ILC6383

PRODUCT SPECIFICATION

6

REV. 1.2.6 6/13/02

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.

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.

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.

Figure 4. Simplified ILC6382 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 ILC6383 monitors the voltage on

the output capacitor to determine how much and how often

to drive the switches.

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

POK

LBO

LB/SD

SEL

GND

ILC6383

PWM/PFM

CONTROLLER

SHUTDOWN

CONTROL

DELAY

+

+

-

-

SW1

SW2

Switch Waveform