sawtooth-shaped volt-
age as seen by the output.]
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 condi-
tions, there is some point where the load current gets to small
to be handled efficiently. If the ILC6370 had an ideal switch,
this would not be such a problem. But an actual switch con-
sumes 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 efficiencies down to
50% and below.
The other limitation of PWM techniques is that, while the
fundamental 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 will vary by application and by actual system
design and layout, so generalizations in this area are diffi-
cult, at best. [For other boost converter techniques, please
see the ILC6380/81 and ILC6390/91 data sheets.]
However, PWM control for boost DC-DC conversion is
widely used, especially in audio-noise sensitive applications
or applications requiring strict filtering of the high frequency
components. Impala’s products give very good efficiencies
of 85% at 50mA output (5V operation), 87% maximum duty
cycles for high load conditions, 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 ele-
ment. Since larger transistors can be selected for this ele-
ment, higher effective loads can be regulated.
Start-up Mode
The ILC6370 has an internal soft-start mode which sup-
presses ringing or overshoot on the output during start-up.
The following diagram illustrates this start-up condition’s
typical performance.
VIN - Vf
VOUT MIN
T SOFT-START
(~10msec)
The other key advantage of the PWM type controllers is that
the radiated noise due to the switching transients will always
t=0
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
©2001 Fairchild Semiconductor Corporation
5
Share Link: 