PULSE FREQUENCY MODULATION (PFM)

Pulse Frequency Modulation is typically accomplished by

switching continuously until the voltage limit is reached and

skipping cycles after that to just maintain it. This results in a

somewhat hysteretic mode of operation. The coil stores

more energy each cycle as the current ramps up to high

levels. When the voltage limit is reached, the system usually

overshoots to a higher voltage than required, due to the

stored energy in the coil (see Figure 3). The system will also

undershoot somewhat when it starts switching again be-

cause it has depleted all the stored energy in the coil and

needs to store more energy to reach equilibrium with the

load. Larger output capacitors and smaller inductors reduce

the ripple in these situations. The frequency being filtered,

however, is not the basic switching frequency. It is a lower

frequency determined by the load, the input/output voltage

and the circuit parameters. This mode of operation is useful

in situations where the load variation is significant. Power

managed computer systems, for instance, may vary from

zero to full load while the system is on and this is usually the

preferred regulation mode for such systems.

CYCLE TO CYCLE PFM

When the load doesn’t vary over a wide range (like zero to

full load), ratio adaptive circuit techniques can be used to

achieve cycle to cycle PFM regulation and lower ripple (or

smaller output capacitors). The key to success here is

matching the duty cycle of the circuit closely to what is

required by the input to output voltage ratio. This ratio then

needs to be dynamically adjusted for input voltage changes

(usually caused by batteries running down). The chosen

ratio should allow most of the energy in each switching cycle

to be delivered to the load and only a small amount to be

stored. When the regulation limit is reached, the overshoot

will be small and the system will settle at an equilibrium point

where it adjusts the off time in each switching cycle to meet

the current requirements of the load. The off time adjustment

is done by exceeding the regulation limit during each switch-

ing cycle and waiting until the voltage drops below the limit

again to start the next switching cycle. The current in the coil

never goes to zero like it frequently does in the hysteretic

operating mode of circuits with wide load variations or duty

cycles that aren’t matched to the input/output voltage ratio.

Optimizing the duty cycle for a given set of input/output

voltages conditions can be done by using the circuit values

in the Application Notes.

LOW VOLTAGE START-UP

The LM4804 can start-up from voltages as low as 1.1 volts.

On start-up, the control circuitry switches the N-channel

MOSFET continuously until the output reaches 3 volts. After

this output voltage is reached, the normal step-up regulator

feedback and gated oscillator control scheme take over.

Once the device is in regulation, it can operate down to

below .8V input, since the internal power for the IC can be

boot-strapped from the output using the Vdd pin.

SHUT DOWN

The LM4804 features a shutdown mode that reduces the

quiescent current to less than a guaranteed 2.5uA over

temperature. This extends the life of the battery in battery

powered applications. During shutdown, all feedback and

control circuitry is turned off. The regulator’s output voltage

drops to one diode drop below the input voltage. Entry into

the shutdown mode is controlled by the active-low logic input

pin EN (pin- 2). When the logic input to this pin is pulled

below 0.15V

logic input to this pin should be above 0.7V

to work in normal step-up mode.

SELECTING OUTPUT CAPACITOR (C

CONVERTER

A single ceramic capacitor of value 4.7µF to 10µF will pro-

vide sufficient output capacitance for most applications. If

larger amounts of capacitance are desired for improved line

support and transient response, tantalum capacitors can be

used. Aluminum electrolytics with ultra low ESR such as

Sanyo Oscon can be used, but are usually prohibitively

expensive. Typical AI electrolytic capacitors are not suitable

for switching frequencies above 500 kHz due to significant

ringing and temperature rise due to self-heating from ripple

current. An output capacitor with excessive ESR can also

reduce phase margin and cause instability.

In general, if electrolytics are used, it is recommended that

they be paralleled with ceramic capacitors to reduce ringing,

switching losses, and output voltage ripple.

INTERNAL CURRENT LIMIT AND THERMAL

PROTECTION

An internal cycle-by-cycle current limit serves as a protection

feature. This is set high enough (2.85A typical, approxi-

mately 4A maximum) so as not to come into effect during

normal operating conditions. An internal thermal protection

circuit disables the MOSFET power switch when the junction

temperature (T

enabled when T

NON-LINEAR EFFECT

The LM4804 takes advantage of a non-linear effect that

allows for the duty cycle to be programmable. The C3 ca-

pacitor is used to dump charge on the FREQ pin in order to

manipulate the duty cycle of the internal oscillator. The part

is being tricked to behave in a certain manner, in the effort to

make this Pulse Frequency Modulated (PFM) boost switch-

ing regulator behave as a Pulse Width Modulated (PWM)

boost switching regulator.

CHOOSING THE CORRECT C3 CAPACITOR

The C3 capacitor allows for the duty cycle of the internal

oscillator to be programmable. Choosing the correct C3

capacitor to get the appropriate duty cycle for a particular

application circuit is a trial and error process. The non-linear

effect that C3 produces is dependent on the input voltage

and output voltage values. The correct C3 capacitor for

particular input and output voltage values cannot be calcu-

lated. Choosing the correct C3 capacitance is best done by

trial and error, in conjunction with the checking of the induc-

tor peak current to make sure your not too close to the

current limit of the device. As the C3 capacitor value in-

creases, so does the duty cycle. And conversely as the C3

capacitor value decreases, the duty cycle decreases. An

incorrect choice of the C3 capacitor can result in the part

prematurely tripping the current limit and/or double pulsing,

which could lead to the output voltage not being stable.

SETTING THE OUTPUT VOLTAGE

The output voltage of the step-up regulator can be set by

connecting a feedback resistive divider made of R

R

R

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