SOT-89 Step-Up PFM Switcher with Auto-Load Sense
The ILC6390 performs boost DC-DC conversion by controlling the
switch element shown in the circuit below.
Because of this, PFM is sometimes called “Pulse Skipping
Modulation.”
The chief advantage of using a PFM technique is that, at low cur-
rents, the switcher is able to maintain regulation without con-
stantly driving a switch on and off. This power savings can be
5mA or more for the ILC6390 versus the ILC6370, and at very
light loads this current difference can make a noticeable impact
on overall efficiency.
However, because the ILC6390 will skip pulses based on
load current, the effective frequency of switching may well
drop into the audio band. This means that the radiated
noise of the ILC6390 may interfere with the audio channel
of the system and additional filtering may be necessary. In
addition, because the PFM on-time is fixed, it usually has
higher output ripple voltage than the PWM switcher, which
dynamically changes the on-time to match the load current
requirements. [Ripple
is due to the output cap constantly
accepting and storing the charge received from the induc-
tor, and delivering charge as required by the load. The
“pumping” action of the switch produces a sawtooth-
shaped voltage as seen by the output.]
On the plus side, because pulses are skipped, overtone content of
the frequency noise is lower than in a PWM configuration. The
sum of these characteristics for PFM converters makes it the ideal
choice for low-current or ultra-long runtime applications, where
overall conversion efficiency at low currents is of primary concern.
[For
other conversion techniques, please see the ILC6370/71 and
ILC6380/81 datasheets.]
Dual-Step Mode
The ILC6390 and ILC6391 have one other unique feature, that
being to automatically switch to a second switching scheme in the
presence of heavy output loading. As we mentioned, the stan-
dard switching scheme for these parts is a 3.55msec, 155kHz,
55% duty cycle part. However, if the device detects that the out-
put load increases beyond a set point (as seen by the voltage
drop on the output capacitor), it switches in a 7.5msec, 100kHz,
75% duty cycle “turbo mode” specifically to keep up with the
increased load demand. This switchover is seamless to the user,
but will result in a change in the output ripple voltage characteris-
tic of the DC-DC converter.
PFM converters are widely used in portable consumer applica-
tions not requiring a high current level and relatively unaffected by
audio noise. Applications such as pagers and PDAs, which need
to operate in stand-by for extended periods of time, gravitate
toward the advantages of PFM since maximum run-time is a chief
differentiating element. The ILC6390 addresses this low-current
requirement, and additionally offers a “turbo” mode which main-
tains output regulation in the presence of heavier-than-normal
load currents, and maintains 0.5mA shutdown currents.
The only difference between the ILC6390 and ILC6391 parts is
that the 6391 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, 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 ILC6390 mon-
itors 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.
P
L
= ƒ(t
ON
, V
IN
)
The ILC6390 and ILC6391 use a PFM or Pulse Frequency
Modulation technique. In this technique, the switch is always
turned on for a fixed period of time, corresponding to a fixed
switching frequency at a predefined duty cycle. For the ILC6390
this value is 3.55msec on time, corresponding to 55% duty cycle
at 155kHz. Because the inductor value, capacitor size, and
switch on-time and frequency are all fixed, the ILC6390 in essence
delivers the same amount of power to the output during each
switching cycle. This in turn creates a constant output voltage
ramp which is dependent on the output load requirement. In this
mode, the only difference between the PFM and PWM techniques
is the duty cycle of the switch.
Once the output voltage reaches the set point, the ILC6390 will
shut off the switch oscillator and wait until the output voltage
drops low again, at which point it will re-start the oscillator. As
you can see in the diagram, the PFM boost converter actually
skips pulses as a way of varying the amount of power being deliv-
ered to the output.
Switch Waveform
V
SET
V
OUT
Impala Linear Corporation
ILC6390 1.1
(408) 574-3939
www.impalalinear.com
Feb 2001
5
IMPALA [ Impala Linear Corporation ]
