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=R2X(V

(5)

FEED-FORWARD COMPENSATION FOR BOOST

CONVERTER

Although the LM4802B’s internal Boost converter is inter-

nally compensated, the external feed-forward capacitor C

required for stability (see Figure 1). Adding this capacitor

puts a zero in the loop response of the converter. The

recommended frequency for the zero fz should be approxi-

mately 6kHz. C

C

(6)

SELECTING DIODES

The external diode used in Figure 1 should be a Schottky

diode. A 20V diode such as the MBR0520 is recommended.

The MBR05XX series of diodes are designed to handle a

maximum average current of 0.5A. For applications exceed-

ing 0.5A average but less than 1A, a Microsemi UPS5817

can be used.

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.

INDUCTANCE VALUE

The first question we are usually asked is: “How small can I

make the inductor.” (because they are the largest sized

component and usually the most costly). The answer is not

simple and involves trade-offs in performance. Larger induc-

tors mean less inductor ripple current, which typically means

less output voltage ripple (for a given size of output capaci-

tor). Larger inductors also mean more load power can be

delivered because the energy stored during each switching

cycle is:

E = L/2 X (lp)2

Where “lp” is the peak inductor current. An important point to

observe is that the LM4802B will limit its switch current

based on peak current. This means that since lp(max) is

fixed, increasing L will increase the maximum amount of

power available to the load. Conversely, using too little in-

ductance may limit the amount of load current which can be

drawn from the output.

Best performance is usually obtained when the converter is

operated in “continuous” mode at the load current range of

interest, typically giving better load regulation and less out-

put ripple. Continuous operation is defined as not allowing

the inductor current to drop to zero during the cycle. It should

be noted that all boost converters shift over to discontinuous

operation as the output load is reduced far enough, but a

larger inductor stays “continuous” over a wider load current

range.

To better understand these trade-offs, a typical application

circuit (5V to 12V boost with a 10µH inductor) will be ana-

lyzed. We will assume:

V

Since the frequency is 1.6MHz (nominal), the period is ap-

proximately 0.625µs. The duty cycle will be 62.5%, which

means the ON-time of the switch is 0.390µs. It should be

noted that when the switch is ON, the voltage across the

inductor is approximately 4.5V. Using the equation:

V = L (di/dt)

We can then calculate the di/dt rate of the inductor which is

found to be 0.45 A/µs during the ON-time. Using these facts,

we can then show what the inductor current will look like

during operation:

During the 0.390µs ON-time, the inductor current ramps up

0.176A 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 33mA,

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

maintained at the typical load current values.

MAXIMUM SWITCH CURRENT

The maximum FET switch current available before the cur-

rent limiter cuts in is dependent on duty cycle of the appli-

cation. This is illustrated in a graph in the typical perfor-

mance characterization section which shows typical values

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

20094198

FIGURE 2. 10µH Inductor Current

5V - 12V Boost (LM4802B)

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