ILC6382-33 데이터 시트보기 (PDF) - Impala Linear Corporation

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ILC6382-33

1-Cell to 3-Cell Boost with True Load Disconnect, 3.3V, 5V or Adjustable Output

Impala Linear Corporation 

1-Cell to 3-Cell Boost with True Load Disconnect, 3.3V, 5V, or Adjustable Output

APPLICATIONS INFORMATION

The ILC6382 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 has to go

somewhere and 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 gets boost-

ed relative to the input.

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.

PL = ƒ(tON, VIN)

Synchronous Rectification

The ILC6382 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 ILC6382, an FET as shown in

figure 4 below.

VIN

LX

SW2

SW1

PWM/PFM

CONTROLLER

ILC6382

VOUT

-

+

POK

GND

SHUTDOWN

CONTROL

+

VREF

-

DELAY

LBO

SYNC

LB/SD

Figure 4: Simplified ILC6382 block diagram

The two switches now open and close in opposition to

eachother, directing the flow of current to either charge the

inductor or to feed the load. The ILC6382 monitors the volt-

age on the output capacitor to determine how much and

how often to drive the switches.

PWM Mode Operation

The ILC6382 uses a PWM or Pulse Width Modulation tech-

nique. 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.

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

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.

The other key advantage of the PWM type controllers is that

the radiated noise due to the switching transients will

always occur at the (fixed) switching frequency. Many appli-

cations do not care much about switching noise, but certain

types of applications, especially communication equipment,

need to minimize the high frequency interference within

their system as much as possible. Using a boost converter

requires a certain amount of higher frequency noise to be

generated; using a PWM converter makes that noise high-

ly predictable thus easier to filter out.

PFM Mode Operation

There are downsides of PWM approaches, especially at

very low currents. Because the PWM technique relies on

constant switching and varying duty cycle to match the load

conditions, there is some point where the load current gets

too small to be handled efficiently. An actual switch con-

sumes some finite amount of current to switch on and off; at

very low currents this can be of the same magnitude as the

load current itself, driving switching efficiencies down to

50% and below. The ILC6382 overcomes this limitation by

automatically switching over to a PFM, or Pulse Frequency

Modulation, technique at low currents. This technique con-

serves 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.

Impala Linear Corporation

ILC6382 1.5

(408) 574-3939 www.impalalinear.com

Oct 1999 6

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