ILC6363 Datasheet, PDF (5/14 Pages) Fairchild Semiconductor – Step-Up DC-DC Converter for One-Cell Lithium-Ion Batteries

PRODUCT SPECIFICATION
ILC6363
Application Information
The ILC6363 performs boost DC-DC conversion by control-
ling the switch element as shown in the simplified circuit in
Figure 3 below.
Figure 3. Basic Boost Circuit
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_L = ƒ(t_ON, V_IN)
Synchronous Rectification
The ILC6363 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 ILC6363, an FET as shown in
Figure 4 below.
VIN
LX
SW2
ILC6363
VOUT
-
SW1
PWM/PFM
CONTROLLER
+
POK
GND
SHUTDOWN
CONTROL
+
VREF _-
DELAY
LBO
SEL
LB/SD
Figure 4. Simplified ILC6383 block diagram
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 ILC6363 monitors the voltage on
the output capacitor to determine how much and how often
to drive the switches.
PWM Mode Operation
The ILC6363 uses a PWM or Pulse Width Modulation
technique. The switches are constantly driven at typically
300kHz. The control circuitry varies the power being
delivered to the load by varying the on-time, or duty cycle,
of the switch SW1 (see Figure 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 maintain 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) types is that the radiated
noise due to the switching transients will always occur at
(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 in those cases
is desirable.
PFM Mode Operation
For light loads the ILC6363 can be switched to PFM. This
technique conserves power 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 ILC6363, the switchover from PWM to PFM mode is
determined by the user to improve efficiency and conserve
power.
The Dual PWM/PFM mode architecture was designed spe-
cifically for applications such as wireless communications,
which need the spectral predictability of a PWM-type
DC-DC converter, yet also need the highest efficiencies
possible, especially in Standby mode.
REV. 1.3.5 5/21/02
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