ILC6390/91

5

©2001 Fairchild Semiconductor Corporation

The ILC6390 performs boost DC-DC conversion by control-

ling the switch element shown in the circuit below.

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 boosted

relative to the input. The ILC6390 monitors 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.

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 differ-

ence 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 delivered to the output.

Because of this, PFM is sometimes called “Pulse Skipping

Modulation.”

The chief advantage of using a PFM technique is that, at low

currents, the switcher is able to maintain regulation without

constantly 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 addi-

tion, because the PFM on-time is fixed, it usually has higher

output ripple voltage than the PWM switcher, which dynam-

ically changes the on-time to match the load current require-

ments. [Ripple is due to the output cap constantly accepting

and storing the charge received from the inductor, and deliv-

ering 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 con-

tent of the frequency noise is lower than in a PWM configu-

ration. The sum of these characteristics for PFM converters

makes it the ideal choice for low-current or ultra-long runt-

ime applications, where overall conversion efficiency at low

currents is of primary concern. [For other conversion tech-

niques, 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 men-

tioned, the standard switching scheme for these parts is a

3.55msec, 155kHz, 55% duty cycle part. However, if the

device detects that the output 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 characteristic

of the DC-DC converter.

PFM converters are widely used in portable consumer appli-

cations not requiring a high current level and relatively unaf-

fected by audio noise. Applications such as pagers and

PDAs, which need to operate in stand-by for extended peri-

ods 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 addi-

tionally offers a “turbo” mode which maintains output regu-

lation 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 tran-

sistor as the switch element. Since larger transistors can be

selected for this element, higher effective loads can be regu-

lated.

V

SET

V

OUT

Switch Waveform