ilc6383 Fairchild Semiconductor, ilc6383 Datasheet - Page 6

• Manufacturer Part Number: ilc6383
• Description: 1-cell To 3-cell Boost Dc-dc Converter With True Load Disconnect, 3.3v, 5v Or Adjustable Output
• Manufacturer: Fairchild Semiconductor

ILC6383
Applications Information
The ILC6383 performs boost DC-DC conversion by control-
ling the switch element as shown in the simplified circuit in
Figure 3 below.
When the switch is closed, current is built up through the
inductor. When the switch opens, this current 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 is boosted relative to the input.
In general, the switching characteristic is determined by the
output voltage desired and the current required by the load.
The energy transfer is determined by the power stored in the
coil during each switching cycle.
P
Synchronous Rectification
The ILC6383 also uses a technique called “synchronous
rectification” which removes the need for the external diode
used in other circuits. The diode is replaced with a second
switch or in the case of the ILC6383, an FET as shown in
Figure 4 below.
The two switches now open and close in opposition to each
other, directing the flow of current to either charge the induc-
tor or to feed the load. The ILC6383 monitors the voltage on
the output capacitor to determine how much and how often
to drive the switches.
6
L
GND
= ƒ(t
L
X
Figure 4. Simplified ILC6382 block diagram
ON
SW1
, V
IN
Figure 3. Basic Boost Circuit
SEL
)
V
IN
CONTROLLER
SHUTDOWN
PWM/PFM
CONTROL
SW2
V
REF
LB/SD
+
-
+
-
DELAY
ILC6383
LBO
POK
V
OUT
PWM Mode Operation
The ILC6383 uses a PWM or Pulse Width Modulation
technique. The switches are constantly driven at typically
300kHz. The control circuitry varies the power being deliv-
ered to the load by varying the on-time, or duty cycle, of the
switch SW1 (see Fig. 5). 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 minimum value of the duty
cycle determines the minimum load current that can main-
tain the output voltage within specified values.
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 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 accept-
ing 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 by the output.
The other key advantage of the PWM type controllers over
pulse frequency modulated (PFM) type 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 applica-
tions, especially communication equipment, need to mini-
mize the high frequency interference within their system as
much as possible. Use of the PWM converter is those cases
is desirable.
PFM Mode Operation
For light loads the ILC6383 can be switched to PFM
technique at low currents. This technique conserves power
loss by only switching the output if the current drain requires
it. As shown in the Figure 5, the waveform actually skips
pulses depending on the power needed by the output. This
technique is also called “pulse skipping” because of this
characteristic.
In the ILC6383, the switchover from PWM to PFM mode is
determined by the user to improve efficiency and conserve
power.
Figure 5. PFM Waveform
PRODUCT SPECIFICATION
Switch Waveform
REV. 1.2.6 6/13/02
V
V
OUT
SET