coefficient dielectrics, such as tantalum or aluminum elec-

trolytic. Capacitors with high-voltage coefficients, such as

ceramics, may result in increased distortion at low frequen-

cies. Other factors to consider when designing the input filter

include the constraints of the overall system. Although high

fidelity audio requires a flat frequency response between

20Hz and 20kHz, portable devices such as cell phones may

only concentrate on the frequency range of the frequency

range of the spoken human voice (typically 300Hz to 4kHz).

In addition, the physical size of the speakers used in such

portable devices limits the low frequency response; in this

case, frequencies below 150Hz may be filtered out.

SELECTING OUTPUT CAPACITOR (C

CONVERTER

A single 100µF low ESR tantalum capacitor provides suffi-

cient output capacitance for most applications. Higher capac-

itor values improve line regulation and transient response.

Typical electrolytic capacitors are not suitable for switching

converters that operate above 500kHz because of significant

ringing and temperature rise due to self-heating from ripple

current. An output capacitor with excessive ESR reduces

phase margin and causes instability.

SELECTING INPUT CAPACITOR (Cs1) FOR BOOST

CONVERTER

An input capacitor is required to serve as an energy reservoir

for the current which must flow into the coil each time the

switch turns ON. This capacitor must have extremely low

ESR, so ceramic is the best choice. We recommend a nomi-

nal value of 2.2µF, but larger values can be used. Since this

capacitor reduces the amount of voltage ripple seen at the

input pin, it also reduces the amount of EMI passed back

along that line to other circuitry.

SELECTING SOFTSTART (C

The soft-start function charges the boost converter reference

voltage slowly. This allows the output of the boost converter

to ramp up slowly thus limiting the transient current at startup.

Selecting a soft-start capacitor (C

off between the wake-up time and the startup transient cur-

rent. Using a larger capacitor value will increase wake-up time

and decrease startup transient current while the apposite ef-

fect happens with a smaller capacitor value. A general guide-

line is to use a capacitor value 1000 times smaller than the

output capacitance of the boost converter (C

start capacitor is recommended for a typical application.

SETTING THE OUTPUT VOLTAGE (V

CONVERTER

The output voltage is set using the external resistors R1 and

R2 (see Figure 1). A value of approximately 13.3k

Ω is rec-

ommended for R2 to establish a divider current of approxi-

mately 92µA. R1 is calculated using the formula:

R1 = R2 X (V

(4)

FEED-FORWARD COMPENSATION FOR BOOST

CONVERTER

Although the LM48520's internal Boost converter is internally

compensated, the external feed-forward capacitor C

quired for stability (see Figure 1). Adding this capacitor puts

a zero in the loop response of the converter. The recom-

mended frequency for the zero fz should be approximately

6kHz. C

C

(5)

SELECTING DIODES FOR BOOST

The external diode used in Figure 1 should be a Schottky

diode. A 20V diode such as the MBRS320T3 is recommend-

ed.

The MBRS320T3 series of diodes are designed to handle a

maximum average current of 3A.

DUTY CYCLE

The maximum duty cycle of the boost converter determines

the maximum boost ratio of output-to-input voltage that the

converter can attain in continuous mode of operation. The

duty cycle for a given boost application is defined as:

Duty Cycle = V

This applies for continuous mode operation.

SELECTING INDUCTOR VALUE

Inductor value involves trade-offs in performance. Larger in-

ductors reduce inductor ripple current, which typically means

less output voltage ripple (for a given size of output capacitor).

Larger inductors also mean more load power can be delivered

because the energy stored during each switching cycle is:

E = L/2 X (I

2

Where “lp” is the peak inductor current. The LM48520 will limit

its switch current based on peak current. With I

creasing L will increase the maximum amount of power avail-

able to the load. Conversely, using too little inductance may

limit the amount of load current which can be drawn from the

output. Best performance is usually obtained when the con-

verter is operated in “continuous” mode at the load current

range of interest, typically giving better load regulation and

less output ripple. Continuous operation is defined as not al-

lowing the inductor current to drop to zero during the cycle.

Boost converters shift over to discontinuous operation if the

load is reduced far enough, but a larger inductor stays con-

tinuous over a wider load current range.

During the TBDµs ON-time, the inductor current ramps up

TBDA and ramps down an equal amount during the OFF-

time. This is defined as the inductor “ripple current”. It can also

be seen that if the load current drops to about TBDmA, the

inductor current will begin touching the zero axis which means

it will be in discontinuous mode. A similar analysis can be

performed on any boost converter, to make sure the ripple

current is reasonable and continuous operation will be main-

tained at the typical load current values.

MAXIMUM SWITCH CURRENT

The maximum FET switch current available before the current

limiter cuts in is dependent on duty cycle of the application.

This is illustrated in a graph in the typical performance char-

acterization section which shows typical values of switch

current as a function of effective (actual) duty cycle.

CALCULATING OUTPUT CURRENT OF BOOST

CONVERTER (I

As shown in Figure 2 which depicts inductor current, the load

current is related to the average inductor current by the rela-

tion:

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