www.fairchildsemi.com
ILC6370/6371
Super Small Switching Regulator with Shutdown
Features
•
•
•
•
•
•
•
85% efficiency at 50mA
900mV Start-up voltage
±2.5% output accuracy
Only 3 external components required
50, 100 and 180kHz versions available
0.5µA supply current in shutdown mode
External transistor option supports up to 1A load currents
Description
The ILC6370 is a compact 50mA boost converter offered in
a 5-lead SOT-89 package. Only three external components
are needed to complete the switcher design, and frequency
options of 50, 100, and 180kHz give the designer the ability
to accommodate a wide range of system objectives, includ-
ing size, electromagnetic interference requirements and so
on. 87% max duty cycle gives conversion efficiencies of
85%.
Standard voltage options of 2.5V, 3.3V, and 5.0V at ±2.5%
accuracy feature on-chip phase compensation and soft-start
design.
The ILC6371 is designed to drive an external transistor for
high current switching regulators applications with all the
features and benefits of the ILC6370 retained.
Applications
•
•
•
•
Cellular Phones, Pagers
Portable Cameras and Video Recorders
Palmtops and PDAs
Battery Powered Systems
Block Diagram
L
X
V
LX
LIMITER
BUFFER
Slow Start
V
DD
V
OUT
V
SS
PWM Controlled
OSC
50/100/180KHz
V
ref
Loop comp
EXT
CE
-
CHIP ENABLE
+
V
DD
is internally connected to the V
OUT
pin.
REV. 1.1.2 4/19/02
ILC6370/6371
Functions and Operation
The ILC6370 performs boost DC-DC conversion by control-
ling the switch element shown in the circuit below.
The other key advantage of the PWM type controllers is that
the radiated noise due to the switching transients will always
occur at a fixed switching frequency. Many applications are
insensitive to switching noise, but certain types of applica-
tions, especially communication equipment, need to mini-
mize the high frequency interference within their system as
much as is possible. Using a boost converter requires a cer-
tain amount of high frequency noise to be generated; using a
PWM converter makes that noise spectrum predictable; thus
it is easier to filter it.
There are downsides of PWM approaches, especially at very
low currents. Since the PWM techniques rely on constant
switching and varying duty cycle to match the load condi-
tions, there is a minimum load current that can be handled
efficiently. If the ILC6370/6371 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 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. Filtering
requirements will vary by application and by actual system
design and layout, so generalization in this area is difficult, at
best.
[For other boost converter techniques, please refer to
the ILC6380/81 and ILC6390/91 data sheets.]
Nevertheless,
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 compo-
nents. Fairchild’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
element. Since larger transistors can be selected for this
element, higher effective loads can be regulated.
When the switch is closed, energy is built up in the inductor.
When the switch opens, this energy is forced to pass through
the diode to the output. As the on and off cycles continue,
the output capacitor voltage builds up due to energy being
transferred from the inductor
Consequently, the output voltage is boosted with respect to
input. The ILC6370/6371 monitors the voltage on the output
capacitor in order to determine how much energy should be
transferred through the switch.
In general, the switching characteristic is determined by the
desired output voltage and the required load current. Specifi-
cally, the energy transfer is determined by the energy stored
in the coil during each switching cycle.
EL = ƒ(t
ON
, V
IN
)
where EL is the energy stored in the inductor, t
ON
is the ON
time and V
IN
is the input voltage.
The ILC6370/6371 use a Pulse Width Modulation (PWM)
technique. The devices are offered with one of three fixed
internal frequencies: 50, 100, or 180kHz. The switches are
continuously driven at these frequencies. The control cir-
cuitry 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 energy built up in the inductor, the
maximum 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. The internal bias (V
OD
) is
provided by the output voltage V
OUT
..
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
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 capacitor con-
stantly 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
voltage as seen at the output.]
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.
V
OUT MIN
V
IN
- V
f
T
SOFT-START
(~10msec)
t=0
REV. 1.1.2 4/19/02
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